New and Refined Methods 

of Trace Analysis Useful in 

Geochemical Exploration

GEOLOGICAL SURVEY BULLETIN 1408

DECS 1.3/5

l^ty-S^^V li&z s -* l *^r .;^^j

New and Refined Methods 

of Trace Analysis Useful in 

Geochemical Exploration

F. N. Ward, Editor

GEOLOGICAL SURVEY BULLETIN 140

A collection of methods additional to 

those presented earlier in U.S. Geological 

Survey Bulletins 1152 and 1289

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

New and refine methods of trace analysis useful in geochemical exploration.

(Geological Survey Bulletin 1408)

Includes bibliographies.

Supt.ofDocs.no.: 119.3:1408

1. Trace elements Analysis. 2. Geochemical prospecting.

I. Ward, Frederick Norville, 1915- II. Series: United States Geological Survey

Bulletin 1408. 

QE75.B9 no. 1408 [QD139.T7] 557.3'083 [622'.13] 75-619319

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-02745-3
CONTENTS

Page

Abbreviations.................................................................................................................. Ill

Introduction, by F. N. Ward .......................................................................................... 1

Atomic absorption determination of antimony in geologic materials, by John B.

McHugh and Eric P. Welsch...................................................................................... 5

Determination of arsenic in vegetation, by T. F. Harms and F. N. Ward................... 13

Flameless atomic absorption method for determination of trace amounts of

mercury in vegetation, by John B. McHugh and Robert L. Turner......................... 21

Atomic absorption determination of molybdenum in plant ash, by H. M.

Nakagawa, J. R. Watterson, and F. N. Ward............................................................. 29

Determination of selenium in vegetation, by T. F. Harms and F. N. Ward................ 37

Atomic absorption determination of bismuth, cadmium, and lead in vegetation

after oxygen flask combustion, by T. F. Harms and F. N. Ward.............................. 43

Determination of mercury in natural waters, plants, and soils by a cold-vapor

procedure, by Margaret E. Hinkle and George L. Crenshaw.................................... 51

Determination of molybdenum in natural waters and brines after separation

using a chelating resin, by H. M. Nakagawa and F. N. Ward.................................. 65

Determination of fluorine in soils and rocks by known-increment addition and

selective-ion electrode detection, by George L. Crenshaw and F. N. Ward.............. 77

Atomic absorption determination of silver, bismuth, cadmium, cobalt, copper,

nickel, lead, and zinc in calcium- and iron-rich geologic materials, by

H. M. Nakagawa......................................................................................................... 85

Use of emission spectroscopy for the semiquantitative analysis of trace elements

and silver in native gold, by Elwin L. Mosier........................................................... 97

ABBREVIATIONS

A.................. ampere (A is closed up to its value, such as 2.176A).

A.................. angstrom (A is separated by a space from its value, such as 372 A).

A/min.......... angstroms per minute.

a.c................ alternating current.

A.C.S........... American Chemical Society A measure of purity of reagent; purity

	equals or exceeds that specified by a Committee of the American

	Chemical Society. 

APDC,......... ammonium 1-pyrrolidine dithiocarbamate.

APDC/MIBK ammonium 1-pyrrolidine dithiocarbamate-methyl isobutyl ketone.

y ................. activity coefficient.

B.O.D.......... Biochemical Oxygen Demand.

°C................. degree Celsius.

cm................ centimetre.

cm3 ............... cubic centimetre.

cmVmin ...... cubic centimetres per minute.

DAB............. diaminobenzidine.

DADC.......... diethylammonium diethyldithiocarbamic acid.
IV CONTENTS

DADC/MIBK diethylammonium diethyldithiocarbamic acid-methyl isobutyl ketone.

DAN............ diaminonaphthalene.

d.c................ direct current.

g................... gram.

> ................. greater than.

HC............... hollow cathode lamp.

< ................. less than.

1................... litre.

1/min .......... litres per minute.

Ib/in2g......... pounds per square inch, gage.

pH ............... measure of hydrogen ion activity.

j^g............... microgram.

/Jg/g........... micrograms per gram.

fj.g/1 ........... micrograms per litre.

Hg/ml......... micrograms per millilitre.

/J m.............. micrometre.

mA............... milliampere.

mg............... milligram.

MIBK........... methyl isobutyl ketone, the common name for 4-methyl-2-pentanone.

ml................ millilitre.

mm.............. millimetre.

mv................ millivolt.

M ................. molar; a molar solution contains one gram mole of solute in 1,000 ml

	of solution. Except for common acids and bases, the units of concentra- 

	tion are expressed as weight/volume in which a given weight of the 

	reagent is dissolved in a specified volume of water or other solvent.

ng ................ nanogram =1 x 10" 9 gram.

ng/ml.......... nanograms per millilitre.

nm ............... nanometre = lx!0~9 metre.

N.................. normal; a normal solution contains one equivalent weight of solute in

	1 litre of solution. One equivalent weight is that number of grams 

	that react with 1.008 grams of hydrogen.

IX ................ one times refers to scale expansion in atomic absorption procedures.

O.D.............. outside diameter.

ppb.............. parts per billion = 1 part in 109 parts.

ppm............. parts per million = 1 part in 106 parts = micrograms per gram or micro- 

	grams per millilitre for most aqueous solutions.

r/min........... revolutions per minute.

"§......... ......... standard taper.

TISAB ......... total ionic strength adjustment buffer.

v....... : ........... volt.
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

INTRODUCTION

By F. N. WARD

This bulletin is composed of 11 short papers which describe one or 

more methods developed during the last 5 years and found useful by 

the U.S. Geological Survey for determining trace amounts of 14 elements 

in geologic materials, including soils, rocks, brines, and vegetation. The 

elements are antimony, arsenic, bismuth, cadmium, cobalt, copper, 

fluorine, lead, mercury, molybdenum, nickel, selenium, silver, and zinc. 

As with methods given in previous publications in this series (Ward 

and others, 1963; Ward and others, 1969), these methods either have been 

adapted from existing procedures or have been developed in the U.S. 

Geological Survey laboratories to increase the analytical capacity of the 

Survey in its continuing effort to develop means of identifying both large 

and small exploration targets composed of different minerals that have 

economic potential and that are commonly concealed or buried beneath 

the present surface.

Some of the methods have been modified for specific applications; 

for example, the method described by Nakagawa was developed to measure trace amounts of eight elements in a single solution of magnetite 

or of a plant ash. Other methods illustrate applications of existing methods to different sample media for example, the analysis of vegetation 

by atomic absorption techniques. A few of the methods are the products 

of original research for example, the atomic absorption determination 

of bismuth, cadmium, and lead in vegetation after oxygen flask combustion. None of the methods should be considered as final; all are subject 

to improvement to obtain better sensitivity, to achieve a greater degree 

of freedom from interferences, and to permit application to different 

types of geologic materials.

With the exception of the spectrographic method described by Mosier, 

all these methods feature a sample breakup partial in some procedures by hot acid digestion, oxygen flask combustion, or fusion with 

appropriate fluxes, followed by solvent extraction, complexation, or distillation to achieve separation and enrichment of the desired element
* TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

in a uniform matrix amenable to atomic absorption or other techniques 

of measurement.

A few of the methods terminate with a colorimetric measurement, 

which may appear to relegate the procedures to some form of alchemy, 

especially in view of the pronounced trend toward instrumental analysis 

since about 1960. The justification for these procedures, if such is required, is simply that, despite the fast communication and rapid transportation of the day, many exploration groups do not have sophisticated 

instruments nor do they wish to buy the skill needed to maintain such 

instruments. On the other hand, several methods described in this bulletin terminate with an atomic absorption measurement. They are 

obviously useful only to groups having such instruments. Of the methods based on atomic absorption spectrometry, both flame and flameless 

procedures are described. The two flameless methods given for mercury 

illustrate the advantage of allowing two different research groups to 

attack the same problem concomitantly.

Methods of chemical analysis for trace elements based on fluorescence 

generally have not been well received by geochemists. Perhaps the lack 

of popularity arises from a combination of the peculiar skill needed by 

the analyst for successful application of fluorescence to geochemical exploration, the cost of instrumentation usually considered to be necessary, 

and the lengthy preparations involved in making fluorescence measurements. Despite these limitations, the advantages of greater sensitivity 

and selectivity often predominate, as is true for the selenium method 

described by Harms and Ward. In fact, that method for determining 

selenium has no peer, except for the procedures requiring activation 

or possibly spark-source mass spectrography facilities.

The use of selective-ion electrodes to measure trace amounts of ions 

in solution is relatively new, and the simplicity of the procedure is deceiving with respect to the complex phenomena represented. The measurement is made by immersing the selective-ion electrode in a solution 

and reading the potential developed against a reference electrode. In 

short, the detection of a given ion is as simple as a pH measurement. 

Explanation of the complex phenomena, as well as the technology involved in the development of sensors in the ion-selective electrodes, is 

beyond the scope of this bulletin, but it will be enough to say that the 

electrode senses the activity of a particular ion in a selective manner 

similar to the way a thin glass membrane senses the activity of hydrogen 

ions. Pure, or properly "doped" crystals of rare earths are one class of 

sensors, and they act the same as the glass membrane by selectively allowing certain ions to behave as if they were passing through the crystal 

and developing a potential. The kind of crystal and doping govern the 

selectivity. Of more than a dozen selective-ion electrodes available today, 

the fluoride electrode is perhaps the most widely used. It consists of a 

crystal of pure lanthanum fluoride sealed in an inert epoxy tube and
INTRODUCTION 3

connected through an internal solution to a lead wire terminating in 

a suitable plug (Ross, 1969). Crenshaw and Ward describe a method for 

determining fluorine in soils and rocks, using such an electrode to sense 

the change in potential caused by a known-increment addition step.

The papers in this report were selected Hby the editor on the basis of 

improvements made in a given procedure, hints and helps to promote 

the usefulness of a method, and demand for, and interest in, an analytical 

procedure by the exploration geochemist. Each paper includes a description of the method, a list of reagents and apparatus necessary for the 

procedure, analytical data, and appropriate documentation. The information presented in this bulletin reflects the thrust of recent research 

in new methodology by the U.S. Geological Survey in the field of geochemical and biogeochemical exploration.

REFERENCES CITED

Ross, J. W., Jr., 1969, Solid-state and liquid membrane ion-selective electrodes, in Ionselective electrodes: [U.S.] Natl. Bur. Standards Spec. Pub. 314, p. 57-88.

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used 

iti geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull. 

1152, 100 p.

Ward, F. N., Nakagawa, H. M., Harms, T. F., and VanSickle, G. H., 1969, Atomicabsorption methods of analysis useful in geochemical exploration: U.S. Geol. Survey Bull. 1289, 45 p.

ATOMIC ABSORPTION DETERMINATION OF 

ANTIMONY IN GEOLOGIC MATERIALS

By JOHN B. McHucn and ERIC P. WELSCH

CONTENTS

	Page 

Abstract'........................................................................................................................... 5

Introduction...........................................................................................?........................ 6

Laboratory reagents and apparatus................................................................................ 6

Reagents................................................................................................................... 7

Apparatus................................................................................................................. 7

Procedure...............................................................................................................:........ 7

Sample preparation................................................................................................. 7

Standard preparation and instrument calibration.................................................. 7

Discussion....................................................................................................................... 8

Interferences.................................................................................................................... 9

Precision and accuracy ................................................................................................... 9

References cited............................................................................................................... 11

TABLES

Page

TABLE 1. Precision of antimony determinations by atomic absorption procedure

and a colorimetric procedure....................................................................... 10

2. Antimony content of rock samples from Humboldt County, Nev................... 10

ABSTRACT

Antimony in soils and rocks is solubilized by fusing the sample with potassium pyrosulfate and digesting the melt from the fusion with 8 N hydrochloric acid. The antimony 

is then extracted into MIBK (methyl isobutyl ketone, which is the common name for 

4-methyl-2-pentanone) and determined by atomic absorption spectrophotometry. As little 

as 1 ppm antimony can be detected in the sample, with a precision and accuracy comparable to that of the rhodamine B colorimetric method.
6 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

INTRODUCTION

Trace amounts of antimony along with other volatile elements commonly occur as halos around base-metal deposits, and the determination 

of these trace amounts of antimony can be a useful tool in the search 

for such deposits, especially where they are hidden by residual or transported materials. For example, Erickson and others (1966) showed that 

antimony is present in the suite of elements associated with the Carlintype gold deposits of Nevada. The determination of small amounts of 

antimony in overlying soils elsewhere may provide a clue to the presence 

of similar deposits.

The rhodamine B colorimetric method (Ward and others, 1963) is now 

used by the U.S. Geological Survey to determine antimony. However, 

because atomic absorption spectrophotometry is widely used and measurements with this equipment are generally fast, simple, relatively 

accurate, and free of interferences, it seemed reasonable to apply atomic 

absorption measurement to antimony. The Perkin-Elmer analytical 

methods manual (1968) described an atomic absorption procedure for 

determining as little as 0.1 jug/ml antimony in aqueous solution. By 

combining the colorimetric sample preparation procedure with atomic 

absorption, a procedure was developed for analyzing 100 samples per 

day as compared with 40 samples by the older colorimetric method.

Briefly, the method consists of sample decomposition by fusion with 

potassium pyrosulfate, digestion in 8 N hydrochloric acid, oxidation 

of the dissolved antimony to the pentavalent state with eerie sulfate, 

extraction of the antimony with MIBK, and atomization of the extract 

in the flame of an atomic absorption spectrophotometer.

LABORATORY REAGENTS AND APPARATUS 

REAGENTS

Standard antimony solution (0.1 percent): Dissolve exactly 1.370 g of 

potassium antimony tartrate in 6 N hydrochloric acid, dilute to 500 

ml with 6 N hydrochloric acid.

Dilute standard antimony solution (0.01 percent): Dilute 10.0 ml of 0.1 

percent standard antimony solution to 100 ml with 6 N hydrochloric 

acid.

Dilute standard antimony solution (0.001 percent): Dilute 10.0 ml of 

0.01 percent standard antimony solution to 100 ml with 6 N hydrochloric acid.

Ceric sulfate solution: Dissolve 52 g eerie sulfate (H4Ce(SO4 ) 4 ) in 1 litre 

of demineralized water containing 30 ml of concentrated sulfuric acid.

Hydrochloric acid, concentrated, reagent grade.

Hydrochloric acid, 6 N: Dilute 500 ml concentrated hydrochloric acid 

to 1 litre with demineralized water.
ANTIMONY IN GEOLOGIC MATERIALS 7

Hydrochloric acid, 8 N: Dilute 660 ml concentrated hydrochloric acid

to 1 litre with demineralized water. 

MIBK, reagent grade. 

Potassium pyrosulfate, powder, reagent grade.

APPARATUS

Atomic absorption spectrophotometer, Perkin-Elmer model 403 or

equivalent. 1 

Centrifuge, International, Size 2, model K, or other centrifuge of similar

performance capable of 1,200 to 8,500 r/min and centrifugal force of

380 to 4,400 times gravity. 

Culture tube, screwcap, 16 by 150 mm. 

Culture tube, screwcap, 25 by 150 mm. 

Fusion rack (Marranzino and Wood, 1956), consisting of a rotating disk

to which as many as 11 tubes can be clipped and their contents fused

as they pass over a group of Bunsen burners.

PROCEDURE

SAMPLE PREPARATION

Weigh 0.25 g of < 80-mesh sample of rock or soil and place in a 

16-by 150-mm screwcap culture tube. Add about 1.0 g of potassium pyrosulfate and mix thoroughly. A 1-g scoop can be devised to expedite this 

addition. Fuse by hand over a Bunsen burner or on a specially designed 

fusion rack (Marranzino and Wood, 1956) until the sample is completely 

attacked by the flux. Remove the tube from the heat and rotate it so 

that the melt will cool in a thin film on the walls of the tube. Add 6 

ml of 8 N hydrochloric acid to the sample tube and place the tube in 

a boiling water bath until the melt is dissolved. Do not boil the sample 

solution. Remove the tube from the water bath and allow to cool. Add 

0.5 ml eerie sulfate solution and mix. Add 2 ml MIBK, cap the tube, 

and shake vigorously for 1 minute. Centrifuge the sample for 5 minutes 

at 1,500 r/min to facilitate MIBK recovery and to insure a clean organic 

layer. Determine the antimony content by aspirating the MIBK layer 

in the flame of an atomic absorption spectrophotometer. Samples must 

be aspirated within 1 day after extraction.

STANDARD PREPARATION AND INSTRUMENT CALIBRATION

Prepare the standards in the same manner as the samples but in sufficient volume for instrument calibration. The potassium pyrosulfate 

fusion is deleted from the standards for the sake of expediency. No difference has been noticed in the resulting standards due to this deletion.

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological 

Survey.
8 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Add 30 ml 8 N hydrochloric acid to each of four 25- by 150-mm screwcap culture tubes. Add 0.2 ml and 1.0 ml of the 0.001 percent antimony 

standard to the first two tubes. Add 0.5 ml and 1.0 ml of the 0.01 percent 

antimony standard to the last two tubes. Add 0.5 ml of the eerie sulfate 

solution to each of the four tubes and mix. Add 10.0 ml of MIBK and 

shake standards vigorously for 1 minute. Centrifuge the standards for 

5 minutes at 1,500 r/min to facilitate phase separation. These standards 

contain 0.2, 1, 5, and 10 g antimony per millilitre, respectively, and 

are stable for 1 day.

Preparation of the blank requires less adherence to the sample preparation procedure. MIBK washed with a small amount of 8 N hydrochloric acid provides an adequate blank once the reagent purity has been 

established. The acid phase may be discarded upon completion of the 

wash.

The Perkin-Elmer model 403 spectrophotometer was operated under 

the following parameters: 

Wavelength: 2,176A. 

Slit: 3 (0.3 mm).

Lamp: Antimony hollow cathode run at 20 mA. 

Air pressure: 50 Ib/in2g on incoming line, 30 Ib/in2g on control box,

flowmeter setting 46. 

Fuel pressure: Acetylene pressure 12 Ib/in2g on gage, 8 Ib/in2g on

control box, flowmeter setting 18. 

Burner: Three-slot (Boling).

Aspiration of the MIBK standard solutions under the preceding parameters produced the following values:

Micrograms Sb/ml MIBK Absorbance

0.2............................................................................... 0.010

1.0............................................................................... .038

5.0............................................................................... .166

10.0............................................................................... .340

DISCUSSION

Selecting the proper agent for adequate decomposition of the sample 

was the first problem in developing a suitable method. Fusion with sodium carbonate or digestion with hydrofluoric acid is unduly vigorous for 

all but a few rare antimony minerals. Sulfuric and nitric acids oxidize 

the MIBK; boiling in hydrochloric acid and aqua regia results in volatilization of antimony chloride. Potassium pyrosulfate was chosen as 

an agent for sample decomposition because of its ability to break down 

sulfides and oxides readily without the above problems. Most antimony 

minerals will decompose and go into solution when fused with potassium pyrosulfate and taken up with 8 N hydrochloric acid (Dolezal and 

others, 1968; Rankama and Sahama, 1950).
ANTIMONY IN GEOLOGIC MATERIALS 9

MIBK extracts about 50 percent of antimony (III) from 8 N hydrochloric acid but extracts 95-100 percent of antimony (V). For this reason 

Ce(IV) is added to oxidize all antimony to the higher valence. From 1 

N acid only 25 percent of antimony(V) is extracted, whereas from 8 N 

acid, virtually all of antimony(V) is extracted.

The method of complexing with ammonium pyrrolidine dithiocarbamate (atph 3) and extraction with MIBK was also used with good results 

but was found to be more time consuming than the 8 N hydrochloric 

acid-eerie sulfate-MIBK method.

Approximately 70 percent of the MIBK is recovered when the following three conditions are met:

Acid temperature of about 25°C. 

Ratio of MIBK to acid between 1:2 and 1:3. 

Acid concentration of 8 N.

The recovery of MIBK increases with higher temperatures, lower MIBKto-acid ratio, and lower acid concentrations. Because 100 percent of the 

antimony is extracted into the recovered MIBK, it is essential that the 

preceding conditions are constant for samples and standards.

The MIBK layer should not be allowed to stand over the acid for more 

than a' day because 8 N hydrochloric acid will eventually react with, 

and become miscible with, the MIBK.

The presence of any aqueous phase in the atomic absorption spectrophotometer system will yield erroneously high values. To prevent this 

error, centrifuge the samples and standards prior to aspiration. At the 

same time, rinse the burner assembly and drain line with MIBK and 

aspirate MIBK for 5 minutes before aspirating the samples.

INTERFERENCES

Mostyn and Cunningham (1967) found no significant interference 

from 16 common metals on antimony absorption.

PRECISION AND ACCURACY

The precision, or repeatability, of the atomic absorption and colorimetric procedures was established by determining the antimony contents 

of five replicates of seven samples (table 1). The precision of the atomic 

absorption method is within acceptable limits for geochemical exploration studies. Two to eight percent relative standard deviation was obtained, except for one low sample in which the values ranged from 3.2 

to 2.4 ppm, giving a relative standard deviation of 16 percent.

The accuracy of the atomic absorption procedure was investigated by 

comparing the antimony content of 27 samples (table 2) determined by 

the colorimetric and atomic absorption methods. The accuracy of the 

atomic absorption method is good, with agreement on all but 3 of the 

27 samples compared.
10 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

TABLE 1. Precision of antimony determinations by atomic absorption procedure and

by colorimetric procedure

[Leaders (....) indicate thai values are too low for significance]

Sample 

No. '

Values of 5 replicates 

(ppm)

Range Mean

Standard 

deviation 

(ppm)

Relative 

standard 

deviation 

(percent)

Atomic absorption procedure

Sb-1.....................

Sb-2.....................

Sb-3.....................

Sb-4.....................

BCI-436...............

BCI-438...............

Universal 

standard-I.........

21.6-27.2

12.0-12.6

4.8- 5.6

4.8- 5.6

<1.0- 1.0

2.4- 3.2

37.6-42.4

24.3 

12.1

5.4 

5.4

2J 

39.9

2.09 

.27 

.31 

.31

'.44 

2.20

8.6 

2.2 

5.8 

5.8

16.3 

5.5

Colorimetric procedure

Sb-1....................

Sb-2....................

Sb-3....................

Sb-4.....................

BCI-436..............

BCI-438..............

Universal 

standard-I........

15.0-20.0

9.0-10.0

2 0- 2 5

4.5- 5.0

<1.0- 1.0

< 1.0- 1.0

44.0-47.0

18.0 

9.5 

2.4 

4.8

45.4

1.94 

.50 

.22 

.28

1.15

10.7 

5.3 

9.3 

5.7

2.5

'Material and location 

Sb-1 through Sb-4: Rock from Nevada. 

BCI-436, BCI-438: Stream sediment from Maine. 

Universal standard-I: Gossan from Colorado.

TABLE 2. Antimony content of rock samples from Humboldt County, Nev.

[Samples supplied by R. L. Erickson, U.S. Geological Survey]

Sample 

No. 

(BBG-)

001 ......

002......

003......

004.......

005.......

009...

fll Q

019

020.......

021

Antimony content (ppm)

Colorimetric 

method1

........ 12

........ 1

<1

<1

1

........ 1

2

........ 2

2

Atomic absorption 

method

6 

2 

2 

3 

2

16 

2 

4 

3 

5

Sample 

No. 

(BBG-)

027 ..-.

028 ......

034 ......

039 ......

048 ......

049

050 -

084 --

086 --..

148 -.-

Antimony content (ppm)

Colorimetric 

method1

......... 19

......... 5

........ 3

4

......... 3

........ i

......... <i

........ 9

......... 18

......... 11

Atomic absorption 

method

16 

6 

6 

6

4

1 

1 

6 

13 

12

'Analyst, James H. Turner, U.S. Geological Survey, Denver, Colo.
ANTIMONY IN GEOLOGIC MATERIALS 11

REFERENCES CITED

Dolezal, J., Povondra, P., and Sulcek, Z., 1968, Decomposition techniques in inorganic

analysis: New York, Am. Elsevier Publishing Co., 224 p. 

Erickson, R. L., VanSickle, G. H., Nakagawa, H. M., McCarthy, J. H., Jr., and Leong,

K. W., 1966, Gold geochemical anomaly in the Cortez district, Nevada: U.S. Geol.

Survey Circ. 534, 9 p. 

Marranzino, A. P., and Wood, W. H., 1956, Multiple-unit fusion rack: Anal. Chemistry,

v. 28, No. 2, p. 273-274. 

Mostyn, R. A., and Cunningham, A. F., 1967, Determination of antimony by atomic

absorption spectrometry: Anal. Chemistry, v. 39, no. 4, p. 433-435. 

Perkin-Elmer Corp., 1968, Analytical methods for atomic absorption spectrophotometry:

Norwalk, Conn., Perkin-Elmer Corp., 522 p. 

Rankama, Kalervo, and Sahama, T. G., 1950, Geochemistry: Chicago, Univ. Chicago

Press, 912 p. 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used

in geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull.

1152, p. 38-39.

NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

DETERMINATION OF ARSENIC 

IN VEGETATION

By T. F. HARMS and F. N. WARD

CONTENTS

	Page 

Abstract........................................................................................................................... 13

Introduction.................................................................................................................... 14

Procedures....................................................................................................................... 15

Preparation of sample solution............................................................................... 15

Reagents............................................................................................................ 15

Dry ignition............................................................................................................. 15

Wet ignition............................................................................................................. 15

Distillation and estimation ..................................................................................... 16

Molybdenum blue reaction.............................................................................. 16

Reagents.................................................................................................... 16

Apparatus.................................................................................................. 17

Silver diethyldilhiocarbamate.......................................................................... 17

Reagents ................................................................................................... 17

Apparatus.................................................................................................. 18

Discussion....................................................................................................................... 18

Experiments.................................................................................................................... 18

Results............................................................................................................................. 19

References cited............................................................................................................... 20

TABLES

Page

TABLE 3. Repeatability of arsenic determinations (molybdenum blue reaction)........ 19

4. Determination of arsenic by molybdenum blue reaction and carbamate

reaction....................................................................................................... 20

ABSTRACT

Arsenic in vegetation is determined by either of two reactions after either a wet digestion of the sample under reflux with a hot mineral acid mixture or a dry ignition 

of the sample with added fixative to minimize loss of arsenic by volatilization. After 

sample preparation by one of the procedures, arsenic is separated by distillation, and 

the evolved arsine is collected in an iodine solution and subsequently reacted with molybdate to form a heteropoly compound, which on reduction forms a blue compound, 

or is alternatively reacted with silver diethyldithiocarbamate to form a red-violet com13
14 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

pound. Sensitivities of about 0.5-1 part per million are achieved by either method of 

measurement, and the relative standard deviation is on the order of 10-20 percent.

INTRODUCTION

The usefulness of arsenic as a pathfinder element in the search for 

concealed ore deposits has long been recognized (Delavault and Manson, 

1971). Pathfinder elements are defined as those elements associated with, 

but more easily determined or identified than, the principal element. 

The association of arsenic with gold is a good example, and arsenic levels, 

which are more easily measured than those of gold, can be used to delineate areas of possible gold occurrence.

Using sampling media other than soils and rocks, Warren, Delavault 

and Barakso (1964) described the pathfinder role of arsenic in biogeochemical prospecting and noted that first-year growth, and twigs considered as second-year growth, of Douglas-fir established in mineralized 

ground contained more than 2 ppm arsenic in the ash. Later, Warren, 

Delavault, and Cross (1967) observed even greater enrichment. The ash 

of Douglas-fir established in a soil over gold-arsenopyrite mineralized 

ground contained 560 ppm arsenic in contrast to 8 other species whose 

ash contained less than 4 ppm arsenic. Such enrichment suggests that 

the tips and twigs of Douglas-fir are useful as a sampling medium to 

locate exploration targets for arsenic and associated elements, such as 

gold and, possibly, others.

A slight variation of the pathfinder concept is shown in work by 

Burbank, Luedke, and Ward (1972), who use arsenic as an indicator 

element of mineralized volcanic pipes in the western San Juan Mountains, Colo.

In this chapter two methods are given for determining trace amounts 

of arsenic in air-dried vegetation. After sample preparation by dry ignition or by wet ignition and the distillation of the arsine from the bulk 

of the sample, the arsenic content is estimated by either one of the following methods:

1. Arsine is collected in an iodine solution which oxidizes the arsenic 

(III) to arsenic (V) which in turn reacts with ammonium molybdate 

in acid medium to form the heteropoly molybdiarsenate, which, 

on subsequent reduction with hydrazine, forms a blue compound 

of unknown composition.

2. Arsine is reacted with silver diethyldithiocarbamate in pyridine to

form a red- to red-violet compound.

The sensitivities achieved by both methods are roughly the same, and 

the choice of which to use has to be governed by other considerations, 

such as facilities available and possible interferences. Both methods are 

adequate to determine anomalous amounts ( > 1 ppm) of arsenic in the 

leaves and twigs of deciduous and evergreen trees, as well as in shrubs, 

grasses, and other vegetal matter.
OIL AND GAS PRODUCTION AND RESERVES 15

DISTRIBUTION OF KNOWN OIL AND GAS 

PRODUCTION BY GEOLOGIC AGE

The known cumulative oil production through 1972 in the Appalachian basin, as categorized by geologic age for each respective State 

and for the total basin, is shown in table 4. The recorded cumulative 

production for the basin from 1859 through 1972 is approximately 2.523 

billion barrels of oil and 362 million barrels of natural gas liquids (a 

total of 2.885 billion barrels).

Table 5 shows the known cumulative natural gas production through 

1972 in the Appalachian basin, categorized by geologic age for each 

State and for the total basin. The recorded cumulative gas production 

for the basin from 1882 through 1972 is approximately 31.2 trillion cubic 

feet.

Three major sources were used for data in these two tables: the 

American Petroleum Institute (API), the American Association of 

Petroleum Geologists (AAPG), and the Independent Petroleum Association of America (IPAA).

DISTRIBUTION OF KNOWN RECOVERABLE 

RESERVES BY GEOLOGIC AGE

The recoverable oil reserves for the Appalachian basin are based 

upon estimates from known producing horizons. The recoverable 

reserves consist of the cumulative oil production through 1972 plus the 

remaining recoverable reserves (as of January 1973) that are expected

YEAR

FIGURE 3. Annual natural gas production for the Appalachian basin, 1882-1972.
TABLE 4. Known cumulative oil and natural gas liquids production in the Appalachian 

[Reported in thousands of barrels. Leaders ( _ _ _ ) indicate no re

Oil production

Pennsylvanian

Pennsylvania _ _ _ _ 

West Virginia _ _ _ _

Eastern Tennessee _ 

Eastern Kentucky _

Totals

Percentage of total 

oil production _ _

1,864 

54,829 

0 

0 

0 

4,595 

19,572

80,860

3.2050

Mississippian

18,967 

282,778 

0 

0 

1,527 

41,629 

25,429

370,330

14.6785

Devonian

1,255,472 

170,073 

0 

223,842 

0 

123,309 

9,536

1,782,232

70.6410

Silurian

326 

0 

0 

0 

62 

18,225 

238,393

257,006

10.1868

Ordovician

1

~281 

0 

275 

6,932 

6,357

13,846

0.5487

'See API references 

-See AAPG references 

: 'See IPAA references

'Lima-Trenton production excluded. 

"'No conversions were made from barrels to metric tons in this report due to the variations in the API gravities of t

API give a range in conversion from 6.7 barrels per metric ton to 7.85 barrels per metric ton (Zaba and Doherty,1956

'Includes some Ordovician.

TABLE 5. Known cumulative gas production in the Appalachian basin, 1882-

[Reported in millions of cubic feet].

Geologic Pennsylvania 

age

Pennsylvanian _ _ _ 

Mississippian _ _ _ _

Early Devonian and 

Late Silurian ___

Total

173,534 

977,562 

7,502,580

0 

23,690 

0 

200

28,677,566

West 

Virginia

1,416,730 

7,792,015 

4,675,209

0 

283,346 

0 

0

2 "14, 167,300

Virginia

1,150 

53,300 

2,875

0 

172 

0 

0

2 :1 57,497

New York

0 

0 

190,630

0 

324,464 

2,726 

16

'2 ''517,836

Eastern 

Tennessee

0 

2,129 

0

0 

0 

164 

0

<2,293

Eastern 

Kentucky

100,726 

945,077 

1,540,683

46,03

138,115 

8,343 

1,947

:|2,780,929

'Findlay gas neld not included. 

-See AAPG references. 

: 'See API references. 

'See IPAA references.
OIL AND GAS PRODUCTION AND RESERVES 17

to be produced in the future if there are no substantial changes in present economic relationships and known production technology. Table 6 

shows the estimated recoverable oil reserves in the Appalachian basin 

categorized by geologic age for each respective State and for the total 

basin. The estimated total recoverable oil reserves for the basin are approximately 2.747 billion barrels. Considering the cumulative production of 2.523 billion barrels, only 8 percent of the total known 

recoverable oil reserves remain to be produced.

Table 7 shows the distribution by State and geologic age of the estimated original oil-in-place in the Appalachian basin. The original oilin-place figures are taken from API (1973) and are defined (AGA and 

others, 1973) as follows:

The estimated number of stock tank barrels of crude oil in known reservoirs prior to any 

production is defined as "original oil-in-place." Known reservoirs include (1) those that 

are currently productive; (2) those to which proved reserves have been credited but from 

which there has been no production; and (3) those that have been depleted.

The estimation of original oil-in-place is based on calculations using volumetric or 

material balance methods when sufficient factual data are available concerning reservoir 

rock, fluid properties, reservoir limits, and production performance. Where such data are 

not available, or are seriously incomplete, the estimation procedure utilizes information 

and performance characteristics from reservoirs believed to be comparable.

The estimated total original oil-in-place for the Appalachian basin is 

approximately 14.249 billion barrels. With an estimated ultimate 

recovery of 2.747 billion barrels for the basin, this presently constitutes 

only a 19.3 percent recovery factor for the basin.

The estimated proved natural gas reserves consist of the cumulative 

natural gas production through 1972 plus the remaining proved reserves 

as of January 1973. Table 8 shows the estimated proved natural gas 

reserves for the Appalachian basin, categorized by geologic age for each 

State and for the total basin. Devonian reservoirs have by far the best 

gas, as well as oil, reserves known in the basin. With a cumulative gas 

production of 31.2 trillion cubic feet (table 5) and an estimated proved 

gas reserve of 36.7 trillion cubic feet, approximately 15 percent of the 

total known natural gas reserves remain to be produced.

Table 9 shows the estimated recoverable reserves of natural gas liquids for the Appalachian basin for four of the eight States and 

categorized by geologic age (data from IPAA, 1973). The total reported 

recoverable reserves of natural gas liquids are 494 million barrels. With 

a cumulative production of natural gas liquids reported to be 362 million barrels (table 4), 27 percent of the recoverable reserves remain.

DISTRIBUTION OF TOTAL HYDROCARBONS BY GEOLOGIC AGE

One of the methods of evaluating the total hydrocarbons found per 

cubic mile of sediment within a specific basin is to convert the number 

of cubic feet of the natural gas reserves to an equivalent in barrels, and 

to add this equivalent to the total barrels of crude oil reserves plus
TABLE 6. Estimated ultimate recoverable oil reserves in the Appalachian basin, 

[Reported in thousands of barrels. All values are based upon proved reserves from kno

Geologic

age

Pennsylvanian _ _

Mississippian _ _

Ordovician _ _

Total

Pennsylvania

1,918

19,522

335

>2

0

1,313,976

West

Virginia

58,506

301,738

181,476

0

0

0

541,720

Virginia

0

0

0

0

299

0

299

New York

0

0

233,088

0

0

0

233,088

Eastern

Tennessee

0

1,923

0

77

347

-100

2,447

East

Kent

4,

44,

133,

19,

7,

210,

'Lima-Trenton production not included.

Includes some Or<iovician.

 'Includes 2,522,943 thousand barrels produced through 1972.

TABLE 7. Estimated original oil-in-place in the Appalachian basin, cat

[Reported in thousands of barrels. All values are based upon API figures (1973) and are for proved p

Geologic 

age

Pennsylvanian _ _ _ 

Mississippian _ _ _ 

Devonian _____

Cambrian _____ 

Total

Pennsylvania

9,646 

98,163 

6,497,666 

1,685 

7 

0

6,607,167

West 

Virginia

244,090 

1,297,530 

1,027,746 

0 

0 

0

2,569,366

Virginia

0 

0 

0 

0 

1,424 

0

1,424

New York

0 

0 

1,116,739 

0 

0 

0

1,116,739

Eastern 

Tennessee

0 

6,409 

0 

259 

1,156 

-334

8,158

E

Ke

1

4

6

'Lima- Trenton field not included. 

-Includes some Ordovician.
OIL AND GAS PRODUCTION AND RESERVES 19

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20 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

reserves in barrels of natural gas liquids. This will give a rough estimate 

of the total recoverable hydrocarbons in the basin.

Table 10 shows the total recoverable reserves of hydrocarbons (oil, 

gas, and natural gas liquids) for the Appalachian basin (10 billion barrels total), categorized by geologic age.

Table 11 is similar to table 10, with one important exception: the estimated values for original oil-in-place have been substituted for the 

recoverable reserve figures for crude oil. The reader should be cautioned 

that although oil-in-place figures may be useful in evaluating producing 

formations or, possibly, source beds, they can be very misleading if they 

are mistakenly considered as representing recoverable reserves. Based 

upon present technology, the average percentage of ultimate 

hydrocarbon recovery in the Appalachian basin is approximately 20 

percent, as compared to an average of 31.4 percent for the entire United 

States (API data, in AGA and others, 1973). The average recovery 

percentages for each State reported on in the Appalachian basin range 

from 13.5 percent for Ohio to 32.4 percent for Kentucky (API, 1973).

By finding the sum of all the known estimated hydrocarbons (oil-inplace, natural gas liquids, and natural gas) within the explored portions 

of the Appalachian basin, the grand total can be estimated at approximately 21.5 billion barrels (equivalent).

All data (production, reserves, and estimated original oil-in-place) 

are based upon known reservoirs. No projections, extrapolations, or extensions into undrilled formations or unexplored portions of the Appalachian basin have been considered in this report.

SPECULATIONS ON WASTAGE

It is impossible to assign any substantiated figures to the total 

amounts of oil and gas wasted1 during the early history of the petroleum 

industry in the Appalachian basin. Reviews of the early literature from 

State surveys, early published histories of petroleum developments, 

and early reports from the U. S. Bureau of Mines convince the reader 

that significant amounts of crude oil were wasted and the tremendous 

wastage of natural gas was on an even greater scale.

Much of the wastage can be attributed to lack of knowledge concerning production techniques, transportation, and usage. However, a 

greater part of the wastage appears to have been due to excessive drilling of oil and gas wells when there were no uses or markets for the 

products, no means of storage or facilities for transportation and poor 

prices. Unnecessary wells were punched into the ground and the gas allowed to escape into the air and the oil to run down the creeks and fill 

the swamps. Fires, explosions, and deaths were common and are

1 "Waste" is defined here as meaning to use up or spend without real need, gain, or purpose; not put to full or proper 

use; produced in excess of what is or can be used; a useless or profitless spending, or consuming; destroyed uselessly or 

dissipated.
OIL AND GAS PRODUCTION AND RESERVES 21

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22 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

i

documented in every State within the Appalachian basin (Derrick's 

Handbook of Petroleum, 1898; 1900).

Wastage in the early history of the Appalachian basin is well ^ 

documented, as the following examples show.

It is simply impossible to ascertain the total production in cubic feet of natural gas in 

the United States. The data do not exist for such a statement. Nor is it essential to ascertain this total production of all wells. The only production that has any commercial importance is that which is utilized. The amount that is wasted has no commercial value, at 

least that can be observed. Only that gas which has been consumed in doing useful work 

will be taken into consideration in this statement of total consumption. The waste cannot v 

be considered.[Joseph D. Weeks, in U.S. Geological Survey, 1885, p. 155.]

The first laws pertaining to the conservation of oil and gas were not enacted until 1878 

[in Pennsylvania], nearly 20 years after the first rush to the fields, late in 1859. The early 

development was attendant with appalling waste and dissipation. [Arnold and Kemnitzer, 1931, p. 96.]

Not withstanding the law upon the statute books requiring the several gas and oil companies operating in the State [West Virginia] to shut in all gas wells and prevent the 

waste of this precious fuel, there is yet an enormous waste going on in several counties of 

the State. * * * In one county alone (Wetzel) it is estimated that the waste of gas, 

counting that which escapes with the oil production, and which the present statute does 

not therefore prohibit, amounts to much more than 100 million cubic feet daily. [White, 

1899, p. 25.]

White later reported (1904, p. 29) that "* * * 200-250 million cubic 

feet is daily wasted through unplugged wells, leaking joints, and from 

producing oil wells," and he estimated that the "* * * State [of West 

Virginia] has lost more than $150,000,000 worth * * *" of natural gas in jt 

the decade from 1894 to 1904. At the 1904 price of 10 cents per 1,000 

cubic feet this would have been an estimated 1.5 trillion cubic feet in 

one decade.

In 1873 J. T. Henry wrote,

Though Western Pennsylvania has produced numerous flowing wells of wonderful 

capacity, there is no quarter of the world where the production attained such prodigious 

dimensions, as in 1862, on Black Creek, in the township of Enniskillen (Ontario). The first 

flowing well was struck there on January 11,1862, and before October not less than thirtyfive wells had commenced to drain a store-house, which provident nature had occupied 

untold thousands of years in filling for the uses not for the amusement of man. There * 

was no use for the oil at that time. The price had fallen to ten cents per barrel. The unsophisticated settlers of that wild and wooded region seemed inspired by an infatuation. 

Without an object save the gratification of their curiosity at the unwonted sight of a combustible fluid pouring out of the bosom of the earth, they seemed to vie with each other in "^ 

plying their hastily and rudely erected "spring poles" to work the drill, that was almost 

sure to burst at a depth of a hundred feet, into a prison of petroleum. Some of these wells 

flowed three hundred, and six hundred barrels per day. Others flowed a thousand, two 

thousand, and three thousand barrels per day. Three flowed, severally, six thousand barrels per day; and the "Black and Matthewson" well flowed seven thousand five hundred 

barrels per day. Three years later that oil would have brought ten dollars per barrel in 

gold. Now, its escape was the mere pastime of fullgrown boys. It floated on the waters of / 

Black Creek to the depth of six inches, and formed a film on the surface of Lake Erie. At 

length the stream of oil became ignited, and the column of flame raged down the winding 

of the creek in a style of such fearful grandeur as to admonish the Canadian Squatter of 

the danger, no less than the inutility of his oleaginous pastimes. From detailed deter-
OIL AND GAS PRODUCTION AND RESERVES 23

minations, Professor Winchell says, "I have ascertained that, during the spring and summer of 1862, not less that five million barrels of oil floated off on the water of Black Creek 

 a national fortune totally wasted."

During the period from 1883 to the close of 1889, natural gas "* * * 

was used in the most extravagant and reckless manner, and it was paid 

for at rates that were in many instances less than one-half the price of 

equivalent coal. Large quantities were allowed to escape and burn from 

the mouths of hundreds of stand pipes from Saturday afternoon until 

Monday morning. It is highly probable that in these 6 years of reckless 

consumption four times the present production was consumed annually." (F. H. Oliphant, in U.S. Geological Survey, 1901, p. 614.)

The annual production of natural gas for the year 1901, the date of F. 

H. Oliphant's statement, was approximately 203 billion cubic feet. If 

four times this amount were consumed annually during the 6 years 

(1883 through 1889), or 812 billion cubic feet, then in 6 years 4.877 trillion cubic feet would have been consumed, according to Oliphant's estimates. That is 13.3 percent of the total estimated proved natural gas 

reserves (36.727 trillion ft3 ) (see table 8) for the entire Appalachian 

basin through 1972. Another 1.5 trillion cubic feet for one decade from 

West Virginia, as cited earlier, added to the above 4.877 trillion cubic 

feet means that more than 6.3 trillion cubic feet, or 17.1 percent of the 

total estimated proved natural gas reserves for the entire basin were 

wasted in these early years.

The U.S. Geological Survey, in 1921, in reporting on the wastage 

problem, cited some of the difficulties encountered in obtaining accurate information. "The Geological Survey has attempted from time 

to time to get figures on the quantity of gas wasted, but the returns 

made to it have been wholly inadequate for this purpose, because the 

operators generally have no idea as to the quantities of gas that are being lost or needlessly consumed." (U.S. Geological Survey, 1921).

The above report was able to give wastage figures for 3 years, 

1919-1921, on the basis of information gathered from the gas-producing 

States. For States reported on within the Appalachian basin these 

figures are as follows (U.S. Geological Survey, 1921, p. 349, 352):

Percent of production wasted

State

Kentucky

New York _ _

Ohio

West Virginia

Average for all

1919

28

.1

12

4

1

7

1920

28

1

18

2

2

7

1921

27

2

19

2

2

9

A very rough estimate of the total wastage of oil and gas during the 

history of their production in the Appalachian basin, based upon old
24 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

published reports and very incomplete and fragmentary data, might be 

as great as 100 million barrels of crude oil and natural gas liquids (approximately 3 percent or more of the known total proved reserves), and 

nearly 10 trillion cubic feet of natural gas (more than 25 percent of the 

total proved natural gas reserves).

DRILLING AND PRODUCTION INFORMATION

The total number of recorded exploration, development, and service 

wells drilled in the Appalachian basin from 1859 through 1972 is 

reported by API and AAPG (1973) to be 608,401. Excluding service 

wells, the total number of wells drilled for oil and gas for the entire Appalachian basin has been reported by the IPAA (1973) as 592,523.

The deepest drilling depths and deepest producing depths for oil and 

gas recorded for each State in the Appalachian basin through 1973 are 

shown in table 12.

TABLE 12. Deepest drilling and production depth for oil or gas in States of the 

Appalachian basin, through 1973

[Data from API (1971) for all States except West Virginia (AAPG, 1973)]

State

Tennessee ______

Deepest drilling

Feet

13,170 

8,700 

13,500 

11,442 

18,834 

10,748 

14,176 

18.554 +

Year

1959 

1966 

1958 

1967 

1966 

1967 

1965 

1973

Horizon

Granite basement 

Helderberg Formation 

Potsdam Sandstone 

Cambrian 

_ _ do _ _ 

do__ 

__do__ (')

Feet

9,665 

8,658 

5,013 

8,114 

11,458 

3,107 

6,158 

8,552

Deepest production

Year

1970 

1966 

1941 

1968 

1964 

1954 

1954 

1970

Horizon

Second St. Peter sand 

Oriskany Sandstone 

Do. 

Cambrian-Ordovician 

Tuscarora Sandstone 

Chattanooga Shale 

Silurian 

Oriskany Sandstone

'Still drilling.

The average footage of wells drilled in each State in the basin was 

recorded for the 22-year period of 1947 through 1968 and is shown on the 

following table (API, 1971):

State

Ohio

Average footage

1,541

5,395

1,499

1,898

2,359

1,169

4,735

2,700

Total number of 

years in which wells 

were drilled

22

10

22

22

22

17

10

22

According to the Interstate Oil Compact Commission, the major part 

of the present production in the Appalachian basin is from stripper 

wells, which they define as a well "capable of producing only 10 barrels 

of oil per day or less during the year under consideration" (COMPACT, 

1973). The percentages of total production from stripper wells in the
REFERENCES CITED 25

Appalachian basin States in 1972, as reported by two sources, were as 

follows:

State

New York

Ohio

Maryland

Percent 

(COMPACT. 1973)

94.08

. _ 100

82.22

100

6.70

100

100

0

Percent 

(IPAA, 1973)

98.5

97.4

70.0

100

7.5

100

100

0

According to these surveys all the major oil production within the Appalachian basin is from stripper wells capable of producing 10 barrels or 

less of oil per day. Within the States of Kentucky, New York, Pennsylvania, Tennessee, and Virginia, the entire remaining proved reserves 

are classified as stripper-well reserves. In West Virginia and Ohio the 

stripper-well reserves make up 53 percent and 70 percent respectively of 

the remaining proved reserves (COMPACT, 1973; API, 1973). In addition, over a third of these stripper-well reserves are being recovered by 

secondary methods of operation.

In light of the energy shortage in the United States, development of 

known reserves is a necessity. It is doubtful that additional significant 

resources will be found in the parts of the Appalachian basin that have 

already been adequately explored; however, through 1974 only 42 wells 

had been drilled in the basin below 10,000 feet, and some hope of deeper 

prospects still encourages additional exploration (Bee, 1974).

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26 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

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28 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

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36 OIL AND GAS PRODUCTION OF THE APPALACHIAN BASIN

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£U.S. GOVERNMENT PRINTING OFFICE: 1975-677-312/57
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

DETERMINATION OF SELENIUM 

IN VEGETATION

By T. F. HARMS and F. N. WARD

CONTENTS

Page

Abstract .......................................................................................................................... 37

Introduction.................................................................................................................... 37

Reagents.......................................................................................................................... 39

Apparatus........................................................................................................................ 39

Procedure........................................................................................................................ 40

Discussion of standard curves...................!..................................................................... 40

Recovery of added selenium........................................................................................... 41

Repeatability................................................................................................................... 41

References cited............................................................................................................... 42

TABLES

Page

TABLE 13. Recovery of added selenium........................................................................ 41

14. Repeatability of selenium determinations .................................................. 42

ABSTRACT

Selenium in vegetation is determined, after wet ignition under reflux with a hot 

mineral-acid mixture, by the reaction of selenium with 2, 3-diaminonaphthalene to 

form 4, 5-benzopiazselenol, whose fluorescence is proportional to the selenium concentration. The reaction is sensitive to about 0.02 microgram of selenium and, accordingly, the lower limit of the method is 0.02 parts per million. Recoveries of added selenium 

are about 90 percent, and the relative standard deviation is on the order of 10-15 percent.

INTRODUCTION

The amount of selenium concentrated in vegetation can be a useful 

indicator of concealed ore bodies in the soils or rocks supporting the

37
38 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

vegetation and can provide a basis for studying the effect of the natural 

environment on man and animals. For example, when the selenium content of forage crops is below 0.04 ppm, animals suffer from a selenium 

deficiency, and, when the selenium content exceeds 4 ppm, animals 

suffer from selenium toxicity (Lakin, 1973; Allaway, 1968). These limits 

indicate a range for which any method of determining selenium in 

vegetation has to be useful, but the upper limit is not restrictive.

The main thrust of our work was to develop a short, simple analytical 

method for determining selenium in vegetation, but the full objectives 

are broader and include setting base levels of selenium in a wide variety 

of vegetation in order to locate areas of enrichment or rejection of 

selenium by a plant.

Previous analytical methods for determining selenium in vegetation 

have traditionally been long and tedious. Such methods require (1) either 

wet ignition or digestion of the vegetation under reflux, or ignition in 

the presence of so-called fixatives (Robinson and others, 1934) to avoid 

losses, and (2) subsequent distillation to separate the volatile bromide 

of selenium.

Under reducing conditions, elemental selenium forms in the bromide 

distillate and colors it light pink to orange. Small amounts of elemental 

selenium remain in solution, but larger amounts, 0.5 mg or more, precipitate and can be filtered. The selenium content was measured by 

visually comparing the color of the solution or the precipitate obtained 

from a sample with that obtained under similar conditions from a 

known material.

The sensitivities of earlier procedures ranged from 1 to 10 ppm of 

selenium. Although this range was considered adequate at the time, the 

recognition that smaller amounts of selenium less than 0.1 ppm in 

animal feed stuff were significant (Schwarz and Foltz, 1957; Muth and 

others, 1958) triggered new research in methodology and improvements 

in reagents and equipment. For example, Cheng (1956) developed a 

colorimetric method based on the reaction of selenium with 3,3'-DAB 

with a sensitivity of 50 ppb using a 1-cm absorption cell. Later, Parker 

and Harvey (1962) suggested that 2,3-DAN was a more effective reagent. 

Several methods based on the reaction of selenium with these reagents 

have been reported. Dye, Bretthauer, Seim, and Blincoe (1963) developed a spectrophotometric and fluorometric procedure using DAB. 

Recently, in an effort to establish a standard procedure for evaluation 

purposes, Olsen (1969) selected the fluorometric method using DAN. 

Wilkie and Young (1970) have improved the existing DAN procedure by 

devising a technique for stabilizing DAN.

The fluorometric procedure, based on the reaction of selenium with 

DAN, is sensitive to about 0.02 ^g of selenium. Such sensitivity seems 

adequate to measure the small amounts in vegetation that are indicative 

of the average composition of solutions percolating through the Earth's
SELENIUM IN VEGETATION 39

crust as well as those concentrations required to evaluate the role of 

selenium in vegetation and its effect on the natural environment of man 

and animals.

REAGENTS

Ammonium hydroxide (1+4). Add 20 ml of concentrated annomium 

hydroxide to 80 ml of water.

2,3-DAN. Prepare reagent in partly darkened room by adding 0.1 g 

of the powder to 10 ml concentrated hydrochloric acid. Allow to stand 

for about 10 minutes and then dilute to 100 ml with water. Extract 

the solution with 10-ml portions of hexane by shaking the mixture 

of solution and hexane vigorously 3 times only, and then by allowing 

the mixture to separate 3-4 minutes only. Examine each hexane extract 

with low-intensity ultraviolet light, and when the fluorescence of the 

extract is negligible, store the aqueous reagent (DAN) in a brown 

bottle under a one-half-cm layer of hexane. Keep refrigerated and use 

in partly darkened area.

Ethylenediamine tetraacetic acid-hydroxylamine hydrochloride (EDTANH2OH«HC1). Dissolve 9.3 g of disodium salt of ethylene-diamine tetraacetic acid and 25 g of hydroxylamine hydrochloride in 1 1 of water.

Formic acid (1+1). Dilute concentrated acid with equal volume of water.

Hexane, reagent grade.

Hydrochloric acid (1+4). Add 20 ml concentrated acid to 80 ml of water.

Nitric acid, reagent grade, redistilled.

Perchloric acid, 70 percent.

Quinine sulfate, 1,000 /ag/ml. In a 100-ml volumetric flask dissolve 

0.1 g quinine (alkaloid base) in mixture of 1 ml sulfuric acid and 2 

ml water. When dissolved, dilute to 100-ml mark with water.

Quinine sulfate, 100 /jg/ml. Dilute 10 ml of the 1,000- /ug/ml standard 

to 100 ml with sulfuric acid (1+99), for example, 1 ml concentrated 

sulfuric acid plus 99 ml water.

Quinine sulfate, 2 ^tg/ml. Dilute 20 ml of 100- jug/ml standard to 100 

ml with sulfuric acid (1+99).

Selenium standard solution, 1,000 jug/ml. In a 50-ml volumetric flask 

dissolve 0.05 g pure selenium in 5 ml of warm perchloric acid. Dilute 

to 50-ml mark with water.

APPARATUS

Fluorometer. Galvanek-Morrisson Mark V or similar instrument. 1

Burner, Fisher type.

Centrifuge, International, size 2, model K or other centrifuge or similar

performance capable of 1,200 to 8,500 r/min and centrifugal force of

380 to 4,400 times gravity.

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological Survey.
40 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Oven Cenco Catalog No. 95106A or similar oven.

pH meter Any one of several laboratory types is suitable.

PROCEDURE

1. Weigh 1.0 g dry pulverized sample into a 30-mMtjeldahl flask and 

add 10 ml of redistilled nitric acid and 1 glass boiling bead.

2. Let stand at room temperature for 4 hours or longer.

3. Heat flask for 15 minutes with low heat, and then gradually increase 

the heat until the nitric acid is refluxing in the neck of the flask.

4. Allow the nitric acid to reflux for 15 minutes, remove heat, and 

allow flask to cool for 5 minutes.

5. Add 2 ml perchloric acid, washing down neck of flask with the acid.

6. Heat the flask slowly until the acids reflux, and allow the refluxing 

to continue for 15 minutes.

7. Increase the heat and boil until the white fumes of perchloric acid 

appear. Continue fuming for 15 minutes.

8. Remove the flask from heat and allow to cool. Add 2 ml water and * 

2 ml of (1+4) hydrochloric acid, using both water and acid to wash 

down the neck and sides of the flask. Mix.

9. Heat the flask in a boiling water bath for 30 minutes and transfer 

the contents to a 50-ml beaker, rinsing the flask with three or 

four 2-ml portions of water, and add the rinse water to the 50- 

ml beaker.

10. Add 2.5 ml of (1+1) formic acid and (1+4) NH4OH or (1+1) HC1 to ^ 

adjust pH of sample solution to 2. (Use pH meter.)

11. Adjust the volume of the solution to 30 ml with water, add 2 ml 

of EDTA-NH2OH«HC1 reagent, and mix.

12. Perform steps 13 through 17 in a partly darkened room.

13. Add 2 ml DAN reagent, mix, and place samples in 80°C oven for y 

20 minutes.

14. Remove samples from oven and allow to cool for 30-40 minutes, *" 

then decant sample solution into a 60-ml separatory funnel. Do 

not rinse beaker.

15. Add 3.0 ml hexane and shake funnel vigorously 3 times only. Allow x 

phases to separate (3-4 min. Drain, discard the aqueous layer, and 

transfer the hexane layer (upper) to a cuvette.

16. Centrifuge tube for 2 minutes and read the fluorescence of the 

sample solution, using 4 ml of a 2 /ag/ml solution of quinine 

to calibrate the fluorometer.

17. Calculate the amount of selenium by comparing fluorescence of ^ 

unknowns with that of standard solutions, observing the specific 

instructions unique to the particular fluorometer used.

^ 

DISCUSSION OF STANDARD CURVES

Standard curves, ranging from 0.02 to 0.5 ppm selenium in original '
SELENIUM IN VEGETATION 41

samples, were prepared in two ways. In the first procedure, the standard 

solutions, which were prepared by dilution of 1,000 /-ig/ml selenium 

standard solution, were taken through the entire determination, including digesting with nitric and perchloric acids. In the second procedure, the calculated amounts of selenium were pipetted from dilutions 

of the 1,000 L(g/ml standard solution into 50-ml beakers followed by 

addition of 1+1 formic acid and pH adjustment, as directed in step 10. 

Complete steps 11 through 17 for the second procedure.

Fluorescence values obtained by the first procedure are more precise 

than those obtained by the second procedure. Values obtained by the 

second procedure tend to be erratic, especially at higher levels, although, 

in fact, either procedure can be used if the differences are not the deciding 

factor.

RECOVERY OF ADDED SELENIUM

Increasing amounts of soluble selenium were added to 1-g portions 

of a sample of alfalfa collected near Delta, Utah, and each portion, including that of the digestion step, was analyzed for selenium (table 13). 

In general, the recoveries are above 80-90 percent, which is adequate 

for most applications.

TABLE 13. Recovery of added selenium 

[Leaders (...) indicate measurement not made]

Selenium

Run No.

1.... .......................

2......... ....... ...........

3...... ................. ....

4...........................

5

6...........................

Added 

(/Jg)

....... 0.02

....... 05

.:..... .1

.2

....... 5

Found

(MS)

0.06

.07

.10

.15

23

.59

Recovered 

(percent)

88

91

94

88

105

- The repeatability of the procedure was tested by making repeat determinations on samples olAtriplex collected in San Juan County, N. Mex. 

These determinations were repeated 4 times on 5 samples and 3 times 

on 4 samples. The number of times of repetition were chosen randomly, 

and the range of the repeat runs, as well as certain derived data, are 

shown in table 14. In general the relative standard deviations are about 

10 percent, although a few are 20 percent or more.

Another test of precision was provided by a geologist who submitted 

for analysis 950 plant samples representing .10 species of trees and shrubs 

from Missouri (J. A. Erdman, oral commun., 1972). Into this group of 

samples he inserted 40 randomly selected samples in duplicate and 

randomly interspersed them without knowledge of the analyst. The
42 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION 

TABLE 14. Repeatability of selenium determinations

Samph

No.

1...........

2...........

3

4...........

5

6...........

7....:......

8...........

9

i

..... 0.12

.64

.67

.60

1.1

1.5

1.5

..... 4.5

..... 4.5

Selenium (ppm)

2

0.13

.66 

.68 

.64 

1.4 

1.5 

1.5 

4.6 

4.5

3

0.14 

.68 

.72 

.90 

1.6 

1.5 

1.7 

4.8 

6.0

4

0'.78

1.8 

1.8

5.2 

6.0

Mean

0.13 

.69 

.69 

.71 

1.5 

1.6 

1.6 

4.8 

5.2

Standard 

deviation

0.02 

.06 

.03 

.16 

.3 

.15 

.11 

.31 

.87

Relative 

standard 

deviation 

(percent)

15 

9 

4 

23 

20 

9 

7 

6 

17

logarithmic variance of the analytical method is 0.00738 which means 

that the analyses are reproducible within a factor of 1.22 at the 68-percent 

level of confidence, or within a factor of 1.49 at the 95-percent level of 

confidence.

The lack of analyzed samples to test the method is an obvious drawback, but the recovery and repeatability data are good enough to suggest 

the potential application of our procedure to the measurement of small 

amounts of selenium in vegetation. These amounts may provide clues 

to concealed ore bodies and information for the assessment of the effect 

of selenium in the natural environment on man and animals. 

REFERENCES CITED

Allaway, W. H., 1968, Control of the environmental levels of selenium, in Missouri 

Univ. Ann. Conf. on Trace Substances in Environmental Health, 2d, Proc.: p. 

181-206.

Cheng, K. L., 1956, Determination of traces of selenium-3, 3-diaminobenzidine as selenium (IV) organic reagent: Anal. Chemistry, v. 28, no. 11, p. 1738-1743.

Dye, W. B., Bretthauer, Erich, Seim, H. J., and Blincoe, Clifton, 1963, Fluorometric

determination of selenium in plants and animals with 3,3'-diaminobenzidine: Anal.

Chemistry, v. 35, no. 11, p. 1687-1693. 

Lakin, H. W., 1973, Selenium in our environment, in Kothny, E. L., ed., Trace elements in the environment: Am. Chem. Soc., Advances in Chemistry Ser. 123, p.

96-111. 

Muth, O. H., Oldfield, J. E., Remmert, L. F., and Schubert, J. R., 1958, Effects of

selenium and vitamin E on white muscle disease: Science, v. 128, p. 1090. 

Olsen, O. E., 1969, Fluorometric analysis of selenium in plants: Jour. Assoc. Official

Agr. Chemists, 52, p. 627-634. 

Parker, C. A., and Harvey, L. G., 1962, Luminescence of some piazselenols A new

fluorimetric reagent for selenium: Analyst, v. 87, p. 558-565. 

Robinson, W. O., Dudley, H. C., Williams, K. T., and Byers, H. G., 1934, Determination

of selenium and arsenic by distillation, in pyrites, shales, soils and agricultural

products: Indus, and Eng. Chemistry Anal. Ed., v. 6, no. 4, p. 274-276. 

Schwarz, Klaus, and Foltz, C. M., 1957, Selenium as an integral part of Factor 3 against

dietary necrotic liver degeneration: Am. Chem. Soc. Jour., v. 79, p. 3292-3293. 

Wilkie, J. B., and Young, Mamie, 1970, Improvement in the 2,3-diaminonaphthalene

reagent for microfluorescent determination of selenium in biological materials: Jour.

Agr. and Food Chemistry, v. 18, no. 5, p. 944-945.
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

ATOMIC ABSORPTION DETERMINATION 

OF BISMUTH, CADMIUM, AND LEAD

IN VEGETATION AFTER 

OXYGEN FLASK COMBUSTION

By T. F. HARMS and F. N. WARD

CONTENTS

Page

Abstract........................................................................................................................... 43

Introduction.................................................................................................................... 43

Reagents and apparatus................................................................. ........... 45

Procedure ........................................................................................................................ 46

Preparation of standards................................................................................................. 46

Experiments and results ................................................................................................. 46

References cited............................................................................................................... 49

TABLES

Page

TABLE 15. Recovery of added element after ignition (aqueous).................................. 47

16. Recovery of added element after ignition (organic solvent)....................... 48

17. Determination of lead and cadmium after sample digestion ..................... -48

ABSTRACT

Samples of vegetation and other organic-rich materials are burned in a closed oxygencontaining flask and the combustion products are collected in dilute nitric acid. After 

pH adjustment, chelates of these elements are formed with ammonium-1-pyrrolidine 

dithiocarbamate and extracted into MIBK (4-methyl-2-pentanone). The organic solvent 

is analyzed for bismuth, cadmium, and lead by atomic absorption spectrometry.

INTRODUCTION

The evaluation of different ways of sample breakdown is essential for 

the development of useful analytical methods for determining trace constituents of vegetation because of the influence of that step on the type

43
44 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

of measurement contemplated. Wet digestion of dried vegetative material with hot oxidizing acids under reflux is especially useful to retain 

volatile elements, such as arsenic, antimony, and mercury, and is also 

useful for multiple analysis, that is, analyzing for more than one element; however, wet digestions are usually long, tedious, and inappropriate for a large number of samples. Ignition of the dried vegetative 

material is more easily automated than wet digestions and usually leaves 

a residue easily solubilized with dilute acids. Without special precaution 

and care, however, volatile elements are lost, thus limiting the applicability. In the mid-1950's Schoniger (1955) devised a technique of sample 

breakdown consisting of burning the vegetative matter or other organic 

material in an atmosphere of pure oxygen contained in a closed flask, 

and dissolving the combustion products in a suitable acid or base. The 

ignition technique, called variously oxygen-flask or Schoniger oxidation, is rapid and complete, and, in its use, no volatile products are lost. 

The acidic or basic collector of the evolved products can be used for 

multiple analysis, within the limits of the measuring procedure.

The oxygen-flask technique has been modified to meet specific needs, 

and Gutenmann and Lisk (1960) and Gutenmann and others (1961) provide examples of such modifications. These workers analyzed a variety 

of vegetative material, including apples, potatoes, cherries, and cabbage, 

for arsenic, bromine, chlorine, manganese, mercury, and nickel. Belcher, 

MacDonald, and West (1958) burned metallo-organic compounds containing one or more of several elements in the oxygen flask and achieved 

total recovery of boron, cadmium, magnesium, and zinc. Recovery of 

lead and bismuth was incomplete, and arsenic appeared to be lost completely. However, other volatile elements, such as selenium, are recovered 

satisfactorily after such an oxidation step (Allaway and Gary, 1964). 

Thus, less volatile elements, such as copper, lead, and perhaps bismuth, 

would likely be retained, depending partly on the treatment after the 

ignition.

The sample size of dried vegetative matter is limited to 1 g or less 

when oxygen flask ignitions are used. This limitation emphasizes a need 

for as much sensitivity as possible for the final measurement of the 

element. If atomic absorption measurements are used, a severalfold increase in sensitivity can be achieved by extracting the metal with a suitable organic solvent and aspirating the solvent in the flame of a laminarflow burner. Mulford (1966) found that the formation of metal chelates 

which are preferentially soluble in suitable organic solvents offers a 

means of increasing the sensitivity. He studied several chelating agents 

and suitable organic solvents and concluded that the APDC had the 

greatest potential. Previously, Malissa and SchSffmann (1955) had 

studied this reagent as well as several other carbamates. They showed 

that several carbamates, including the ammonium salt, will chelate a
BISMUTH, CADMIUM, AND LEAD IN VEGETATION 45

large number of elements over a wide range in pH values. Among the 

elements that are not thus chelated are the alkalis and alkaline earths.

Allan (1961) determined the content of copper in vegetation and soils 

by atomic absorption spectroscopy using APDC and ethyl acetate or 

MIBK to extract the chelated copper. Willis (1962) determined the content of lead, bismuth, mercury, and nickel in urine with an extraction 

step using APDC and methyl n-amyl ketbne. These examples serve to 

illustrate the widespread use of the carbamate-ketone combinations to 

eliminate possible interferences and to increase the sensitivity of the 

measurements of the elements. The predominant use of MIBK probably 

reflects a fact of economy and availability, rather than a demonstrated 

superiority.

Accordingly, our procedure includes a Schoniger oxidation and collection of the combustion products in dilute nitric acid followed by pH 

adjustment, addition of APDC, extraction of chelated metals into MIBK, 

and finally aspiration of the solvent in an atomic absorption instrument.

REAGENTS AND APPARATUS

Ammonium hydroxide, 5 M. Dilute 340 ml concentrated ammonium 

hydroxide to 1,000 ml with metal-free water.

Ammonium 1-pyrrolidine dithiocarbamate (APDC), 1 percent. Dissolve 

1 g of APDC in 100 ml water. Prepare fresh daily.

Bismuth, 1,000 jug/ml. Dissolve 0.5 g metal in 500 ml concentrated 

nitric acid. Prepare 100- /ug/ml standard solution by tenfold dilution 

of concentrated standard solution with water. Prepare less concentrated standard solutions by tenfold dilution with 1 M nitric acid.

Cadmium standard solution, 1,000 ^ig/ml. Dissolve 0.114 g cadmium 

oxide in 10 ml 8 M nitric acid. Dilute to 100 ml with water.. Prepare 

100- /ug/ml and 10- Aig/ml solutions by tenfold dilutions with 1 M 

nitric acid.

Lead standard solution, 1,000 /jg/ml. Dissolve 0.16 g lead nitrate in 

small amount of water. Add 5 ml concentrated nitric acid and dilute 

to 100 ml with metal-free water. Prepare 10- ng/m\ solution by tenfold 

dilution, keeping nitric acid concentration the same as for the more 

concentrated standard solution.

4-methyl-2-pentanone (MIBK).

Oxygen, gaseous.

Magnetic stirrer and Teflon-coated spin bars.

Combustion flasks, oxygen, 2-1 capacity. Thomas-Lisk type with ignition basket, safety outlet, and balloon.

Die 0.5-in. diameter.

pH meter.

Press, hydraulic Range up to 20,000 Ib/in2g.

Perkin-Elmer model 303 or equivalent.
46 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

PROCEDURE

1. Weigh 0.5 g of dried vegetation into the 0.5-in. die, and press into 

a pellet with the hydraulic press.

2. Remove pellet from die, wrap in small filter paper cut with 1-in. 

tail, and place in basket attached to stopper of a 2-1 combustion 

flask.

3. Place a magnet in the flask and add 60 ml 0.5 M nitric acid.

4. Flush the flask with oxygen for approximately 30 seconds, ignite 

tail of paper enclosing vegetation pellet and quickly insert the paper 

with the pellet into flask.

5. After combustion is complete, place flask on a magnetic stirrer and 

stir vigorously for 30 minutes so that the ash remaining in the ignition basket is dissolved in the acid.

6. Transfer the solution to a 150-ml beaker and rinse basket and flask 

3 times with distilled water, adding these portions of rinse water 

to the solution in the beaker.

7. Adjust the pH to 2.2 using 5 M ammonium hydroxide.

8. Transfer the solution to a 125-ml separatory funnel, add 2.5 ml of 

1 percent APDC solution and mix well.

9. Add 10 ml of MIBK and shake vigorously for 2 minutes.

10. Allow the phases to separate, transfer the organic layer to a stoppered 

test tube, aspirate in atomic absorption instrument, and record the 

absorbance reading.

PREPARATION OF STANDARDS

To series of 150-ml beakers each containing 60 ml of 0.5 M nitric acid 

add individually 0.0, 0.1, 0.2, 0.6, and 1.0 ml of 0.001 percent standard 

cadmium solution. In the same order, add individually 2.0, 1.4, 0.8, 0.2, 

and 0.0 ml of 0.01 percent standard lead solution. Adjust pH of each 

solution to 2.2 with 5 M ammonium hydroxide and add 2 ml APDC. 

Extract aqueous solutions with 20 ml of MIBK. These standard solutions 

will contain 0.0, 0.05, 0.1, 0.3, and 0.5 jug/ml of cadmium and 10, 7, 

4, 1, and 0/ug/ml of lead in the organic layer. Aspirate the ketone 

(organic layer) as in step 10 of procedure.

EXPERIMENTS AND RESULTS

Recoveries of added amounts of bismuth, cadmium, and lead were 

tested first by adding increasing amounts of these elements to filter 

papers and burning these papers in an oxygen flask and absorbing the 

combustion products in 0.5 M nitric acid. Then, these solutions were 

atomized directly in an atomic absorption instrument, and the meter 

readings were compared with those obtained from dilute nitric acid solutions to which similar amounts of standard solutions had been added
BISMUTH, CADMIUM, AND LEAD IN VEGETATION 47 

TABLE 15. Recovery of added element after ignition (aqueous)

[Leaders (...) indicate that data are missing]

Meter reading

Element

Lead......................

element in final

volume (ppm)

5

10

0.5

1.0

1.5

2.0

1 .

2

5

10

30

After direct

addition

10

20

50

100

5

30

90

After ignition

10

20

22

47

71

96

3

6

14

28

directly (table 15). The meter readings in the right-hand column show 

recoveries of 96-100 percent, and one can conclude that bismuth, cadmium, and lead are recovered after combustion.

Similar experiments were made to test the recovery when solvent 

extraction is used to enrich the analyte and thereby achieve greater 

sensitivity. Enrichment may be as great as tenfold with bismuth and 

lead; fivefold with cadmium. Accordingly, increasing amounts of 

bismuth, cadmium, and lead were pipetted from standard solutions onto 

filter paper, allowed to dry, and combusted in an oxygen flask. After 

ignition, the acidic solution of the combustion products was shaken 

with an MIBK solution of APDC, and the carbamates of bismuth, cadmium, and lead were extracted into the organic solvent. The MIBK solutions were aspirated in an atomic absorption instrument, and the meter 

readings were compared with those obtained from MIBK solutions of 

the metal carbamates extracted directly from nitric acid solutions containing similar amounts of standard solutions as added to filter papers 

(table 16).

Similar meter readings after ignition and after direct addition show 

that the losses of bismuth, cadmium, and lead during combustion are 

insignificant, and one can conclude that sample solution of vegetation 

can be prepared in this manner. The recovery data in tables 15 and 16 

show also that insignificant losses of bismuth, cadmium, and lead occur 

during combustion of organic materials in an oxygen flask. Other elements tested, such as cobalt, nickel, and zinc, were not recovered uniformly and, hence, are not included in this paper.

Determinations of bismuth, cadmium, and lead in various organicrich materials after oxygen flask combustion were compared with those 

obtained after hot-acid digestion. The bismuth values were below the 

limit of detection, but the cadmium and lead were measurable, and the 

comparison is given in table 17.
48 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

TABLE 16. Recovery of added element after ignition (organic solvent)

[Leaders (...) indicate that reading is missing]

Meter reading

Elemeni 

added

Bismuth................

Cadmium ..............

Lead.......................

Concentration of

element in final 

volume (ppm)

0.5

1.0

2.0

4.0

.1

.2

.5

.1

.5

1

2

5

10

After direct 

addition

11

24

45

78

20

42

98

5

10

21

50

100

After 

ingition

9

18

34

73

23

46

101

3

5

10

19

50

98

TABLE 17 . Determination of lead and cadmium after sample digestion

Lead 

(ppm)

No.

1 ...........

2 ...........

3 ...........

4 ...........

5 ...........

6 ...........

7 ...........

8 ...........

9

10...........

11...........

12.. .........

13...........

14...........

15

16...........

17.. .........

18...........

19...........

20...........

21

..... Cedar wood....................

Cedar bark.....................

Oak leaves......................

.... Cedar leaves...................

Asparagus......................

Oak leaves......................

Onions............................

.... ...do.................................

.... ...do.................................

Yellow birch twigs.........

Hot acid

<2.5

2.5

<2.5

5

5

2.5

5

7 5

10

15

10

10

12

18

20

38

30

50

55

95

160

Oxygen flask1

<5

<5

<5

5

5

5

8

8

10

10

11

12

15

18

20

32

35

52

54

94

160

Cadmium 

(ppm)

Hot acid

<1

2

2.6

<1

5.4

<1

6.8

<1

38

40

5.6

5.8

44

12

1.1

17

61

40

13

22

13

Oxygen flask 1

<1

2

3

<1

6

1

5

1.4

40

38

5

6

46

12

0.8

15

64

42

12

22

12

'Our procedure.

The wet digestion of dry material is with a perchloric-nitric acid mixture (Ward and McHugh, 1964). Cadmium was measured in the acidic >- 

sample solution by atomic absorption spectrometry; lead was measured 

either by atomic absorption spectrometry or by colorimetry (Ward and 

others, 1963) depending on the lead level.
BISMUTH, CADMIUM, AND LEAD IN VEGETATION 49

Data in tables 15-17 indicate that multiple analyses can be made on 

a single small sample of organic-rich material, using our procedure.

REFERENCES CITED

Allan, J. E., 1961, The determination of copper by atomic absorption spectrophotometry:

Spectrochim. Acta, v. 17, p. 459-466. 

Allaway, W. H., and Gary, E. E., 1964, Determination of submicrogram amounts of

selenium in biological materials: Anal. Chemistry, v. 36, no. 7, p. 1359-1362. 

Belcher, Ronald, MacDonald, A. M. G., and West, T. S., 1958, The determination of

metals in organic compounds by the closed flask method: Talanta, v. 1, p. 408-410. 

Gutenmann, W. H., and Lisk, D. J., 1960, Rapid determination of mercury in apples

by modified Schbniger combustion: Jour. Agr. Food Chemistry, v. 8, no. 4, p. 306-308. 

Gutenmann, W. H., Saint John, L. E., Barry, D. L., Jones, E. D., and Lisk, D. J.,

1961, Modified Schoniger combustion for determination of residues of arsenic,

bromide, chloride, manganese, and nickel in pesticide-treated plant material: Jour.

Agr. Food Chemistry, v. 9, no. 1, p. 50-51. 

Malissa, H., and Schoffmann, E., 1955, Uber die Verwendung von substituierten

Dithiocarbamaten in der Mikroanalyse. Ill: Mikrochim. Acta, 1, p. 187-202. 

Mulford, C. E., 1966, Solvent extraction techniques for atomic absorption spectroscopy:

Atomic Absorption Newsletter, v. 5, p. 88-90. 

Schoniger, W., 1955, Eine mikroanalytische Schnellbestimmung von Halogen in

organischen Substanzen: Mikrochim. Acta, 1, p. 123-129. 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used

in geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull.

1152, 100 p. 

Ward, F. N., and McHugh, J. B., 1964, Determination of mercury in vegetation with

dithizone A single extraction procedure, in Geological Survey research 1964: U.S.

Geol. Survey Prof. Paper 501-D, p. D128-D130. 

Willis, J. B., 1962, Determination of lead and other heavy metals in urine by atomic

absorption spectroscopy: Anal. Chemistry, v. 34, no. 6, p. 614-617.
4
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

DETERMINATION OF MERCURY IN

NATURAL WATERS, PLANTS, AND SOILS

BY A COLD-VAPOR PROCEDURE

By MARGARET E. HINKLE and GEORGE L. CRENSHAW

CONTENTS

Page

Abstract........................................................................................................................... 52

Introduction.................................................................................................................... 52

Equipment...................................................................................................................... 53

Reagents.......................................................................................................................... 55

Procedure for determinations......................................................................................... 56

Elemental and ionic mercury in water ................................................................... 56

Mercury in water samples having an H2S odor...................................................... 56

Mercury in plants.................................................................................................... 56

Mercury in water samples of high organic content or which contain

ammonia gas........................................................................................................ 57

Mercury in soils, rocks, and sediments................................................................... 57

Standards......................................................................................................................... 57

Ionic mercury........................................................................................................... 57

Elemental mercury................................................................................................... 57

Discussion....................................................................................................................... 58

Results............................................................................................................................. 60

References cited............................................................................................................... 63

ILLUSTRATION

Pago

FIGURE 1. Diagram of mercury detector...................................................................... 54

TABLES

Page

TABLE 18. Concentrations of elements causing loss in mercury recovery.................. 60

51
52 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Page

TABLE 19. Precision of elemental mercury measurement........................................... 61

20. Precision of ionic mercury measurement................................................... 61

21. Results of mercury analyses........................................................................ 62

ABSTRACT

The recent interest in mercury and its behavior in the natural environment has 

triggered the development of new, rapid, and sensitive analytical procedures for mercury. 

In our method, the elemental and ionic mercury fractions of natural waters and acid 

digestates of plants and soils can be determined. After ionic mercury is reduced to the 

elemental form with stannous chloride, a stream of nitrogen flushes the mercury vapor 

from the reaction flask into the mercury-vapor absorption detector. From 10 to 400 

nanograms of mercury can be detected in the sample solution. Mercury amounts determined by this method were as little as 0.05 micrograms per litre in water samples and 

as much as 49,000 parts per billion in mud samples.

INTRODUCTION

During the last few years, interest has increased rapidly in the associations and behavior of mercury in the natural environment. Mercury has 

been used in geochemical exploration as a pathfinder element in the 

search for concealed ore deposits (Hawkes and Webb, 1962, p. 73; 

McCarthy, Gott, and Vaughn, 1969). Studies of the geochemical associations of mercury are leading to the understanding of mercury transport 

and deposition (White, 1967; Dickson and Tunell, 1968). Recently, 

mercury lost from industrial processes has spurred the study of mercury 

in the environment (U.S. Geological Survey, 1970).

Several analytical procedures are now in use for determination of mercury in various kinds of samples. The most sensitive methods involve 

measurement of mercury vapor by a mercury-vapor absorption detector 

utilizing the 2,537-angstrom line of mercury. The mercury in samples 

of water is often concentrated in some way prior to measurement, either 

by amalgamation on silver (Hinkle and Learned, 1969; Fishman, 1970) 

or by extraction with dithizone (Chau and Saitoh, 1970). Omang (1971) 

treated water samples with KMnO4 and H2SO4 to decompose organic 

interferences and, then, reduced the mercury to elemental form and measured it directly from the solution by flameless atomic absorption 

spectrophotometry. The most convenient method of soil analysis is by 

direct heating of the sample in an induction furnace and subsequent 

measurement of evolved mercury (Vaughn and McCarthy, 1964). Extensions of this method, useful for organic-containing soils, include acid 

digestions of the sample, followed by reduction with stannous ion 

(Hatch and Ott, 1968), and dithizone extraction of the acid digestate 

(Pyrih and Bisque, 1969) prior to measurement of the mercury by atomic 

absorption spectrophotometry. The most thorough method of sample 

dissolution for analysis of mercury in plants is digestion of 0.5-1 g
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 53

ground air-dried plant material under reflux with a mixture of nitric, 

sulfuric, and perchloric acids (Ward and McHugh, 1964); the mercury 

is then determined colorimetrically with dithizone.

In our procedure, mercury is measured in a variety of samples waters, 

soils, and plants by one generalized method. A special feature of our 

method is the use of a bulbous-shaped reaction flask, which permits 

more efficient mixing of sample solution and reagents than conventional 

reaction bottles. This feature is especially useful for plants and for soils 

and sediments containing organic materials. Our procedure eliminates 

the tedious dithizone color development of acid-digested plants and soils.

Elemental and ionic mercury species in natural waters are separated 

and measured by this procedure. Mercury-containing solutions disproportionate into mercuric, mercurous, and elemental mercury forms, 

depending on such conditions as availability of complexing agents and 

temperature. The concentrations of different species occurring in a 

natural water may indicate the mode of transport of the mercury.

Elemental mercury is measured by flushing mercury vapor from an 

acidified water sample into the mercury-vapor absorption chamber in 

a stream of nitrogen. The ionic mercury is reduced to the elemental 

form with stannous chloride, and the elemental mercury thus produced 

is swept into the detection unit with nitrogen. Acid digestates of soil 

and plant samples are also treated with stannous chloride and the reduced mercury flushed into the vapor absorption detector for measurement.

From 10 to 400ng can be measured in a 50-ml sample solution.

EQUIPMENT

Reaction flask train (fig. 1):

Reaction bubbler flask. Pyrex, commercially available from Ace 

Glass, Inc., 1 Vineland, N.J.: The reaction flask is 8 in. high and 

has a S^-in.-long neck with 129/42 fitting. The body of the flask 

expands to a 2M-in. diameter, and will hold 50 ml when half filled. 

Below the body, the flask narrows to a %-in. diameter for the remaining S^-in. of total length.

Reaction bubbler flask head. Pyrex, commercially available from 

Ace Glass, Inc., Vineland, N.J.: The reaction bubbler flask head 

holds both the gas inlet and outlet tubes and fits into the 129/42 

fitting of the reaction bubbler flask. The 9-in.-long gas inlet tube 

extends down nearly to the bottom of the narrow part of the reaction flask and ends in a fritted glass dispersion plug. The gas 

outlet tube is fitted with a ball-joint connection.

Outlet to dryer and mercury detector. Ball-joint connection

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological Survey.
54 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

JL

rr

Mercury-vapor absorption detector^

Reaction bubbler 

Gas flowmeter 

^Nitrogen inlet / ,

0

y

Mitrogen tank

1

 T- <~

i '

 /

fl

cp

Outlet to dryer and ^\ 

mercury detector ^.^s\

I v\

J

V

1

Drying tube

\2,»

rt Reaction bubbler flask

->J U-%

FIGURE 1. Diagram of mercury detector. Not to scale.

Reaction flask train (fig. 1) Continued

Outlet Continued

attached to Tygon tubing. The ball joint is connected to the gas 

outlet of the reaction bubbler flask head, and the Tygon tubing 

is connected to the drying tube. Wash with sulfuric acid, water, 

and acetone and dry thoroughly under a heat lamp between determinations.

Drying tube. Bend an approximately 7-in. length of Pyrex glass 

tubing (O.D. 18 mm standard wall) into a V shape. Make at least 

six of these drying tubes. Place a 0.5-g scoop of Mg(C104 ) 2 drying 

agent in each bent tube. Attach one end of a drying tube to the 

reaction flask with the outlet connector and attach the other end 

of the drying tube to the mercury detector with Tygon tubing. 

The glass drying tube should be washed, rinsed with acetone, and 

dried under a heat lamp between determinations.

Mercury-vapor absorption detector. An atomic-absorption 

instrument utilizing the 2537-angstrom line of mercury (described 

by Vaughn and McCarthy, 1964) was used. A chamber with quartz 

windows can be placed in beam of a hollow cathode lamp, mercury passed into the chamber and measured as in flame spectroscopy.

Nitrogen inlet (from gas flowmeter to reaction flask). Tygon 

tubing with ball-joint connection.

Gas flowmeter. Nitrogen should be delivered to the sample solution in the reaction flask at a reproducible rate of about 1,000 

cmVmin. Attachments are Tygon tubing and a ball-joint connection.
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 55

Heat lamp, or drying oven.

Syringe, 10-cm3 capacity.

Special equipment for solution of plants:

250-ml round-bottom flasks.

Heating mantles for flasks.

Variable rheostats.

Water-cooled condensers for the round-bottom flasks.

Glass boiling beads. 

Special equipment for solution of rocks, soils, and sediments:

Alcohol lamp or hotplate

Pyrex test tubes or flasks with $ necks

f - Standard taper. 12-in. air condensers to fit test tubes or flasks.

REAGENTS

All reagents should be prepared with demineralized water. 

Hydrochloric acid, reagent grade. 

Sulfuric acid, reagent grade.

Stannous chloride solution: Dissolve 10 g of SnCl2 in 100 ml 6 N HC1. 

Magnesium perchlorate, anhydrous. 

Nitrogen, dry.

Mercury standard solution: Dissolve 0.1354 g of reagent-grade mercuric 

chloride in 100 ml of 1 N sulfuric acid. This standard contains 1,000 

/jg/ml mercury. By dilution with water, prepare fresh a 1- jug/ml 

(1,000-ng/ml) standard as needed.

Mercury-saturated air: Place a few grams of mercury in a Dewar flask 

Stopper the flask with a 2-hole rubber stopper. Through one hole insert a thermometer, and through the other hole insert a glass tube 

fitted with a rubber membrane stopper through which the syringe can 

be inserted to withdraw mercury-saturated air. 

Additional reagents for waters containing H2S:

Sodium chlorate: Dissolve 10.6 g of NaClO3 in 1,000 ml water. 

Hydroxylamine hydrochloride: Dissolve 50 g NH2OH«HC1 in 100

ml water.

Additional reagents for solution of plants: 

Nitric acid, reagent grade. 

Perchloric acid, reagent grade. 

Hydrogen peroxide, 30 percent. 

Antifoaming agent, Dow-Corning Antifoam B; solution of 25 ml

diluted to 100 ml with water.

Additional reagents for solution of rocks, soils, and sediments: 

Potassium bromide, reagent grade.

Potassium persulfate, reagent grade, saturated aqueous solution. 

Phenol solutions: Dissolve 1 g of reagent in 100 ml water. 

Sulfuric acid, 18 N: Mix equal volumes of acid and water.
56 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

PROCEDURE FOR DETERMINATIONS

ELEMENTAL AND IONIC MERCURY IN WATER

[Limited to maximum volume of 50 ml water]

Place 5 ml of hydrochloric acid in the reaction flask. Bubble N2 

through it for 1-2 minutes to remove mercury contamination and undissolved gases. Add 50 ml of water sample or a suitable smaller aliquot 

of sample diluted to 50 ml with mercury-free water. Attach the outlet 

and drying tubes, connecting the reaction flask to the mercury detector. 

Bubble N2 through the system and record the meter reading. This 

reading represents the elemental Hg content.

To determine ionic Hg in the same sample aliquot, turn off the nitrogen flow, replace the outlet and drying tubes in the delivery train, remove 

the reaction flask head and add 2 ml of SnCl 2 reagent to the sample. 

Reassemble the train and again bubble N2 through the apparatus. 

Record the meter reading.

Run a blank on the SnCl2 solution after each determination.

MERCURY IN WATER SAMPLES HAVING AN H2 S ODOR

H2S (Hydrogen sulfide), if present, must first be oxidized because it 

causes a false reading on the meter. Place 5 ml of hydrochloric acid in 

the reaction flask and bubble N 2 through it for 1-2 minutes. Remove 

reaction flask head and add 5 ml NaClO3 solution. Add the sample solution and mix well. Add 5 ml hydroxylamine hydrochloride solution and 

mix well. Attach the delivery train connecting the reaction flask to the 

mercury meter. Raise the reaction flask head and add 4 ml SnCl 2 solution. 

Replace the reaction flask head, bubble N2 through the system, and 

record the meter reading.

Run blanks on the solutions of SnCl 2 , NaClO3 , and NH2OH»HC1.

MERCURY IN PLANTS

Place 1 g of ground air-dried plant material in a round-bottom flask. 

Add 5 ml each of nitric, sulfuric, and perchloric acids. Digest under reflux for 4 hours. Cool slightly, add 2 ml of 30 percent hydrogen peroxide, 

cool to room temperature, and dilute with water to 100 ml.

Place 5 ml hydrochloric acid in the reaction flask and bubble N2 

through it for 1-2 minutes. Remove reaction flask head, and place sample 

aliquot in the reaction flask. Hydrochloric acid is not necessary for 

aliquots of acid-digestales 20 ml and over. Add water to 50 ml volume. 

Add 1-2 drops of antifoaming agent, if necessary. Add 2 ml SnCl 2 solution for sample aliquots 25 ml and under; add 4 ml SnCl2 for aliquots 

greater than 25 ml. Attach outlet and drying tubes, connecting the reaction flask and mercury meter. Bubble N2 through the system. Record 

meter reading.
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 57

Run blanks on SnCl 2 solution, acid digestion-mixture, and antifoaming agent.

MERCURY IN WATER SAMPLES OF HIGH ORGANIC CONTENT 

OR WHICH CONTAIN AMMONIA GAS

Use 25-ml aliquots of water samples. Follow the same procedure as 

that for the determination of mercury in plants.

MERCURY IN SOILS, ROCKS, AND SEDIMENTS

The sample is attacked as described by Ward and Bailey (1960).

Place 0.1-0.2 g of -80-mesh sample in flask or tube. Add 0.05 g KBr. 

Add 3 ml (1 + 1) H2SO4 . Attach 12-in. air condenser. Heat over alcohol 

flame or on hotplate to incipient boil. Add 0.5 ml saturated K2S2O8 solution through the top of the air condenser. Alternately, heat to incipient 

boil and remove the flask from the heat so that Br2 fumes do not escape 

out the top of the condenser for 1 minute, keeping the flask as hot as 

possible without losing fumes. Add another 0.5 ml saturated K2S2O8 

solution and heat for 1 more minute. Cool the flask. When cool, wash 

down the air condenser with water. Add 0.5 ml aqueous phenol solution 

to remove excess bromine.

To measure mercury, pour entire sample solution and solid materials 

into the reaction flask. Rinse out test tube with water and add rinsings 

to the reaction flask. Add water to about 50 ml. For samples containing 

more than 400 ng of mercury, the sample solution in the test tube should 

be diluted with water to a specific volume, and aliquots taken of this 

dilution. Add 2 ml SnCl 2 solution. Attach outlet and drying tubes, connecting the reaction flask to the mercury detector. Bubble N2 through 

the system. Record the meter reading.

Run blanks of SnCl2 solution and digestion reagents.

STANDARDS 

IONIC MERCURY

To a series of reaction bubbler flasks each containing a mixture of 

5 ml hydrochloric acid, 2 ml stannous chloride solution, and about 40 

ml water which has been previously degassed with nitrogen, add individually 10,20, 40, 80, 100, 200, and 400 ng of Hg +2 . Successively, connect 

each flask to the nitrogen inlet and to the outlet to dryer, and bubble 

nitrogen through the flask. Record meter readings for the different 

amounts of mercury.

ELEMENTAL MERCURY

Place about 50 ml water in the reaction flask. With a syringe, inject 

a given volume of mercury vapor into the outlet tube. At any given tem-
58 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

perature, mercury-saturated air contains a definite concentration of 

mercury. The concentration of mercury in 1 cm3 of mercury-saturated 

air can be calculated from knowledge of the vapor pressure of mercury 

at different temperatures (Weast, 1968, p. Dill) and use of the ideal gas 

law PV=nRT: 1 cm3 of mercury-saturated air contains 13 ng of Hg at 

20°C, 18 ng of Hg at 24°C, and 25 ng of Hg at 28°C.

Attach the outlet tube of the reaction flask and bubble N2 through 

the system to sweep the mercury into the detector unit. Record the meter 

reading, and repeat for different volumes of mercury-saturated air. Plot 

meter readings versus amounts of mercury on log-log paper. 

Calculation

For soils, sediments, plants:

tig _ Total volume (ml) nanograms measured

 Hg= x 

1 aliquot (ml) sample weight (grams)

10- 3 Mg 103ml

X X 

ng 1 

For water:

tig ng measured 10'3 jug 103ml

 Hg= x x 

1 ml of sample ng 1

DISCUSSION

Elemental mercury standards must be prepared by injecting Hg vapor 

directly into the inlet to the drying tube and detector part of the reaction 

train. The standards cannot be prepared by injecting Hg vapor into 

aqueous solutions, because some of the elemental mercury dissociates 

and is not swept out of solution by the nitrogen flow. The dissociated 

Hg can be recovered by adding SnCl2 and be reduced to the elemental 

form, but this reduced mercury is then considered to be ionic. About 

25-35 percent of elemental Hg was lost by dissociation in attempts to 

prepare standards by injecting Hg vapor by syringe through a rubber 

membrane stopper into water and 1 N HC1 solutions. In contrast, all 

the Hg vapor injected into solutions can be recovered by amalgamation 

on silver screens (Hinkle and Learned, 1969).

The special shape of the reaction flask is beneficial for good mixing 

when nitrogen is bubbled through the solution. Thorough but rapid 

mixing of SnCl 2 with the sample solution is needed to quickly reduce 

ionic Hg to elemental Hg, so that the Hg vapor will be pushed through 

the detector in a plug. A flow rate of 1,000 cmVrnin N2 will sweep a 

plug of elemental Hg vapor from this flask and, thus, give a sharp 

response from the detector. Mercury standards measured by reaction in 

the bulbous-shaped flask gave approximately twice the scale deflection 

on the mercury detector as did the same standards measured in a reagent 

bottle-type reaction flask.
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 59

Nitrogen is used in this procedure because it is free of mercury, and 

no special filters are necessary as would be needed with an air stream. 

A tank of N2 is also convenient to transport to the field, and the slow 

flow rate of 1,000 cmVmin makes Ng economical to use. The flow rate 

of N2 should be kept constant because the standard curve changes for 

different flow rates.

The tube through which the plug of Hg vapor is pushed into the 

detector must be kept dry. Small amounts of moisture along the interior 

walls of the tubing will absorb Hg vapor and cause analytical results 

to be low. Even when no moisture is visible in the tubing, the Mg(ClO4 ) 2 

dehydrating agent will become warm due to water absorption. About 

0.5 g of Mg(ClO4 ) 2 absorbs moisture adequately for each analysis; smaller 

amounts are inadequate; larger amounts, such as 1 g, tend to absorb 

Hg vapor as well as the moisture.

Hydrochloric acid is commonly contaminated by mercury. In addition 

to the mercury contamination, HC1 and C12 fumes from the acid often 

cause high blank values. Both of these problems are overcome by degassing the hydrochloric acid with nitrogen for 1-2 minutes prior to 

adding the sample to the reaction flask; the acid should give a zero 

instrument reading. Usually, the 2-4 ml of SnCl 2 will have a negligible 

blank value, but sometimes the SnCl2 solution absorbs mercury from 

the air; hence, a blank should always be run. Also, blanks should be 

run on other reagents used in any particular analysis. Hydrochloric acid 

is preferred over sulfuric or nitric acid in this procedure, to help avoid 

oxidation of elemental Hg to ionic Hg.

Ammonia gas in a water sample will cause a high mercury reading 

on the mercury detector. Conversely, dissolved organic substances, such 

as petroleum, will prevent any Hg present from leaving the water sample 

and, thus, will cause low mercury values. Both ammonia and organic 

interferences can be eliminated by an acid reflux of the water sample. 

When organic interference in a sample is suspected, aqueous Hg standards cannot be recovered, then the sample should be refluxed with nitric, 

sulfuric, and perchloric acids.

H2S in a water sample will also cause a high Hg reading on the 

detector. In addition, mercury in a water sample may be in the form 

of an HgS" 1 complex which breaks down to elemental sulfur on acidification; mercury may adsorb on the sulfur and not be swept out by the 

nitrogen. Strong oxidizing agents, such as Br2 , rapidly oxidize S~2 to 

SO4 - 2 (Latimer, 1952, p. 71). Addition of 10 ml of freshly prepared 

chlorine-water solution immediately removes all evidence of H2S or elemental sulfur in a sample; excess C1 2 is reduced by addition of hydroxylamine hydrochloride, and also by doubling the usual volume of SnCl 2 . 

Sodium chlorate solutions stronger than 0.1 M cannot be used in preparation of the chlorine water, because the mercury recovery drops,
60 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

either from chlorine-mercury complex formation or from oxidizing 

conditions too strong to be reduced by the hydroxylamine hydrochloride 

and stannous chloride.

Halides generally do not interfere with mercury determinations on 

natural water samples. As much as 1 M (35,000 jug/ml) chloride ion, 

in addition to the chloride from the hydrochloric acid and chlorine water, 

can be tolerated before mercury recovery is affected. All added mercury 

can be recovered from solutions containing 1,000 jug/ml bromide, much 

more bromide than is found in natural brines. In unpolluted natural 

waters, iodide ion is the only halide which may occur in sufficient concentrations to interfere with mercury recovery. As much as 30 /jg/ml 

iodide ion (concentration of a brine) in a water sample can be tolerated; 

however, 40-50 /ug/ml iodide ion causes a 50-percent drop in mercury 

recovery. Any natural water sample with a concentration of 40-50 /jg/ml 

iodide must be diluted with iodide-free water before analysis.

To check out possible interferences from other elements, aliquots of 

various dissolved metals were added to a solution of 100 ng Hg standard 

in 50 ml 1 N HC1 and carried through the reduction procedure. No interferences were seen from 2,000 tig Cd or from 5,000 n g As, Sb, or Pb 

in the 50-ml sample solution. These concentrations are unlikely in 

natural water samples and in diluted acid digestions of most plant 

materials and soils. Metals easily reduced to their elemental form by 

SnCl2 were found to lower mercury recovery by adsorbing the elemental 

Hg; a few other metals also cause low mercury recovery (table 18).

TABLE 18. Concentrations of elements causing loss in mercury recovery

[jjg in 50-ml sample solution]

Pd ...................

Te

Se....................

Pt. ....... ............

Au. ..................

...................0.02

05

................... .5

...................2

...................5

TL...................

Cu(II)...............

In .....................

Ag.......... ..........

Zn .....................

................ 100

................ 200

................ 300

...............'1,000

................2,000

'AgCl precipitate.

The presence or absence of undigested soil, sediment, and rock 

materials in the reaction flask has no effect on mercury recovery when 

the sample is freshly digested. If the acid digestate has been standing 

for 1 or 2 days before analysis, some mercury might adsorb onto the 

undigested solid material and cause low mercury recovery if the solid 

material is not placed in the reaction flask along with the sample solution. Analysis of the sample soon after digestion is the best procedure.

RESULTS

The precision of our method was checked for both elemental and ionic 

mercury. To check the precision for elemental Hg, different volumes
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 61

of mercury vapor were repeatedly injected into the detector, following 

the procedure for preparation of the standard curve. The relative standard deviation was 10 percent or less for approximately 6.5 ng of injected 

elemental Hg (table 19). The precision of ionic Hg was checked by 

running aqueous mercury standards through the procedure. The relative 

standard deviation ranged from 40 percent at the threshold value to 4 

percent at the midrange value (table 20), showing acceptable precision 

for the quantities of ionic Hg measured. Total mercury includes both 

elemental and ionic mercury and replicate analyses of total mercury 

content of waters, soils, and plants also yielded acceptable data (table 

21).

To check the accuracy of our procedure, various kinds of samples were 

analyzed by our method as well as by the following methods: (1) Water 

samples were analyzed by the silver screen method (Hinkle and Learned, 

1969); (2) sediment samples were run by direct heating in an induction 

furnace and measurement of evolved Hg vapor .in a mercury detector 

(Vaughn and McCarthy, 1964); and (3) plant samples were run by the 

complete acid digestion dithizone colorimetric method (Ward and 

McHugh, 1964). The mercury content of samples analyzed by our procedure compares favorably with that of samples analyzed by other procedures (table 21).

Our procedure has proved useful for the determination of mercury 

in a variety of samples, such as water and aqueous solutions of plants,

TABLE 19. Precision of elemental mercury measurement

Mercury recovered (ng)

added

(ng)

6.5

13

26

65

determinations

5

11

9

3

High

7

14

29

66

Low

6.5

11

23

62

Mean and

standard deviation

6.7±0.3

13.8±1.4

26±2

64 ±2

Relative

standard

deviation

(percent)

4.5

10.1

7.7

3.1

TABLE 20. Precision of ionic mercury measurement

Mercury recovered (ng)

He + !

added.

(ng)

5

10

20

50

100

200

400

Number of

determinations

7

5

5

5

9

5

6

Low

2

8

19

45

84

180

360

High

9

12

24

54

115

200

420

Mean and

standard deviation

5±2

10±2

20±2

50±4

lOOtlO

195±8

400±19

Relative

standard

deviation

(percent)

40

20

10

8

10

4

4.8
TABLE 21. Results of mercury analyses, in part

[Leaders (...) indicate sample not analyzed; Nd, not 

Induct

Cold vapor method (this paper) d

Sample 

No.

Sample type Number 

of 

determinationsLow High

Mean and

standard Numbe

deviations determ

nation

Total mercury found

FRS-6. ... ......

MRS-4. ....... .

PBD-3 .........

FPP-5.. ........

NA-5

NA-2 ...........

NA-3 ...........

H-17. ........ ...

H-2. .............

H-5. ...... .......

71-2207........

H-ll. ........ ...

71-2207 ........

H-ll............

71-2207........

H-ll............

Stream sediment,

Stream sediment..............

...do ..................................

Mud pot in Yellowstone

Plant................................

...do.......... ........................

...do..................................

Water, high H2S content. 

Water, high NH3 content 

Water, high H2S content.

...do....... ........... ................

Water ...............................

...do... ...... .........................

...do..................................

2

2 

3

3 

2 

7 

2 

6 

3 

6 

4 

4

2 

3

2 

3

300 

640 

950

48,000 

400 

1,000 

2,900 

0.05 

.2 

.8 

.3 

.7

0.5 

.2

Nd 

0.3

500 

1,025 

1,450

50,000 

470 

2,500 

3,100 

0.16 

.4 

1.7 

.6 

1.2

0.6

.4

0.06 

.6

400±100 3 

830±140 6 

1,210±205 6 

49,000± 1,000 6 

435±35 ... 

1,700±720 ... 

3,000± 1,000 ... 

0.1±0.04 

0.3±0.08 

1.2±0.3 

0.5±0.1 

1.0±0.2

Ionic mercury

0.55±0.05 

0.3±0.1

Elemental mercury

<0.05 

0.5±0.2

"Vaughn and McCarthy (1964). 'Ward and McHugh (1964). 'Hinkle and Learned (1969).
MERCURY IN NATURAL WATERS, PLANTS, AND SOILS 63

soils, and sediments. Measurement of mercury is quick, and the data 

obtained by our method are comparable to data obtained by older 

methods.

REFERENCES CITED

Chau, Yiu-Kee, and Saitoh, H., 1970, Determination of submicrogram quantities of

mercury in lake waters: Environmental Sci. and Technology, v. 4, no. 10, p. 839-841. 

Dickson, F. W., and Tunnell, G., 1968, Mercury and antimony deposits associated with

active hot springs in the western United States, in Ore deposits of the United States, 

1933-1967, v. II: Am. Inst. Mining, Metall., and Petroleum Engineers (Graton-Sales

Volume), p. 1673-1701. 

Fishman, M. J., 1970, Determination of mercury in water: Anal. Chemistry, v. 42, no.

12, p. 1462-1463.

Hatch, W. R., and Ott, W. L., 1968, Determination of submicrogram quantities of mercury by atomic absorption spectrophotometry: Anal. Chemistry, v. 40, no. 14, p.

2085-2087. 

Hawkes, H. E., and Webb, J. S., 1962, Geochemistry in mineral exploration: New York,

Harper and Row, 415 p. 

Hinkle, M. E., and Learned, R. E., 1969, Determination of mercury in natural waters

by collection on silver screens, in Geological Survey research 1969: U.S. Geol. Survey

Prof. Paper 650-D, p. D251-D254 [1970]. 

Latimer, W. M., 1952, The oxidation states of the elements and their potentials in

aqueous solutions [2d ed.]: New York, Prentice-Hall, 392 p. 

McCarthy, J. H., Jr., Gott, G. B., and Vaughn, W. W., 1969, Distribution and abundance

of mercury and other trace elements in several base- and precious-metal mining

districts, in Exploration for mineral resources Idea Conf., 4th Ann. 1968: New

Mexico Bur. Mines and Mineral Resources Circ. 101, p. 99-108.

Omang, S. H., 1971, Determination of mercury in natural waters and effluents by flameless atomic absorption spectrophotometry: Anal. Chim. Acta, v. 53, no. 2, p. 415-420. 

Pyrih, R. Z., and Bisque, R. E., 1969, Determination of trace mercury in soil and rock

media: Econ. Geology, v. 64, no. 7, p. 825-828. 

U.S. Geological Survey, 1970, Mercury in the environment: U.S. Geol. Survey Prof.

Paper 713, 67 p. 

Vaughn, W. W., and McCarthy, J. H., Jr., 1964, An instrumental technique for the

determination of submicrogram concentrations of mercury in soils, rocks, and gas,

in Geological Survey research 1964: U.S. Geol. Survey Prof. Paper 501-D, p.

D 123-D 127 [1965]. 

Ward, F. N., and Bailey, E. H., 1960, Camp and sample-site determination of traces

of mercury in soils and rocks: Am. Inst. Mining, Metall., and Petroleum Engineers

Trans., v. 217, p. 343-350. 

Ward, F. N., and McHugh, J. B., 1964, Determination of mercury in vegetation with

dithizone A single extraction procedure, in Geological Survey research 1964: U.S.

Geol. Survey Prof. Paper 501-D, p. D128-D130 [1965]. 

Weast, R. C., ed., 1968, Handbook of chemistry and physics, 49th ed.: Cleveland, Ohio,

Chemical Rubber Co. 2,092 p. 

White, D. E., 1967, Mercury and base-metal deposits with associated thermal and mineral

waters, in Geochemistry of hydrothermal ore deposits: New York, Holt, Rinehart

and Winston, p. 575-631.
V C
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

DETERMINATION OF MOLYBDENUM IN

NATURAL WATERS AND BRINES AFTER

SEPARATION USING A CHELATING RESIN

By H. M. NAKAGAWA and F. N. WARD

CONTENTS

Page

Abstract........................................................................................................................... 65

Introduction...............................;.................................................................................... 66

Reagents and apparatus................................................................................................... 67

Procedure........................................................................................................................ 68

Experimental work and results ....................:................................................................. 69

References cited............................................................................................................... 74

ILLUSTRATION

Page

FIGURE 2. Graph showing recovery of molybdenum after different initial pH

adjustments with hydrochloric and nitric acid...................................... 70

TABLES

Page

TABLE 22. Molybdenum recovery at different flow rates of feed................................. 72

23. Molybdenum recovery from different column lengths.............................. 72

24. Molybdenum retention on column ............................................................ 73

25. Repeatability of molybdenum determinations........................................... 73

26. Determination of molybdenum in water and water residues.................... 74

ABSTRACT

The use of the molybdenum content in streams, other natural waters, and brines 

as a guide to mineralization requires a rapid field method for determining 1 microgram 

per litre or less of molybdenum in the water. After a preliminary collection on the

65
66 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

chelating resin A-l, and subsequent elution with dilute alkali, the molybdenum is measured by a field-applicable colorimetric procedure based on the formation of the thiocyanate complex of molybdenum(V). Experiments show that a resin column 5 centimetres 

long and 1.2 centimetres in diameter is adequate for collection of as much as 300 micrograms of molybdenum from a sample adjusted to a pH of about 1 and that the flow 

rate through the column should not be greater than 20 millilitres per minute. The 

alkaline elution removes the collected metal and regenerates the resin simultaneously. 

The method provides for the direct determination of molybdenum in the field and 

eliminates the need for transporting bulky water samples to the laboratory.

INTRODUCTION

Inasmuch as molybdenum in the form of molybdenite is a minor constituent of many base-metal deposits, one promising method of geochemical prospecting for such deposits is based on the detection of traces 

of molybdenum in the dispersion products of a base-metal deposit. 

Under mild alkaline conditions molybdenum is relatively mobile, and 

small amounts ultimately reach the natural-water systems. The measurement of the small amounts of molybdenum in natural waters and 

brines thus provides a means of searching out molybdenum-rich ore 

bodies, and such measurements require a rapid and moderately accurate 

method for determining small amounts of the element.

The kind of analytical method needed has a direct relation to the concentration of molybdenum in natural waters, and the range in concentration can only be tentatively established because of the scarcity of published data. In a study limited to surface waters of Colorado, Voegeli 

and King (1969) detected molybdenum in 89 percent of samples collected. 

The amount detected ranged from 1 to 3,800 y. g/1. On the basis of these 

data and other meager data available, the lower limit of a really useful 

method should be near 1 IJL g/1. The achievement of such a limit demands 

a sensitive method, and the ultimate field application limits the type 

of method.

Molybdenum reacts with a number of reagents to form colored complexes which are necessary for the simple colorimetric determinations 

widely used in the field (Hawkes, 1957). The reaction with thiocyanate 

is extremely sensitive, and the amber-colored molybdenum thiocyanate 

is readily extracted into an immiscible solvent, such as isopropyl ether. 

In the laboratory, the absorbance of as little as 5X10~9 g of molybdenum 

extracted into isopropyl ether as the thiocyanate complex is readily measured. Thus, with a favorable partition coefficient, 5 ml of a water or 

brine containing 1 /^ g/1 of the molybdenum is enough for easy measurement in the laboratory. Using visual comparisons of absorption, we 

established that 20 times more of the sample is required for positive measurement in the field. For example, a volume of at least 100 ml is needed 

for the field measurement of molybdenum, but only 5 ml is needed for 

the laboratory measurement. Solvent extraction of a large volume with
MOLYBDENUM IN NATURAL WATERS AND BRINES 67

only 1 ml of solvent is impracticable; hence, some other means of concentration of the molybdenum is needed. Moreover, solvent extraction 

of natural waters is often undesirable because of the color imparted to 

the solvent by small amounts of dissolved organic matter and certain 

inorganic salts.

Ion-exchange resins offer a better means of concentrating and separating the molybdenum than does the direct application of solvent extraction. Such resins (Morachevskii and Gordeeva, 1957; Canney and 

Hawkins, 1960) have been used to collect traces of metals from natural 

waters, but these resins often failed with brines and other waters containing large amounts of dissolved solids. In our method, the chelating 

resin we used (A-l, also known as Chelex 100) 1 differs primarily from 

more familiar resins, such as Dowex 1 or Dowex 50, by the kinds of 

active groups attached to the styrene divinyl-benzene copolymer matrix. 

Chelating resin A-l contains active iminodiacetate groups, and the arrangement of these groups results in a resin showing a marked affinity 

for heavier elements, such as copper, lead, or zinc. The resin is particularly suited to removing traces of heavy metals from sea water (Riley 

and Taylor, 1968) and from solutions of concentrated salts (Forberg and 

Lundgren, 1960), as well as from industrial waste (Biechler, 1965). These 

applications suggest the use of the chelating resin A-l for collecting 

small amounts of molybdenum from natural waters in which the salt 

content may range from that of fresh waters from a mountain spring 

to that of saline waters from a desert environment.

REAGENTS AND APPARATUS

Chelating resin A-l, 50-100 mesh. Available as Chelex 100 from BioRad Laboratories, Richmond, Calif.

Ferrous ammonium sulfate, reagent grade, 1 percent. Dissolve 1 g of 

salt in 100 ml of 0.1 M sulfuric acid.

Hydrochloric acid, reagent grade, concentrated.

Isopropyl ether, practical grade, is suitable if it is free of peroxides. 

To test for peroxides, shake 5 ml of the ether with 5 ml of an acidified 

aqueous solution of potassium iodide. If the aqueous solution shows 

more than a faint tinge of the yellow color, which is due to the free 

iodine produced by the reaction of peroxides with iodide, the ether 

contains appreciable quantities of peroxides and is unsatisfactory for 

use. Immediately before use of isopropyl ether, shake 25 ml ether with 

2 ml each of potassium thiocyanate and stannous chloride reagents.

Phenolphthalein indicator. Dissolve 0.05 g reagent in 50 ml alcohol 

and add 50 ml water.

Potassium thiocyanate, 10 percent aqueous. Dissolve 10 g salt in 100

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological Survey.
68 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

ml water. Prepare fresh daily.

Sodium hydroxide, 3 M. Dissolve 120 g reagent in 1 litre water.

Standard molybdenum solution, 100 /jg/ml. Dissolve 0.075 g pure 

molybdic anhydride in dilute sodium hydroxide, dilute with water 

to about 50 ml, add hydrochloric acid until solution is just acid, and 

make up to 500 ml with water. Less concentrated standard solutions 

are prepared as needed by a tenfold dilution of the concentrated standard solution with water.

Stannous chloride solution, 10 percent. Dissolve 10 g dihydrate in 17.2 

ml concentrated hydrochloric acid and dilute to 100 ml with water.

Water, rendered metal free by passing it through any of several commercially available resins.

Columns, glass. Approximate dimensions, length 30 cm; inside diameter, 

1.3 cm. The columns drain through a Teflon valve attached by an 

aluminum coupling. Several columns available commercially from 

SGA Scientific, Inc., are satisfactory.

Glass wool.

pH meter or other device to measure pH; pH papers are unsatisfactory.

Separatory funnels, 125 ml, Squibb type.

Borosilicate glass evaporating dish.

Hotplate.

PROCEDURE

Pretreatment of resin and separation of molybdenum: May be done in 

the laboratory or in the field at temporary quarters, such as trailer 

camps.

Pretreatment of resin: Prepare a slurry by soaking the chelating resin 

A-l overnight in about VA times its volume of 3 M sodium hydroxide. 

Pack glass wool into the bottom of the resin column until the thickness is about 2 cm, then pour a sufficient amount of the resin slurry 

into the column to produce a resin column 5 cm in length. Wash 

resin with 125 ml water, and drain all but about 2 ml of the wash 

water.

Separation of molybdenum: Adjust pH of the sample solution 

usually 100 ml to 1.1 ±0.5 with dilute hydrochloric acid, and percolate 

sample solution downward through the resin column at a flow rate 

not exceeding 20 ml per minute. Wash resin with 25 ml water, and 

discard wash water.

Elute the resin column with 10-20 ml of 3 M sodium hydroxide and 

wash resin with water 10-20 ml until the column length is 5 cm. 

Neutralize the combined eluant and wash water the volume should 

not exceed 40 ml with concentrated hydrochloric acid, using a drop 

of phenolphthalein as an indicator, and, finally, make the volume 

up to 50 ml with water.
MOLYBDENUM IN NATURAL WATERS AND BRINES 69

Estimation of molybdenum: Transfer the 50-ml sample solution to a 

125-ml separatory funnel and add the following reagents, shaking 

after each addition:

1 ml ferrous ammonium sulfate solution, 

5.5 ml concentrated hydrochloric acid, 

3 ml potassium thiocyanate solution, and 

3 ml stannous chloride solution. 

In laboratory:

Cool mixture to about 25°C, add 4 ml isopropyl ether, and shake 

funnel for 30 seconds. Allow phases to separate, then drain aqueous 

phase into a 150-ml beaker. Transfer ether to 5-ml volumetric flask 

and return the contents of the beaker to the funnel. Add 2.5 ml isopropyl ether and repeat the extraction procedure. Allow phases to 

separate; drain and discard aqueous phase and combine ether extracts. 

Make up to mark on volumetric flask with fresh ether.

Mix contents of volumetric flask and determine molybdenum content by measurement of absorbance at 470 nm and subsequent reference to a previously established standard curve. 

In field:

Cool mixture to about 25°C, add 1 ml isopropyl ether and shake 

funnel for 1 minute. When phases separate, hold funnel vertically and 

view the shallow layer of the organic solvent at right angles to the 

funnel. Compare color of sample solution with that of standard solutions, prepared as follows:

To a series of 125 ml separatory funnels each containing 50 ml water 

. add individually 0.1, 0.2, 0.5, 1.0 and 10.0 jug molybdenum from 

standard molybdenum solution. To each funnel add ferrous ammonium sulfate solution, hydrochloric acid, thiocyanate, and stannous 

chloride as directed under estimation. Cool mixtures in each funnel 

to about 25°C, add 1 ml isopropyl ether to each, and shake each funnel 

for 1 minute.

EXPERIMENTAL WORK AND RESULTS

Collection of molybdenum on resin: In order to maintain uniform conditions, the pH of each water sample is adjusted to 1.1 ±0.5 and the 

resin column is washed with a definite volume of water before percolation of the sample through the resin. Either nitric acid or hydrochloric 

acid can be used to adjust the pH of each water sample, but with the 

nitric acid the optimum pH range for best recovery of molybdenum 

is small, as measured by our procedure. A difference of as little as 

0.1 of a pH unit often resulted in losses of as much as 2 /jg of the 

added molybdenum. In contrast to the pH adjustment of the sample 

with nitric acid, the hydrochloric acid adjustment is less critical. 

Aliquots of water sample containing 10 /jg molybdenum were ad-
70 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

justed to different pH values, and the aliquots were passed down 

through a resin column. Satisfactory recoveries were obtained with 

the initial pH values, ranging from 0.7 to 1.5, as shown in figure 2. 

The optimum initial pH range is wide; hence, the close adjustment 

of the pH is less critical when hydrochloric acid is used in place of 

nitric acid.

12.0

1.5 

INITIAL pH

FIGURE 2. Recovery of molybdenum after different initial pH adjustments with hydrochloric acid (circle) and nitric acid (dot).

Elution of molybdenum: Nitric acid and hydrochloric acid are both 

useful for eluting the collected molybdenum, but each has certain disadvantages. When nitric acid is used, excellent recoveries are made 

from new batches of resin, but with successive use and regeneration 

of a given batch of resin, the recoveries of added molybdenum vary 

erratically. Nitric acid appears to have a degenerative effect on the 

resin.
MOLYBDENUM IN NATURAL WATERS AND BRINES 71

Hydrochloric acid is satisfactory except for the fact that molybdenum is subsequently determined by its reaction with thiocyanate, 

and this reaction proceeds maximally in a narrow concentration range 

of hydrochloric acid. Start, preferably, with a neutral solution and 

add the requisite amount of acid to produce the optimum acid concentration. The acid concentration of the eluent could conceivably 

differ from time to time, and such differences would require a careful 

adjustment for maximum reaction between the molybdenum and the 

thiocyanate.

Because the resin is easily converted to the sodium form upon contact with dilute sodium hydroxide, we assumed that the sodium ion 

would displace any molybdenum or other metal ions on the resin. 

Indeed, this reaction does proceed rapidly and has the further advantage of eluting the collected molybdenum and regenerating the 

resin in the same step. The alkaline effluent poses no problem in the 

subsequent analysis.

Elution time: The alkaline elution causes the resin to swell, which, in 

turn, affects the flow rate of the elutriant. The swelling occurs rapidly 

enough to be the chief factor in controlling the flow of the effluent, 

and other factors, such as hydrostatic head, are insignificant. In fact, 

the flow increases slightly with swelling.

Interferences: Of the elements which Sandell (1959, p. 646) listed as 

possible interferences in the thiocyanate method chromium, cobalt, 

copper, uranium, vanadium, niobium, rhenium, and tungsten only 

the last three could cause appreciable interference in our procedure. 

The tungsten interference can be eliminated by the addition of sodium 

citrate or tartrate. The rhenium interference can be removed by a 

modification introduced by Hoffman and Lundell (1939) in which 

molybdenum and rhenium are separated by a preferential reduction 

of the molybdenum with mercury and extraction of the mblybdenum(V) thiocyanate into an immiscible solvent. Rhenium is not 

reduced and, consequently, does not form an extractable species. 

Mercurous nitrate has also been proposed for the preferential molybdenum reduction (Tarayan and Musheggan, 1958).

Niobium may possibly interfere in our method. The recovery of 

5 Mg of added molybdenum in the presence of 25, 50, or 100 ;ug of 

niobium is approximately 80 percent. However, the occurrence of 

niobium in natural water is so infrequent that this element can 

probably be ignored.

Flow rate of sample solution: In order to measure the optimum flow 

rate for complete recovery of the molybdenum in a water sample, 

10 /ng of the element was added to each of several 100-ml portions 

of water; then the portions were passed through a 6-in. column at 

different rates. Data in table 22 show the molybdenum recovery at
72 « TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

different flow rates of feed. Complete recoveries are observed at flow 

rates of 20 ml per minute or less; thus, 1 litre of sample passes 

through the resin column in less than 1 hour.

Volume of sample: If one stipulates 0.1 jug as the minimum amount 

of molybdenum definitely perceptible under the conditions of our procedure, theoretically, he could analyze waters containing 1 ppb of 

molybdenum with only a 100-ml sample. He could analyze waters 

containing 0.1 ppb of molybdenum using a 1-litre sample. In a field 

trial of the procedure, we analyzed natural waters containing as 

little as 0.2 ppb.

Length of resin bed: Sample solutions containing 10 /jg of molybdenum 

were adjusted to a pH of 1.1 with hydrochloric acid, and each solution 

was passed through resin columns of different length, as shown in 

table 23. A 2.5-cm column shows poor recovery and a 5-cm column 

shows satisfactory recovery; columns of greater length are unnecessary 

except when testing samples containing relatively large amounts of 

molybdenum. Such amounts can be determined directly without 

separation on a resin.

TABLE 22. Molybdenum recovery at different flow rates of feed

Sample 

No.

1..... ......

2...........

3...........

4...........

Molybdenum 

added

(us)

........... 10

........... 10

........... 10

........... 10

Flow rate of feed 

(ml/min)

33

25

20

10

Molybdenum 

recovered 

(MS)

8.7

9 5

10

10

TABLE 23. Molybdenum recovery from different column lengths

Sample 

No.

1 ...........

o

2

4

5.

Molybdenum 

added

(US)

............ 10

............ 10

............ 10

............ 10

............ 10

Length 

of column

(cm)

2 5

5

7 5

10

15

Molybdenum 

recovered 

(US)

8.8

9.8

10

10

10

Preliminary experiments show that a 15-cm column collects as much 

as 10,000 /jg of molybdenum from a water sample before permitting 

a breakthrough for example, failure to react with the metal. At the 

10- /jg level, a 5-cm column is adequate, and it is of interest to determine 

the maximum amount of molybdenum that may be collected on that 

column. Such data are shown in table 24, and one may readily conclude
MOLYBDENUM IN NATURAL WATERS AND BRINES 73

that in the range from 10 to 1,000 jug of added molybdenum, about 

90 percent is collected on the 5-cm column. Long before the quantity 

of molybdenum in a 50- to 100-ml water sample exceeds 1,000 jug, the 

determination can be made directly (L. C. Huff, oral commun., 1966). 

To date, no brines have contained more than 1,000 /jg of molybdenum 

in a 100-ml sample. Thus, the 5-crn column is entirely satisfactory in 

practice.

TABLE 24. Molybdenum retention on column

Sample 

No.

1.........

2.........

3.........

4.........

5.........

Molybdenum 

added

(MB)

.......... 10

.......... 20

50

.......... 100

.......... 200

Molybdenum 

recovered 

from column, 

(Mg)

10

19

52

100

194

Sample 

No.

6.... ......

7. .........

8.. ........

9..........

 Molybdenum 

added

(MS)

........ 300

........ 400

........ 500

........ 1,000

Molybdenum 

recovered 

from column,

(MB)

310

370

450

870

Precision and accuracy: In order to measure the overall precision of 

the separation and estimation, standard solutions one containing 

2 /ug, another 5 /ag, .and another 10 jug of molybdenum were prepared and passed through the column. The data for several repeat 

determinations are given in table 25.

TABLE 25. Repeatability of molybdenum determinations

Molybdenum found (M g)

Sample Molybdenum

No. added

(MB)

l. .........:. 2

2............ 5

3............ 10

determinations

5

9

38

Low

1.6

4.3

8.8

High

2.

5.3

10.7

Mean and

confidence

limits at

95-percent level

1.8*0.2

4.9*0.3

10.0*0.2

Relative

standard

' deviation

(percent)

11.1

8.2

5.0

The overall accuracy of the method was tested in the following 

manner: Appropriate volumes of different samples of natural water 

were evaporated to dryness in a borosilicate glass evaporating dish 

placed on a hotplate. Spectrophotometric (Ward and others, 1963) and 

spectrographic determinations were made on these residues and compared with Spectrophotometric determinations made after resin collection (table 26). Except for sample SM-1, the data obtained by all 

three methods are comparable, and the agreement is good enough to 

suggest that the Spectrophotometric method after resin collection is
74 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

TABLE 26. Determination of molybdenum in water and water residues

[Leaders (...) indicate data not available]

Molybdenum in material analyzed 

(fjig/\ water)

Sample 

no

Location pH

Residue analyzed

Spectrographic Spectrophotometric

TB-3.. ......

SM-12......

SM-10......

SM-8........

SM-1........

SM-5.........

SM-3.........

3420..........

SM-11.......

TB-1.........

3415..........

... ...do.................

... ...do.................

... ...do.................

... ...do.................

... ...do.................

... ...do.................

.. ...do.................

8.8 

8.6 

7.9 

8.0

8.2 

7.8 

8.0

8.4 

8.3 

7.9 

8.2

5

7 

9

8 

12 

23

27 

39 

40 

73

5 

6 

8 

12

20 

18 

30

44 

55

Water 

(resin collection)

Spectrophotometric

8

6

io

15 

16 

31

32 

46 

60 

85

applicable to waters containing different amounts of total dissolved

solids, varying fourfold or fivefold.

Molybdenum bound in an uncharged organic complex would not be 

collected on the ion-exchange resin; hence, one might expect the quantities collected on the resin. All the data obtained to date are on waters 

from the Western States, and significant differences are not observed, 

possibly because of the small amount of organic matter present in these 

samples.

The method is relatively short and has already proved to be useful 

in defining the distribution of molybdenum in several areas of the Southwestern States.

REFERENCES CITED

Biechler, D. G., 1965, Determination of trace copper, lead, zinc, cadmium, nickel, and

iron in industrial waste waters by atomic absorption spectrometry after ion exchange

concentration on Dowex A-l: Anal. Chemistry, v. 37, no. 8, p. 1054-1055. 

Canney, F. C., and Hawkins, D. B., 1960, Field application of ion-exchange resins in

hydrogeochemical prospecting, in short papers in the geological sciences: U.S. Geol.

Survey Prof. Paper 400-B, p. B89-B90. 

Forberg, Sevald, and Lundgren, Svante, 1960, Activation analysis of trace metals in

sodium triphosphate: Anal. Chemistry, v. 32, no. 9, p. 1202-1203. 

Hawkes, H. E., 1957, Principles of geochemical prospecting: U.S. Geol. Survey Bull.

1000-F, 355 p. 

Hoffman, J. I., and Lundell, G. E. F., 1939, Separation and colorimetric determination

of rhenium and molybdenum: U.S. Natl. Bur. Standards Jour. Research, v. 23, p.

497-508.
MOLYBDENUM IN NATURAL WATERS AND BRINES 75

Morachevskii, Yu. V., and Gordeeva, M. N., 1957, Separation of molybdenum from

iron, aluminum, and calcium by means of ion-exchange anionates: Zavodskaya Lab.,

v. 23, p. 1066-1067. 

Riley, J. P., and Taylor, D., 1968, Chelating resins for the concentration of trace elements

from sea water and their analytical use in conjunction with atomic absorption

spectrophotometry: Anal. Chim. Acta, v. 40, p. 479-485. 

Sandell, E. B., 1959, Colorimetric determination of traces of metals [3d ed.]: New York,

Intersci. Publishers, 1032 p. 

Tarayan, V. M., and Musheggan, L. G., 1958, New method of separation of rhenium

from molybdenum: Akad. Nauk Armyan. SSR Doklady, v. 27, p. 157-160. 

Voegeli, P. T., and King, R. U., 1969, Occurrence and distribution of molybdenum

in the surface water of Colorado: U.S. Geol. Survey Water-Supply Paper 1535-N,

32 p. 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used

in geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull.

1152, 100 p.

NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

DETERMINATION OF FLUORINE

IN SOILS AND ROCKS BY

KNOWN-INCREMENT ADDITION AND

SELECTIVE-ION ELECTRODE DETECTION

By GEORGE L. CRENSHAW and F. N. WARD

CONTENTS

Page

Abstract......................................................;..............................................................-..... 77

Introduction.................................................................................................................... 77

Reagents and apparatus.................................................................................................. 80

Procedure........................................................................................................................ 80

Experiments and results ................................................................................................. 81

References cited............................................................................................................... 83

TABLE

Page

TABLE 27. Fluorine content in U.S. Geological Survey standard rocks, in samples 

reported by other workers, and in rocks and soils analyzed by methods 

in this report........................................................................................... 82

ABSTRACT

In the determination of fluorine in soils and rocks by known-increment addition 

and selective-ion electrode detection, the sample is sintered with a sodium carbonatepotassium nitrate flux, and the sinter is leached with water. The leachate is treated 

with a citric acid-sodium citrate buffer, and fluorine is measured in the buffered solution 

by a selective-ion electrode connected to an expanded scale meter equipped with a knownincrement scale for direct addition of known amounts of fluoride.

INTRODUCTION

In the early 1900's Hillebrand (1919) determined fluorine in silicate 

and carbonate rocks by fusing the powdered sample with alkali carbon77
78 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

ate to render fluorine water-soluble, separating alumina and other interferences by ammonium carbonate precipitation, and measuring the solubilized fluorine either by weighing precipitated calcium fluoride or by 

noting its bleaching effect on a yellow color produced by the action of 

hydrogen peroxide on titanate solution. In the early 1930's Willard and 

Winter (1933) reported a method in which they separated the fluorine 

from the acid digestate of the sample by steam distillation. The distilled 

fluorine was measured by titration with thorium nitrate in the presence 

of alizarin as an indicator. The impact of their work was so great that 

the Willard and Winter procedure became the method of choice, and 

subsequent variations or improvements attest to its caliber. Reynolds 

and Hill (1939) incorporated many of these improvements in a method 

for determining fluorine in natural phosphates and phosphatic fertilizers. Later, Peck and Smith (1964) offered an improved fluorine measurement based on the stable colored complex formed between fluorine 

and a zirconium-SPADNS (sodium salt of 2- (4-sulfophenylazo)-l,8-dihydroxynaphthalene-3,6-disulfonic acid) reagent. Their method retained 

the steam distillation step, especially for silicate rocks in which the 

fluorine contents are in the range of a few hundred parts per million.

Trace amounts of fluorine have also been separated by gaseous diffusion of hydrogen fluoride and collected on an alkali-impregnated 

support (Singer and Armstrong, 1954; Hall, 1963; and Stuart, 1970). Measurements of the separated fluorine involved titration and colorimetry, 

except in the method of Stuart which made use of the selective-ion electrode that had been introduced previously by Frant and Ross (1966) for 

sensing fluoride ion activity. Singer and Armstrong (1968) also utilized 

the capability of the fluoride electrode for measuring the fluorine content of bone.

Measurement of the fluoride activity by the fluoride electrode grew 

in popularity because of the high degree of selectivity and the ease of 

making the measurements by immersing the electrode into an aqueous 

solution and reading the developed potential on a sensitive voltmeter. 

Because of the good selectivity of the electrode, separation of fluorine 

from the bulk of the sample appeared unnecessary, and once again attention focused on fusions to effect sample solution. Typical examples of 

such application to silicate rocks and other geologic materials are those 

described by Ficklin (1970) and Ingram (1970).

Selective-ion electrodes sense activity. Except in dilute solutions, the 

activity and concentration are seldom equivalent. They are related to 

each other in the following way:

A = VCj, 

where

A is the activity of fluoride ion, 

Ct is concentration of free fluoride ion, and 

y is the activity coefficient.
FLUORINE IN SOILS AND ROCKS 79

The activity coefficient varies with total ionic strength, approaching 

unity in dilute solutions and decreasing with increased ionic strength. 

From an analytical perspective one is interested in concentration, and, 

ideally, measurements with a selective-ion electrode should be made on 

dilute solutions. Measurements on concentrated solutions require some 

means of estimating the ionic strength so as to relate activity and concentration. Again, ideally, selective-ion electrode measurements on 

dilute solutions are not always feasible because of the innate sensitivity 

of the sensing element of the electrode, and measurement on concentrated solutions is hindered by the difficulty of estimating ionic strength 

because of the possible assortment of ions present in solutions of geologic materials. At low pH values, 3-4, fluorine appears to form a 

complex species not sensed by the electrode (Srinivasan and Rechnitz, 

1968), and at high pH values hydroxyl ion interferes, especially with 

fluoride levels less than about Ixl0~5 M. Thus, a practicable method 

requires pH control and knowledge of, or ability to repeat, ionic strength 

levels. Achieving the latter by dilution is not so practical as the addition 

of a high ionic strength solution, called swamping, so as to nullify relatively small variations due to different kinds of geologic materials.

The pH requirement and the need to achieve similar ionic strength 

levels suggest the use of appropriate swamping agents and buffers, such 

as a mixture of acetic acid and its salts, which provide a great buffer 

capacity. In order to meet these conditions, Frant and Ross (1968) devised 

an acetate buffer, a mixture of acetic acid, sodium chloride, and sodium 

citrate, which when properly balanced for acidity and mixed with the 

sample solution in equal volumes not only automatically adjusted the 

pH to the optimum value but also swamped the ionic potential so that 

variations from sample to sample exerted no appreciable effect on the 

response of the electrode. The buffer is known by the acronym TISAB, 

which stands for total ionic strength adjustment buffer. This buffer not 

only maintained an optimum pH but also contained a small amount 

of sodium citrate or disodium salt of cyclohexylene dinitrilotetra-acetic 

acid to inhibit the formation of aluminum fluoride not sensed by the 

electrode. The buffer also contained a relatively large amount of sodium 

chloride to maintain a large ionic potential so that variations in the 

ionic potential of the different sample solutions have no significant 

effect on the total ionic potential.

The composition of the TISAB used in our procedure is listed under 

"Reagents." We modified this by increasing the citrate content, but it 

did not prove to be advantageous.

The sample is sintered with an alkali carbonate-nitrate flux, and the 

water-soluble fluorine is mixed with the acetate buffer to accomplish 

the pH adjustment and to provide a nearly constant ionic strength in 

the analyte solution. The potential due to fluoride-ion activity is measured by adding a known amount of fluoride standard solution and 

observing the change caused by the addition.
80 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

REAGENTS AND APPARATUS

Citric acid-sodium citrate reagent. Dissolve 193 g of citric acid in 1 litre 

of solution. Mix with an equal volume of sodium citrate solution, 

which is prepared by dissolving 294 g of the salt in 1 litre of solution. Adjust the pH of the mixture to 3 with 6 M hydrochloric acid.

Flux. Mix two parts by weight of sodium carbonate with one part of 

potassium nitrate.

Hydrochloric acid, 6 M. Mix equal volumes of concentrated acid and 

fluorine-free water.

Sodium fluoride standard solution. 1,000 /jg/ml. Dissolve 2.21 g of 

reagent grade salt in 1 litre of fluorine-free water. Prepare appropriate 

dilute standard solutions by dilution of stock solution.

Sodium hydroxide, 5 M. Dissolve 200 g of sodium hydroxide in 1 litre 

of water.

TISAB (Frant and Ross, 1968). Prepare by dissolving 171 ml glacial 

acetic acid in 1,500 ml water and adding 174 g sodium chloride. Add 

0.9 g sodium citrate and adjust the pH to 5.5 with 5 M sodium hydroxide solution. Dilute to 3 1 with water.

Expanded-scale pH meter with known-increment scale. Orion model 

407 was used. 1

Selective-ion electrode for fluoride. Orion model 94-09 was used with 

Orion model 90-01 single-junction reference electrode.

Beakers, polypropylene, 100 ml.

Burner.

Crucibles, nickel.

PROCEDURE

1. Weigh 0.25 g finely ground sample into nickel crucible and mix 

with 1.25 g flux.

2. Sinter mixture over burner until bubbling subsides. Cool.,

3. Add 10 ml water to the crucible and let stand overnight.

4. Stir contents of crucible with small stirring rod and transfer to a 

100-ml volumetric flask.

5. Rinse crucible twice with 10-ml portions of water and add to flask. 

Rinse once with 10 ml citric acid-sodium citrate reagent and add 

to the flask.

6. Swirl flask to liberate the carbon dioxide formed by reaction of acid 

with carbonate and dilute to volume with water.

7. Transfer contents of flask to a polypropylene beaker immediately 

and let stand at least one-half hour, or until carbon dioxide ceases 

evolving.

8. Transfer a 50-ml aliquot to a clean polypropylene beaker and mix 

with 50 ml TISAB.

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological Survey.
FLUORINE IN SOILS AND ROCKS 81

9. Immerse the electrodes into the solution and stir slowly for 1 minute. 

Adjust meter to midscale.

10. Add 1 ml 100- /ag/ml fluorine standard solution and continue 

stirring for 3 minutes.

11. Read parts per million fluorine from known-increment scale and 

convert to parts per million in sample as follows:

_, . , scale readingx 104 F, ppm in sample = ° 

sample weight x aliquot

EXPERIMENTS AND RESULTS

Although this procedure is usable with any expanded-scale ion meter, 

it is designed around the Orion meter model 407, which has a special 

scale to facilitate known-increment addition and subtraction techniques. 

The useful range of this scale is from about 0.2 to 2 ppm fluoride in 

the analyte solution and, in the 100-ml volume suggested, the molarity 

of the fluoride ion is, respectively, 10~5 to 10~ 4 . At such molarities, the 

response of the fluoride electrode is essentially constant over a pH range 

of 5 to 8 (Frant and Ross, 1966). Ficklin (1970) recommended a pH range 

of 5.5 to 6.5 and, fortuitously with the materials he tested, a 0.5-M citric 

acid solution was effective in maintaining the suggested pH range. Geologic materials vary widely in composition, and, conceivable, the citric 

acid may fail to adjust the resulting pH to the optimum range primarily 

because of the relatively low buffer capacity of citric acid and its salts.

Citrate is, however, advantageous for complexing aluminum and preventing complex formation between aluminum and fluoride. The 

fluorine in such a complex is not sensed by the electrode; hence, the 

formation of that complex would lead to low results for fluorine. 

Edmond (1969) and Ingram (1970) have been notably successful in 

complexing aluminum with citrate so that the fluoride activity can be 

measured without a lengthy distillation to separate fluorine from the 

bulk of the sample. Increasing the amount of citrate to complex even 

larger amounts of aluminum than had been complexed in the past 

seemed to provide a logical way to promote repeatability, as well as to 

provide greater flexibility.

In one of the experiments, we increased the citrate by substituting 

the citric acid-sodium citrate reagent for TISAB and adding it to the 

sample solution in equal proportions. The results are given in table 

27.

With some of the samples we used no buffer at all, and with other 

samples we used TISAB and the citric acid-sodium citrate reagent to 

adjust the pH. The results using the excessive amount of citrate seemed 

to be greater, and we modified the TISAB by increasing the citrate about 

tenfold. Illustrative results on U.S. Geological Survey standard rocks 

by TISAB alone, citric acid-sodium citrate alone, and modified TISAB
TABLE 27. Fluorine content in U.S. Geological Survey standard rocks, in samples repo

by the methods described in present re

[Leaders (...) indicate data not available; numbers have been rounded to two significant figures. Each numb

Fluorine content (ppm)

Sample 

No.

AGV-1.......

BCR-1 .......

G-2............

GSP-1. . ......

pp_979

HC-2-67....

BCT-128....

BCT-146....

Material

Granodiorite..

. Soil... ..............

. ...do. ...............

. ...do................

U.S. Geological 

Survey rocks, 

recommended 

values'

435 

470 

1,290 

3,200

Ficklin 

(1970)2

360 (5) 

510 (5) 

1,120 (5) 

3,500 (5) 

1,573 (5) 

742 (5)

Peck and 

Smith

(1964)'

400 

500 

1,300-1,400 

3,800-3,900

W. D. Goss

mun., 1969) TISA

500 

550 

1,600 

3,800(

2,400 2,300 

1,000 1,400 

1,400 

900 

'1972 values for international geochemical reference samples, Flanagan (1973). 

Selective ion electrode. 

'Distillation and titration.
FLUORINE IN SOILS AND ROCKS 83

(increased citrate content) are compared in table 27 with those obtained 

by Ficklin's procedure. The modified TISAB proved to be detrimental 

and was subsequently abandoned. The substitution of citric acid-sodium 

citrate reagent did not provide any remarkable advantage and was likewise abandoned in favor of the original formulation of TISAB. The final 

procedure includes the addition of enough citric acid-sodium citrate 

reagent to provide a citrate concentration in the final volume of about 

10,000 jug/ml and sufficient TISAB to adjust the pH of the sample 

solution to the optimum range and to provide a high ionic strength 

medium.

A comparison of the fluorine contents of several U.S. Geological 

Survey standard rocks and of other rocks and soils, as determined by 

the known-increment addition method with the amounts as determined 

by several other methods, is also given in the table. Fluorine was seprated by distillation prior to measurement on all of the values reported 

by Peck and Smith (1964) and W. D. Goss (oral commun., 1969).

The favorable comparison is adequate to recommend the knownincrement addition method, and the ease of making the addition along 

with the few operations required combine to validate the recommendation.

REFERENCES CITED

Edmond, C. R., 1969, Direct determination of fluoride in phosphate rock samples, using

the specific ion electrode: Anal. Chemistry, v. 41, no. 10, p. 1327-1328. 

Ficklin, W. H., 1970, A rapid method for the determination of fluoride in rocks and

soils, using an ion-selective electrode, in Geological Survey research 1970: U.S. Geol.

Survey Prof. Paper 700-C, p. C186-C188. 

Flanagan, F. J., 1973, 1972 values for international geochemical reference samples: Geochim. et Cosmochim. Acta, v. 37, p. 1189-1200. 

Frant, M. S., and Ross, J. W., Jr., 1966, Electrode for sensing fluoride ion activity in

solution: Science, v. 154, p. 1553-1554.

 1968, Use of a total ionic strength adjustment buffer for electrode determination

of fluoride in water supplies: Anal. Chemistry, v. 40, no. 7, p. 1169-1171. 

Hall, R. J., 1963, The spectrophotometric determination of sub-microgram amounts

of fluorine in biological specimens: Analyst, v. 88, no. 1043, p. 76-83. 

Hillebrand, W. F., 1919, The analysis of silicate and carbonate rocks [a revised and

enlarged edition of Bulletin 422]: U.S. Geol. Survey Bull. 700, 285 p. 

Ingram, B. L., 1970, Determination of fluoride in silicate rocks without separation of

aluminum using a specific ion electrode: Anal. Chemistry, v. 42, no. 14, p.

1825-1827. 

Peck, L. C., and Smith, V. C., 1964, Spectrophotometric determination of fluorine in

silicate rocks: Talanta, v. 11, p. 1343-1347. 

Reynolds, D. S., and Hill, W. L., 1939, Determination of fluorine, with special reference

to analysis of natural phosphates and phosphatic fertilizers: Indus, and Eng.

Chemistry, Anal. Ed., v. 11, no. 1, p. 21-27. 

Singer, Leon, and Armstrong, W. D., 1954, Determination of fluoride: Anal. Chemistry,

v. 26, no. 5, p. 904-906.

 1968, Determination of fluoride in bone with the fluoride electrode: Anal. 

Chemistry, v. 40, no. 3, p. 613-614.
84 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Srinivasan, K., and Rechnitz, G. A., 1968, Activity measurements with a fluoride-selective 

membrane electrode: Anal. Chemistry, v. 40, no. 3, p. 509-512.

Stuart, J. L., 1970, A simple diffusion method for the determination of fluoride: Analyst, 

v. 95, no. 1137, p. 1032-1038.

Willard, H. H., and Winter, O. B., 1933, Volumetric method for determination of 

fluorine: Indus, and Eng. Chemistry, Anal. Ed., v. 5, no. 1, p. 7-10.
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

ATOMIC ABSORPTION DETERMINATION 

OF SILVER, BISMUTH, CADMIUM, 

COBALT, COPPER, NICKEL, LEAD,

AND ZINC IN CALCIUM- AND 

IRON-RICH GEOLOGIC MATERIALS

By H. M. NAKAGAWA

CONTENTS

Page

Abstract........................................................................................................................... 85

Introduction.................................................................................................................... 86

Reagents.......................................................................................................................... 87

! Apparatus........................................................................................................................ 88

Procedure........................................................................................................................ 88

Experiments.................................................................................................................... 90

Results............................................................................................................................. 93

References cited............................................................................................................... 95

TABLES

Page

TABLE 28. Amounts of iron causing interference in the absence or.presence of

analyte elements...................................................................................... 91

29. Amounts of calcium causing interference in the absence or presence

of analyte elements.................................................................................. 91

30. Recovery of 1 ppm of each element in the presence of varying amounts

of iron and calcium................................................................................. 93

31. Comparison of two methods for determination of several elements

in diverse materials ................................................................................. 94

32. Precision data for eight elements in magnetite and limestone................. 94

ABSTRACT

An atomic absorption method is presented for the determination of as little as 1-2 

parts per million silver, bismuth, cadmium, cobalt, copper, nickel, lead, and zinc in

85
86 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

ferruginous and calcareous materials. In other procedures the presence of different 

amounts of iron and calcium causes erroneous results, but in the atomic absorption 

procedure these interfering elements are removed prior to the analysis of the trace elements. Iron is removed by extraction of ferric bromide with methyl isobutyl ketone; 

calcium is removed by extraction of the carbamates of the elements of interest into 

methyl isobutyl ketone. The trace elements are then measured by atomic absorption 

spectrometry.

INTRODUCTION

Speed, brevity, simplicity, and versatility are factors largely responsible for the overwhelming application of atomic absorption methods 

to the analysis of trace elements in geologic materials. Formerly, methods based on molecular absorption (colorimetry) were widely used, but 

such methods required a high degree of skill on the part of the analyst 

as compared with that required by atomic absorption methods. In the 

application of atomic absorption methods to geochemical exploration, 

the trace elements are determined directly by atomizing a dilute acid 

solution of sample of soil, rock, or plant ash into an acetylene-air flame 

of a conventional atomic absorption instrument (Ward and others, 1969).

The present report is concerned with the simultaneous determination 

by atomic absorption of silver, bismuth, cadmium, cobalt, copper, nickel, 

lead, and zinc from a single sample solution. Large-size (1-5 g) samples 

from material, such as gossan, laterite, magnetite, calcareous rocks, or 

plant ash, are needed to furnish threshold amounts of the elements, 

particularly for bismuth and lead which are the least sensitive.

Analytical results from iron- or calcium-rich sample media are subject 

to error (either positive or negative). Several methods were considered, 

either to minimize the error caused by large amounts ( >1 percent) of 

iron and (or) calcium or to remove them from the sample solution. Some 

of these methods included the addition to the sample solution of complexing agents (such as fluoride for iron), reducing agents to reduce 

iron(III) to iron(II), and the use of ion-exchange resins. Other methods 

tried were the precipitation of the iron or calcium from the sample solution, the use of nonabsorbing lines, and the addition of overwhelming 

amounts of iron and calcium to the standard solutions in order to simulate the chemical composition of the unknown material. None of these 

methods were adequate because of the large amounts of extraneous salts 

formed during complexation or reduction and because of the danger 

of loss of analyte by coprecipitation with calcium, iron, or other major 

components of the sample solution.

Obviously some other means of removing the iron and calcium or 

means of minimizing the effect of the iron and calcium had to be devised. 

Solvent extraction is the technique of choice, and the manner of application is given below.

In previous work with gold and tellurium, large amounts of iron had 

to be separated (Thompson and others, 1968; Nakagawa and Thompson,
ELEMENTS IN Ca- AND Fe-RICH GEOLOGIC MATERIALS 87

1968) and this separation was accomplished by extraction. Ferric 

bromide extracts into MIBK, and using this extraction Denaro and 

Occleshaw (1955) successfully separated iron(III) from manganese, 

aluminum, cobalt, and nickel contained in a hydrobromic acid-ammonium bromide solution. In addition, they observed that zinc and copper 

extracted only slightly, and at low concentrations of hydrobromic acid 

the iron(III) bromide extracted more efficiently than the zinc. Also, 

Menis and Rains (1960) observed that even microgram quantities of iron 

could be quantitatively extracted with MIBK. Moreover, Scott, Roberts, 

and Cain (1967) found that other solvents, such as isobutyl acetate, were 

efficient in separating iron(III) from minor constituents of ore samples. 

Isobutyl acetate is considerably more expensive than MIBK, and sheer 

economics precludes its use except in special applications. In the method 

described in this paper, hydrobromic acid is added after sample attack, 

and iron(III) bromide is removed by extraction into MIBK.

Iron is conveniently removed as the bromide by extraction into MIBK 

and the elements of interest are removed from calcium by chelation and 

extraction into MIBK containing 1 percent DADC. The MIBK is atomized into a flame of an atomic absorption instrument.

REAGENTS

Standard silver solution, 1,000 ppm: Dissolve exactly 0.158 g silver

nitrate in 100 ml 1 + 1 HC1. 

Standard bismuth solution, 1,000 ppm: Dissolve 0.1 g bismuth metal

in 50 ml concentrated nitric acid and dilute to 100 ml with demineralized water. 

Standard cadmium solution, 1,000 ppm: Dissolve 0.1142 g cadmium

oxide in 100 ml 1+1 HC1. 

Standard cobalt solution, 1,000 ppm: Dissolve 0.394 g cobalt chloride

(CoCl2 .6H2O) in 100 ml of 1+1 HC1. 

Standard copper solution, 1,000 ppm: Dissolve 0.0393 g of clear uneffloresced crystals of copper sulfate (CuSO4 »5H2O) in 100 ml of 1 + 1

HC1. 

Standard nickel solution, 1,000 ppm: Dissolve 0.496 g nickel nitrate

[Ni (NO3 )2 -6H2O] in 100 ml 1+1 hydrochloric acid. 

Standard lead solution, 1,000 ppm: Dissolve 0.16 g lead nitrate in 100

ml of 1+1 HC1. 

Standard zinc solution, 1,000 ppm: Dissolve 0.1 g zinc metal in 100 ml

of 1+1 HC1. 

Combined dilute standard solution, 10 ppm: Dilute 1 ml of 0.1 percent

standard solution of each element to 100 ml with 1+1 HC1. 

Ammonium hydroxide, concentrated reagent grade. 

Hydrobromic acid, concentrated reagent grade. 

Hydrochloric acid, concentrated reagent grade. 

Hydrofluoric acid, 48 percent, reagent grade.
88 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Nitric acid, concentrated reagent grade.

DADC, 1 percent in MIBK: Dissolve 1 g DADC in 100 ml MIBK.

MIBK, reagent grade.

Cresol red indicator: Dissolve 0.1 g in 26 ml of 0.01 M NaOH and dilute

to 250 ml with demineralized water. 

Tartrate buffer: Dissolve 18.6 g tartaric acid and 23 g sodium tartrate

in 1 litre demineralized water.

APPARATUS

Atomic absorption instrument, Perkin-Elmer model 303 or equivalent. 

Teflon beakers. 

Hotplate. 

pH meter.

PROCEDURE

SAMPLE SOLUTION

Silicate rocks:

Place the weighed sample (up to 1.0 g) in a Teflon beaker having 

a capacity of at least 50 ml. Moisten the sample with 2 ml demineralized water. Add 2 ml of concentrated nitric acid and 25 ml of hydrofluoric acid. Let stand at least 2 hours, or overnight preferably, and 

evaporate the contents over moderate heat to dryness. Repeat the 

addition of nitric and hydrofluoric acids, let stand for 2 hours, and 

again evaporate to dryness. Add 1 ml of concentrated nitric acid, 

15 ml of 6 M hydrochloric acid and warm the beaker and contents 

to dissolve the residue. Generally, this treatment is adequate for 

most siliceous-type materials. Transfer the contents of the beaker 

to a 25-ml volumetric flask; wash the beaker 3 times with 2-ml portions of 6 M hydrochloric acid and add each wash to the volumetric 

flask. Adjust the volume to 25 ml with 6 M hydrochloric acid. The 

acidity of the sample solution is 6.5 M.

Magnetite:

Place the weighed sample (up to 1.0 g) in a suitable beaker having 

a capacity of at least 50 ml. Moisten the sample with 2 ml demineralized water, add 10 ml concentrated hydrochloric acid, and evaporate 

to dryness using moderate heat. Add 2 ml concentrated nitric acid 

and again evaporate to dryness. Add 1 ml concentrated nitric acid, 

15 ml 6 M hydrochloric acid and warm the beaker and contents to 

dissolve the residue. Transfer the contents of the beaker to a 25- 

ml volumetric flask, wash the beaker three times with 2-ml portions 

of 6 M hydrochloric acid and add each wash to the volumetric flask. 

Adjust the volume to 25 ml with 6 M hydrochloric acid. The acidity 

of a magnetite solution is 7 M.
ELEMENTS IN Ca- AND Fe-RICH GEOLOGIC MATERIALS 89

Plant ash:

Follow the same directions as given for magnetite. The final acidity 

of a plant ash solution is 6.5 M.

IRON REMOVAL

Transfer the entire sample solution to a 125-ml separatory funnel, 

add 0.5 ml concentrated hydrobromic acid, 10 ml MIBK, and shake for 

5 seconds. Drain the aqueous phase into a second 125-ml separatory 

funnel and discard the spent MIBK phase. Repeat the MIBK extractions 

until the organic layer remains colorless. Generally, three extractions 

are sufficient to remove as much as 1,000 mg of iron. If it is necessary 

to use three MIBK extractions for complete removal of iron, then all 

the samples and standard solutions in a given job lot must be treated 

in a similar manner.

For sample solutions of magnetite only, add 2 ml demineralized water 

before adding other reagents necessary for iron removal. The addition 

of 2 ml demineralized water lowers the acidity of the magnetite solution 

from 7 to 6.5 M, which would make its acidity identical to the acidity 

of solutions of other materials and of standard solutions.

If iron is not present in the original sample solution (calcites, plant 

ash, and so forth), the preceding step may be omitted.

EXTRACTION

To the iron-free aqueous solution in the separatory funnel add 5 ml 

tartrate buffer, 3 drops cresol red indicator solution, and enough concentrated ammonium hydroxide to cause the solution to become pale 

yellow or colorless. Let the solution cool to ambient temperature and 

adjust the final pH to a meter reading of 2 using either 1 M hydrochloric 

acid or 1 M ammonium hydroxide. Add 10 ml of 1 percent DADC/ 

MIBK solution and shake for 1 minute. The pH (2.8) of the sample solution at this point will be at the optimum. Discard most of the aqueous 

phase and transfer the remaining aqueous and organic solutions to a 

clean test tube.

ESTIMATION

Atomize the organic extract out of the sample solution and convert 

reading to parts per million using a standard curve which was calculated 

from tracings obtained by using an automatic null recorder readout 

accessory and a 10-mv strip-chart recorder. The burner used was a 

narrow-slot air-acetylene premix burner. For each element the analyst 

should use the recommended settings as supplied by the manufacturer 

of the instrument.

PREPARATION OF STANDARDS

Aliquots of 0.0, 0.1, 0.2, 0.5, 1.0, 2.0 and 3.0 ml of the combined standard solution are diluted to 25 ml with 6 M hydrochloric acid in volumetric
90 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

flasks each containing 1 ml of concentrated nitric acid. Transfer to 125- 

ml separatory funnels, add 0.5 ml concentrated hydrobromic acid and 

10 ml MIBK to each funnel and shake for 5 seconds. Drain the aqueous 

phase from each funnel into clean separatory funnels and repeat the 

MIBK extractions the same number of times as the samples. Discard the 

organic phase after each extraction. To the aqueous solution add 5 ml 

of tartrate buffer, 3 drops of cresol red indicator solution and adjust the 

pH to 2. Let cool and extract the metals with 10 ml DADC/MIBK. The 

organic layer in each separatory funnel contains respectively, 0.0, 0.1, 

0.2, 0.5, 1.0, 2.0, and 3.0 ppm each of the following elements: Bismuth, 

cadmium, cobalt, copper, lead, nickel, silver, and zinc. Atomize each 

standard in the atomic absorption instrument and prepare a standard 

curve from the absorbance measurements.

EXPERIMENTS

EFFECT OF IRON AND CALCIUM ON ATOMIC ABSORPTION MEASUREMENTS

The determination of trace constituents of geologic materials is 

hampered by large ratios of macroconstituents, such as iron and (or) 

calcium to the trace constituents. Iron-rich materials, such as magnetite, 

contain up to 70 percent elemental iron, and the ratio of iron to a trace 

constituent at the 0.001 percent level (10 ppm) is 70,000. The determination of the trace constituent in such materials is difficult. Similar relations hold in other geologic materials, expecially in plant ash in which 

the calcium content can be as much as 40 percent. Accordingly, the effect 

of such ratios is of importance and the following experiments were made 

to evaluate the effect.

In the procedures for silicate rocks, magnetite, and plant ash, sample 

weights up to 1 g are suggested. With a 1-g sample, a conversion factor 

of 10 changes the concentration of the analyte measured in the organic 

solvent to the concentration in the original sample. In the absence of 

all elements of interest, increasing amounts of iron were added to control 

sample solutions so that the amounts of iron present in the organic 

extract corresponded to 1-100 percent. Each solution was atomized into 

an air-acetylene flame, and the absorption was measured for each element of interest in the control sample solution. The iron concentration 

in the original sample at which the interference enhancement or depression equaled or exceeded the sensitivity of the element of interest 

is given in table 28.

A similar experiment was made with increasing amounts of iron in 

the presence of a fixed amount of each element of interest corresponding 

to 10 ppm in a control sample. The iron concentration in the control 

sample at which the interference enhancement or depression equaled 

or exceeded the sensitivity of the element of interest is shown in table 

28.
ELEMENTS IN Ca- AND Fe-RICH GEOLOGIC MATERIALS 91

TABLE 28. Amounts of iron causing interference in the absence or presence of analyte

elements

Element

Silver..........................

Cobalt....... ..................

Nickel.........................

Sensitivity,

(/jg/gof 

sample)

1 

5 

1 

1 

1 

1 

5 

1

Iron in sample causing 

interference (percent)

Analyte 

absent

10 

5 

10 

5 

10 

5 

5 

5

Analyte 

present

10 

10 

5 

5 

50 

1 

10 

1

Relatively small amounts of iron (1-5 percent) interfere in the measurement of cadmium, cobalt, nickel, and zinc. As little as 10 percent 

iron causes interference in the measurement of silver, bismuth, and lead. 

The measurement of copper is relatively free of interference from iron.

In the absence of any of the elements of interest, increasing amounts 

of calcium were added to solutions corresponding to calcium concentrations in a control sample ranging from 1 to 100 percent. Each solution 

was atomized into an air-acetylene flame and the absorption measured 

for each element of interest. The observations are shown in table 29.

TABLE 29. Amounts of calcium causing interference in the absence or presence of analyte

elements 

[Leaders (...) indicate entry not available]

Element

Silver..........................

Cadmium ...................

Cobalt.........................

Copper........................

Nickel.........................

Sensitivity, 

this report 

( M §/£ °f

sample)

1

5

1

1 1 

1

5 

1

Calcium in sample causing 

interference (percent)

Analyte 

absent

5 

5 

5 

5 

50 

5

"5

Analyte 

present

50 

5 

1 

10 

10 

1 

1 

1

Small amounts of calcium (1-5 percent) interfere with the measurement of bismuth, cadmium, nickel, lead, and zinc. Cobalt and copper 

measurements are affected by 10 percent calcium, and only the silver 

measurement is relatively free from calcium interference.

Thus far, no attempt has been made to describe the nature of the iron 

or calcium interference. In some samples, the interference is one of 

enhancement and in others one of depression. In a few examples, both 

occur and so the effect can be graphically described as concave or convex,
92 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

depending on which enhancement or depressions occurs at lower 

concentrations of the interferent.

EFFECT OF HYDROCHLORIC ACID ON 

EXTRACTION OF IRON BROMIDE INTO MIBK

The extraction of iron, silver, and cadmium with MIBK is critically 

dependent on the hydrochloric acid concentration of the solution, but 

the extraction of such elements as bismuth, cobalt, copper, nickel, lead, 

and zinc is slight and not appreciably affected by different acid concentrations. Large amounts of iron are extracted by MIBK from a strong 

acid concentration, such as 8 M hydrochloric acid, but MIBK is appreciably soluble in the acid, and at slightly greater acid concentrations 

than 8 M, the ketone is mutually soluble. The resulting single phase 

defeats the objective, and higher acid concentrations have to be avoided. 

From 8 M hydrochloric acid, about half of the cadmium extracts along 

with iron; however, at this acid concentration the retention of silver in 

the aqueous phase is maximum. At the other extreme for example, 

1 M hydrochloric acid approximately 75 percent of the silver extracts 

with iron into the ketone, but most of the cadmium remains behind 

in the aqueous phase. The use of 6.5 M hydrochloric acid seems to be 

a satisfactory compromise; the extraction of iron is appreciable, and the 

extraction of silver and cadmium is minimized. Compensation for the 

small losses of cadmium is made by treating samples and standards 

similarly. Small losses of the other elements occur also, especially with 

repeated MIBK extractions, but generally three extractions are sufficient 

to separate iron from the analyte elements. Under the conditions just 

described, three such extractions remove as much as 1,000 mg of iron, 

and that amount is more than adequate for any conceivable iron-rich 

geologic samples.

EXTRACTION OF ANALYTE FROM 

LARGE AMOUNTS OF CALCIUM

Many investigators, including Sprague and Slavin (1964), Mansell 

(1965), and Mulford (1966), have successfully used the APDC/MIBK 

system to remove trace amounts of several elements such as copper, 

bismuth, lead, cadmium, cobalt, nickel, iron, and manganese from 

various media, such as concentrated potassium chloride solution and 

other brines. The extraction is somewhat pH dependent; and, although 

the APDC/MIBK system is applicable over a fairly wide range, Mansell 

(1965) experienced difficulties in preparation of aqueous solutions of 

APDC even at a 1-percent level. Another system, DADC/MIBK has a 

wider range, especially for the elements of interest. Moreover, DADC 

is completely soluble in MIBK and is less sensitive to strong acids (Stary, 

1964).
ELEMENTS IN Ca- AND Fe-RICH GEOLOGIC MATERIALS 93

Accordingly, the pH of the iron-free sample solution is adjusted to 

optimum (2.8) and shaken with a relatively immiscible solution of 

DADC/MIBK. The phases are separated, and the ketone is atomized in 

an atomic absorption instrument. The efficiency of this extraction is 

shown by the data in table 30.

TABLE 30. Recovery of 1 ppm of each element in the presence of varying amounts

of iron and calcium

Parts per million

Milligrams

Ag Cd Co Cu Pb Zn

Iron (III) 3.5..............

10 ..............

100 ..............

500 ..............

1,000 ..............

Calcium(II)5 ..............

10 ..............

100 ......./.....

500 ..............

1,000 ..............

0.9

9

1.0

1.0

1.1

.9

.9

9 1

1.1

1.0 1

0

0

0

0

0

q

.0

0.90

.90

1.0

1.1

1.2

1.0

1.0

1.0

1.1

1.0

0.9

.9

9

.9

.8

1.0

1.0

9

1.0

9

0

1

0

,0

.0

1

1

1

1

0

0.9

.9

9

.9

1.1

1.1

1.0

1.1

1.1

9

1.1

1.0

1 i

.9

1.0

1.0

1.0

1.0

1.0

1.0

0

8

0

,1

1

1

1

1

1

.2

Recoveries of analyte are satisfactory in the presence of overwhelming 

quantities of iron and (or) calcium. Moreover, because no perceptible 

enhancement or depression occurs when analyte metals were added, the 

iron and calcium solutions used in this study appear to be free of such 

metals. Data shown in tables 28 and 29 result from the interference of 

iron and calcium.

It was found by experimenting with different commercially available 

atomic absorption instruments that the iron or calcium levels causing 

significant interference in the absence or presence of analyte differ, 

depending on the particular instrument.

Foster (1971) observed that the effects of calcium on lead, zinc, nickel, 

copper, and manganese determinations by atomic absorption spectrometry became more noticeable with shorter wavelengths, except for zinc. 

He also observed that the use of a continuum lamp largely eliminated 

the background interference.

RESULTS

Table 31 compares the results of the determinations of several elements 

obtained by the atomic absorption procedure, described in this report, 

and by semiquantitative spectrographic analysis. In general, the agreement between the two methods is good. Special effort was made to achieve 

high sensitivity for bismuth in magnetite only. This was accomplished
TABLE 31. Comparison of two methods for determination of severa

[AA, atomic absorption; spec, semiquantitative spectrographic; M, major constituent; Nd, not detected at limit of d

leaders (...) indicate data not available; results are in parts pe

Sample 

No.

E-590...............

E-243...............

PC-859............

E-276...............

E-94. ................

M-238..............

VC-3.. ..............

Material

Caliche ............

Quartz

'Results in percent. 

[Leaders (

Material

Magnetite ...... 

Limestone......

Values

High............

Mean 1 ...........

RSD.............

High............

RSD.............

AA

........ 17

........ 550

........ 3

........ 23

........ 25

........ 2

Ag

Spec AA

20 <2

500 3,300 

1 <2 

20 525

35 <2 

12 ND .70 

3 <2

Bi

Spec AA

<5 130 

>1,000 35 

<5 65 

300 35

<5 145 

<5 

<5 5

Cd

Spec AA

70 1. 

70 <1 

70 400 

50 <1

200 6 

,45 Nd 65 

<20 3

Co

Spec 

5 <5 

<5 

100 

<5 

20 

50 

5 

TABLE 32. Precision data for eight elements in magn

...) indicate data not calculable; results are in parts per million, except for RSD

Ag

0.17 

.10 

.13±0.03 

22 

18 

16 

17±0.9 

4

Bi

1.0

.60 

.76±0.20 

23 

<2 

<2

Cd

0.51 

.26 

.4110.11 

23 

130 

105 

120H3 

9

Co

75 

40 

65118 

22 

1.7 

1.1 

1.410.3 

18

'Conficence limits at 95-percent level.
ELEMENTS IN Ca- AND Fe-RICH GEOLOGIC MATERIALS 95

by reducing the volume of the extractant DADC/MIBK from 10 ml to 

3 ml. As little as 0.6 ppm bismuth in a 1-g sample can be detected by 

this special treatment.

All samples in this study, with the exception of the magnetite and 

plant ash samples, were treated with hydrofluoric and nitric acids before 

being solubilized in hydrochloric acid. The magnetite and plant ash 

samples were treated with nitric and hydrochloric acids, as described 

on pages 88 and 89. Materials such as limestone, limonite, gossan, caliche 

and quartz monzonite were subjected to the same treatment as magnetite, but low results were obtained in many samples. The low results 

are probably attributable to those metals being encompassed in the crystal lattice of the minerals and not completely solubilized without the 

use of hydrofluoric acid.

Precision was obtained by analyzing iron- and calcium-rich samples 

five times at different intervals, and the results are shown in table 32. 

The relative standard deviation and the confidence limits at the 95- 

percent level were calculated by the method given by Youden (1951, p. 

20). The relative standard deviation of the magnetite sample is higher 

than that of the limestone; however, a pure magnetite sample is difficult 

to obtain.

A method of analysis has been developed for the determination of 

several trace elements in iron and (or) calcium-rich materials. Iron interference is removed by a simple MIBK extraction and the trace elements 

chelated with DADC. Calcium does not form a chelate with DADC and 

is eliminated from interfering. Although the method is designed primarily for use with sample media, such as magnetite, calcite and plant ash, 

it can be used, in part or in its entirety, for nearly all types of geologic 

materials.

REFERENCES CITED

Denaro, A. R., and Occleshaw, V. J., 1955, The extraction of some metallic bromides

from aqueous solutions with methy ethyl ketone and methyl iso-butyl ketone: Anal.

Chim. Acta, v. 13, p. 239-247. 

Foster, J. R., 1971, The reduction of matrix effects in atomic absorption analysis and

the efficiency of selected extractions on rock-forming minerals, in Geochemical

exploration (Internal. Geochemical Explor. Symposium, 3d, Proc.): Canadian Inst.

Mining and Metallurgy Spec. Vol. 11, p. 554-560. 

Mansell, R. E., 1965, Notes on the extraction of manganese with chelating agents and

MIBK: Atomic Absorption Newsletter, v. 4, no. 5, p. 276. 

Menis, O., and Rains, T. G, 1960, Extraction and flame photometric determination

of iron: Anal. Chemistry, v. 32, no. 13, p. 1837-1841. 

Mulford, C. E., 1966, Solvent extraction techniques for atomic absorption spectroscopy:

Atomic Absorption Newsletter, v. 5, no. 4, p. 88. 

Nakagawa, H, M., and Thompson, C. E., 1968, Atomic absorption determination of

tellurium, in Geological Survey research 1968: U.S. Geol. Survey Prof. Paper 600-B,

p. B123-B125.
96 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

Scott, T. C., Roberts, E. D., and Cain, C. A., 1967, Determination of minor constituents

in ferrous materials by atomic absorption spectrophotometry: Atomic Absorption

Newsletter, v. 6, no. 1, p. 1. 

Sprague, S., and Slavin, W., 1964, Determination of very small amounts of copper and

lead in KC1 by organic extraction and atomic absorption spectrophotometry: Atomic

Absorption Newsletter, no. 20, p. 11. 

Stary, J., 1964, The solvent extraction of metal chelates: New York, The Macmillan

Co., p. 164. 

Thompson, C. E., Nakagawa, H. M., and VanSickle, G. H., 1968, Rapid analysis for

gold in geologic materials, in Geological Survey research 1968: U.S. Geol. Survey

Prof. Paper 600-B, p. B130-B132.

Ward, F. N., Nakagawa, H. M., Harms, T. F., and VanSickle, G. H., 1969, Atomicabsorption methods useful in geochemical exploration: U.S. Geol. Survey Bull.

1289, 45 p. 

Youden, W. J., 1951, Statistical methods for chemists: New York, John Wiley & Sons,

126 p.
NEW AND REFINED METHODS OF TRACE ANALYSIS 

USEFUL IN GEOCHEMICAL EXPLORATION

USE OF EMISSION SPECTROSCOPY 

FOR THE SEMIQUANTITATIVE ANALYSIS

OF TRACE ELEMENTS 

AND SILVER IN NATIVE GOLD

By ELWIN L. MOSIER

CONTENTS

Page

Abstract........................................................................................................................... 97

Introduction.................................................................................................................... 98

Method of analysis.......................................................................................................... 98

Apparatus and spectrographic operating conditions..................................................... 98

Procedure........................................................................................................................ 100

Standards......................................................................................................................... 100

Precision and accuracy ................................................................................................... 102

(inclusion...................................................................................................................... 103

Acknowledgment............................................................................................................ 104

References cited............................................................................................................... 104

TABLES

Page

TABLE 33. Analytical lines and detection limits.......................................................... 101

34. Concentration range of elements in standards........................................... 101

35. Electrode charge for the first four standards of standard No. 1................ 102

36. Results of homogenized and natural-occurring gold samples from

the same source area................................................................................ 103

ABSTRACT

A spectrographic method for the semiquantitative determination of trace elements 

and silver in native gold has been developed. Silver, because it is commonly found 

alloyed with native gold as a major constituent, is determined in percent. Other elements 

(As, B, Ba, Be, Bi, Ca, Cd, Co., Cr, Cu, Fe, Ga, Ge, Hg, In, La, Mg, Mn, Mo, Nb, 

Ni, Pb, Pd, Pt, Sb, Sc, Si, Sn, Sr, Ta, Te, Ti, V, W, Y, Zn, and Zr) are determined 

as trace impurities and reported in parts per million. Direct-current arcanode excitation 

was chosen because of its simplicity and sensitivity. Five-milligram samples are arced

97
98 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

in an aluminum oxide-graphite mixture at 15 amperes current. The resulting spectra 

are recorded on Spectrum Analysis No. 1, 35-millimetre film and visually compared 

with standard films prepared by arcing mixtures of spectrographically pure powders 

in a graphite base, pure gold, and aluminum oxide. By analyzing the gold in its native 

state, this method avoids the necessity of complicated sample preparation.

INTRODUCTION

Semiquantitative determination of trace elements in gold by emission 

spectrography is hindered by the difficulty of preparing adequate standards. Much of the previous work on the trace-element analysis of native 

gold has been qualitative (Crook, 1939; Warren and Thompson, 1944), 

but Mertie (1940) discussed quantitative determinations made on native 

gold by R. E. Stevens and E. T. Erickson of the U.S. Geological Survey. 

Antweiler and Sutton (1970) described a semiquantitative spectrochemical method in which they dissolved native gold in aqua regia, 

evaporated the solution to dryness, and analyzed the dried residue. However, they were unable to analyze for the highly volatile elements, such 

as arsenic, mercury, tellurium, and zinc, because these elements were 

lost during sample preparation. The method also lacked adequate sensitivity for many elements. The desirability for increased sensitivity and 

the importance of obtaining data on volatile elements closely associated 

with gold emphasize the need for a new semiquantitative procedure.

Selection of a method suitable for the analysis of trace elements in 

native gold was made after considering the following criteria: (1) Samples of native gold are generally small; therefore, the method should 

provide for the analysis of a large number of elements from a single 

electrode charge; (2) the detection limits of all elements should be as 

low as possible; (3) sample preparation should cause minimal change 

in the natural chemical composition of native gold; (4) standards should 

be prepared to encompass wide ranges of concentrations; and (5) for the 

purpose of identifying the genetic origin of gold, the precision and 

accuracy should be such that one would be able to assign a signature 

to native gold based on its trace-element composition.

The following procedure meets these criteria.

METHOD OF ANALYSIS

The instrumentation employed and operating parameters are listed 

below:

APPARATUS AND SPECTROGRAPHIC 

OPERATING CONDITION

Spectrograph: Jarrell-Ash 1.5-m Wadsworth-mounted, having a dispersion of 5.45 A/mm over a spectral range from 2,100 A to 4,800 A 

in the second order.

Source: Jarrell-Ash Spectro Varisource (Model 43650).
TRACE ELEMENTS AND SILVER IN NATIVE GOLD 99

Discharge: 15 amps d.c. arc.

Sample Electrode: Ultra Carbon 101L anode.

Counter Electrode: Ultra Carbon 5001.

Analytical Gap: 5 mm held constant.

Exposure Time: 96 seconds no preburn.

Slit Width: 10 microns.

Filter: 100 percent transmission over 36 percent transmission 0.8 cm

aperture mask on grating. 

Film: 35-mm Kodak Spectrum Analysis No. 1, processed with D-19

developer for 2 minutes 15 seconds at 20°C, stopped in cold running

water (30 sec), fixed for 3 minutes, rinsed in cold water, and air dried. 

Comparator: Jarrell-Ash console microphotometer (Model 21-000).

Campbell, Mosier, and Antweiler (1973) reported that laboratory treatments of native gold with various acid washes, roasting, or amalgamation may affect the chemical composition of the gold. To prevent altering 

the chemical composition of the gold, and to achieve the maximum 

amount of data possible, the gold is arced in its metallic state. Therefore, 

after retrieval from its source, no sample preparation is required other 

than handpicking under a binocular microscope and, if desired, cleaning 

in an acid wash to remove surface coating. Direct-current arc-anode 

excitation was chosen because of its simplicity and the sensitivity it provided for a wide range of elements.

Experience showed that a total energy burn was required to obtain 

reliable data on many of the elements. For this reason, an electrode with 

a small crater size and undercut design (Ultra Carbon 101L) 1 was selected 

to create greater heat intensity in the electrode cavity. Thus, a total 

energy burn and a fast volatilization of the sample are achieved without 

burning excessive graphite from the electrode.

A means of buffering the sample was found to be necessary. When gold 

is arced alone it has a tendency to "jump out" of the electrode cavity. 

Arcing gold in graphite results in an erratic burn of the arc and high 

background radiation. Also, the time required for total volatilization 

is greater than 2 minutes. Several codistillation agents were tried; of these, 

a mixture of aluminum oxide and graphite gave the best results. The 

addition of aluminum oxide resulted in a smooth even burn throughout 

the exposure period, whereas graphite prevented the aluminum oxide 

from forming a bead.

Time-burn studies showed that volatile elements (As, Bi, Cd, Ga, Hg, 

In, Pb, Sb, Te, and Zn) are completely volatilized in 30 seconds, whereas 

aluminum, gold, and less volatile elements are codistilled and volatilized 

in 90 seconds. Therefore, to insure a total energy burn, the exposure 

time was set at 96 seconds. The electrode charge was empirically determined to be 5 mg gold, 5 mg graphite, and 2.5 mg aluminum oxide.

'Use of a specific brand name does not necessarily constitute endorsement of the product by the U.S. Geological Survey.
100 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

To achieve the sensitivity desired and to maintain a high contrast 

between line intensity and background radiation, a technique developed 

for the analysis of plant ash proved successful (Mosier, 1972). The technique utilizes a Hartmann diaphragm and step-filter (100 percent over 

36 percent) assemblage which enables the simultaneous recording of 

volatile elements from a 30-second burn at 100-percent transmission and 

of less volatile elements from a total energy burn (96 seconds) at 36- 

percent transmission. Thus, from the same electrode charge used for a 

total energy burn, volatile elements are detected at low concentration 

levels. In essence a dual analysis is achieved from one electrode charge.

PROCEDURE

Five milligrams gold, 5 mg graphite (grade SP-1), and 2.5 mg aluminum oxide are weighed and manually mixed in a weighing pan. The 

mixture is transferred into the cavity of a Ultra Carbon 101L electrode 

(or equivalent) with the aid of a Teflon funnel and packed tightly with 

a venting tool. Eleven test samples and one standard sample are weighed 

for each film.

The arcing sequence is as follows: (1) Set the fishtail aperture on the 

slit for a 2-mm-high spectrum and the automatic timer at 30 seconds; 

(2) initiate an arc at 3A and immediately increase to 15A; (3) when the 

arc stops after the 30-second burn, position the Hartmann diaphragm 

to block out the top half (100-percent portion) of the spectrum, reset 

the timer for 66 seconds and reignite the arc at 3A, increasing immediately to 15A; (4) on completion of the burn, rack the camera 2 mm 

and repeat the sequence for the next sample. This technique produces 

a 2-rnm-high spectrum with the top half representing a 30-second burn 

at 100-percent transmission and the bottom half representing a total 

energy burn (96 seconds) at 36-percent transmission. It should be noted 

that the optics of a Wadsworth-mount spectrograph produces an inverse 

spectrum, and, when projected in the suggested comparator, the bottom 

half is the 100-percent portion.

The film is processed and the results are obtained by visual comparison with master films made by exactly the same technique as that 

described, except that synthetic standards are substituted for unknowns. 

The analytical lines used and the detection limits obtained are given 

in table 33 (Meggers and others, 1961). When possible, the total energy 

portion of the spectrum is used for making comparisons. The shortburn portion of the spectrum is used only for volatile elements whose 

concentrations are below the detection limits of the total energy burn.

STANDARDS

Because it would be virtually impossible to achieve adequate standards 

in a metallic gold matrix, two sets of standards were prepared in a graph-
TRACE ELEMENTS AND SILVER IN NATIVE GOLD

TABLE 33. Analytical lines 1 and detection limits

101

Element

Ag.. ...............

As..................

B... ................

Ba .................

Be..................

Bi...... ............

Ca.................

Cd.................

Co. ................

Cr..................

Cu.................

Fe

Ga.................

Ge... ..............

He

In..................

La.................

Mg................

Mn ................

Wavelength (A)

2,721.8

2,780.2

2,497.7

4 554 0

3,131.1

3,067.7

4,226.7

3,261.1

3,044.0

2 843 3

S 274 0

2,599.6

2,943.6

3 039 1

. 2,536.5

3,039.4

3,337.5

9 7Q 1! ^

2,798.3

Detection 

limit (ppm)

0.5

20

1

10

1

1

5

10

10

5

1

5

1

5

10

5

20

5

1

Element

Mo...............

Nb...... ..........

Ni ................

Pb ................

Pd ................

Pt...... ...........

Sb......... ........

Sc.................

Si .................

Sn ................

Sr .................

Ta..... ...........

Te

Ti.................

V........ .... ......

W. ...... ..........

Y............ ......

Zn ................

Zr... ..............

Wavelength (A)

3,170.4

3,163.4

3.101.6

2,833.1

3,242.7

3,064.7

2,598.1

3,353.7

2,881.6

2,840.0

3,464.5

2,653.3

2,385.8

3 242 0

.. 3,102.3

9 947 4

3 242 3

3,302.6

3,392.0

Doled ion 

limit (ppm)

5

10

10

1

5

10

10

2

5

5

100

50

20

5

5

20

5

50

10

'Meggers, Corliss, and Scribner (1961).

TABLE 34. Concentration range of elements in standards

[Results are rounded to reporting values and are in parts per million except where noted. Leaders (...) indicate 

the element is not present in the standard]

Standard No. 1 Standard No. 2

Ag 

(percent)

50...............

20...............

10...............

5...............

2

1.. ..............

.5.............

.2.............

.1.............

.05.....:.....

.02...........

.01 ...........

Hg

. 50,000

. 20,000

. 10,000

. 5,000

. 2,000

. 1,000

500

200

100

50

20

10

Cu,Pb,Si

20,000

10,000

5,000

2,000

1,000

500

Bi.La.W, 

Y,Zr

10,000

5,000

2,000

1,000

500

200

As,Cu,Fe,Pb, 

Te.Ti.Zn

5,000

2,000

1,000

500

200

100

50

20

10

5

2

1

Be.La.Mn.Pd, 

Pt.Sb.Si.Ta

500

200

100

50

20

10

5

2

1

B,Ba,Bi,Ca,Cd, 

Ce,Cr,Ga,Ge,In,

Mg,Mo,Nb,Ni,Sc, 

Sn,Sr,V,W,Y,Zr

200

100

50

20

10

5

2

1

ite matrix. Spectrographically pure compounds were added in specified 

amounts to the graphite to obtain the desired concentration of each element. Table 34 gives the concentration range of the standards for the 

38 elements sought.

To obtain standards in a geometric progression desirable for semiquantitative analysis, the reciprocal of the cube root of 10 (0.464) was
102 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

used as the dilution factor (Myers and others, 1961; Grimes and 

Marranzino, 1968). By use of the 0.464 dilution factor, the succeeding 

standards prepared contain concentrations in order of magnitudes of 

100, 46.4, 21.5, 10, and so forth. For semiquantitative use these numbers 

are rounded to 100, 50, 20, 10, and so forth, when comparisons are made 

with sample films. Those lines which are estimated to fall between the 

above values are arbitrarily given values of 70, 30, 15, 7, and so forth 

(Ward and others, 1963).

To obtain uniformity between the standard and sample electrode 

charge, 5 mg of 99.999 percent purity gold, 5 mg of graphite base standard, and 2.5 mg of aluminum oxide are weighed, mixed, and packed 

into the electrode cavity. The first four standards of standard set No. 

1 are an exception to this. These standards contain less than 5 mg gold 

to compensate for their high silver content. Pure graphite is added to 

bring the weight to 15 mg. Table 35 illustrates the amounts to be weighed 

for these four standards.

TABLE 35. Electrode charge for the first four standards of standard No. 1

[Values are reported in milligrams]

Gold wire

2.7

3.9

4.5

4.8

Pure graphite

2.3

1.1

.5

.2

Aluminum oxide

2.5

2.5

2.5

2.5

Standard

5

5

5

5

PRECISION AND ACCURACY

The nature of gold, which varies greatly from grain to grain as well 

as from deposit to deposit (Gay, 1963), creates a problem in determining 

the analytical precision on a natural gold sample. To obtain a homogeneous sample for the purpose of determining the precision of the 

method, a nugget of native gold was cleaned in hydrofluoric acid to 

remove the surface coating and then heated at 1,300°C for 30 minutes. 

Ten splits of the homogenized sample and ten splits of a natural occurring sample from the same source area were then analyzed; the results 

are shown in table 36. Assuming that the procedure was successful in 

homogenizing the sample, the results show the analytical variance is 

generally within a factor of 2, whereas the natural variance can be as 

great as a factor of 20. The differences in the two sets of data are probably 

due to the influence of the acid wash and heating during the homogenizing process.

Accuracy proved to be more difficult to determine than precision, 

because there was no homogeneous gold sample available with known 

amounts of several trace impurities. However, a gold standard was obtained that had been annealed with 1-percent copper. To obtain a
TRACE ELEMENTS AND SILVER IN NATIVE GOLD 103

TABLE 36. Results of homogenized and natural-occurring gold samples from the same

source area 

[Except for silver, concentrations are in parts per million. Leaders (...) indicate the element was not detected]

Ag 

(percent) Ca Cu Fe Hg Mg Mn Pb Ti

Homogenized sample

10...........

10...........

10...........

10...........

10...........

10............

10............

10............

10............

10............

........ 1.5

........ 2

........ 2

........ 2

........ 2

........ 1.5

........ 2

........ 2

........ 1

........ 1.5

20

5

30

10

10

10

30

30

200

10

500

500

500

500

500

500

500

500

500

500

500 ...

500 ...

500

500

500

500

500

500

500 ...

500 ...

10

10

20

15

10

10

20

20

20

10

5

7

7

7

7

5

5 '<

1 i

1

5

,000

,000

,500

,000

,000

,000

>,000

1000

,000

,000

30

15

20

15

15

15

20

30 ...

20 ...

15

Natural sample

10...........

15............

10............

10............

15............

15............

10............

10............

10............

10.. ..........

15

......... 10

......... 2

......... 10

......... 10

3

3

........ 2

1 5

........ 2

70

200

50

50

100

50

50

15

30

20

200

200

150

200

200

150

500

200

200

150

500

1,000

700

300

700

500

500

200

500

150

2,000

2,000

1,500

1,000

1,500

2,000

100

2,000

1,500

1,000

200

200

200

150

200

200

50

100

150

100

10

15

10

7

10

10

5

5

7

5

700

500

200

500

500

100

1,000

150

30

100

2,000

2,000

2,000

1,500

5,000

7,000

500

1,000

1,500

1,500

50

70

100

50

100

70

5

20

70

20

standard with 2,000 ppm copper in gold for the purpose of establishing 

some understanding of the accuracy, each of five electrodes was charged 

with 1 mg of the 1-percent copper standard and 4 mg pure gold. The 

five replicate samples were then analyzed and comparisons made with 

a standard film. The comparisons gave 2,000-ppm copper reading for 

all five samples, proving the method accurate, at least for copper. The 

same accuracy cannot be assumed for all elements sought; however, 

experience has shown that the combined accuracy and precision is 

sufficient to distinguish differences in the chemical composition of 

native gold originating from various source areas.

CONCLUSION

The study of trace element and silver content of native gold for the 

purpose of establishing the genetic origin of the gold requires the analysis of a wide range of elements from samples that may be extremely 

small. Preparation of samples for analysis must not result in the alteration of the natural chemical composition of gold by loss of certain 

volatile elements. A fast effective semiquantitative spectrographic 

method has been developed to meet these requirements. To eliminate
104 TRACE ANALYSIS IN GEOCHEMICAL EXPLORATION

the necessity of complicated sample-preparation techniques which may 

result in loss of volatile elements or otherwise alter the natural chemical 

composition of gold, the gold is analyzed in its metallic state by use 

of direct-current arc anode excitation. Thus, after the gold is retrieved 

from its source, no further preparation is required other than handpicking under a binocular microscope and, if desired, cleaning in an 

acid wash to remove the surface coating. A split-slit technique utilizing 

a Hartmann diaphragm and step filter assemblage enables a dual analysis from one electrode charge by simultaneously recording volatile 

elements from a short burn at 100-percent transmission and less volatile 

elements from a total energy burn at 36-percent transmission on separate 

portions of the spectrum.

A major difficulty in analyzing native gold is the acquisition of 

adequate standards with which to make comparisons. In this procedure 

standards are effected by the admixture of spectrographically pure compounds in a graphite base and volatilized in the presence of pure gold. 

Aluminum oxide is added to both sample and standard electrode charges 

as a codistillation agent to produce a smooth even-burning arc throughout the exposure period.

The analysis of several hundred gold samples from various source 

areas has shown the method to be effective in the determination of distinguishing chemical characteristics of gold. Therefore, the method is 

of value in aiding the understanding of the geologic principles involved 

during the deposition of gold deposits (J. C. Antweiler and W. L. 

Campbell, oral commun., 1971).

ACKNOWLEDGMENT

I thank Mr. J. C. Antweiler for the assistance given in the preparation 

of this paper.

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