U.S. GEOLOGICAL SURVEY CIRCULAR 948 

Analytical Methods Used in 

Geochemical Exploration, 1984 

Analytical Methods Used in 

Geochemical Exploration, 1984 

By Richard M. O'Leary and Allen L. Meier 

U.S. GEOLOGICAL SURVEY CIRCULAR 948 

1986 
United States Department of the Interior 

DONALD PAUL HODEL, Secretary 

U.S. Geological Survey 

Dallas L. Peck, Director 

Free on application to the Branch of Distribution, Eastern Region, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
Introduction . . . . . . . . . . . . . . 

Antimony, atomic absorption method 

Arsenic, confined-spot method . . . . 

Arsenic, spectrophotometric method . 

Bismuth, cadmium, copper, lead, silver, and zinc, organic extraction method . . . . . . . . . . . . . 

Calcium, lithium, magnesium, potassium, and sodium, 

atomic absorption method . . . . . . . . . . . . . . 

Cobalt, copper, lead, nickel, and zinc, atomic absorption 

method ................. . 

Copper, acid-extractable method .... . 

Fluorine, ion-selective electrode method . 

CONTENTS 

Page 

1 Gold, flame atomic absorption method . . . . . . . . 

4 Gold, flameless atomic absorption method . . . . . . 

6 Heavy metals, ammonium citrate-extractable method . 

8 Indium and thallium, atomic absorption method 

Molybdenum, atomic absorption method . . . . . . . . 

11 Selenium, flameless atomic absorption method . . . . 

Tellurium, flame and flameless atomic absorption 

14 methods .................. . 

Tin, atomic absorption method . . . . . . . . 

17 Tungsten, visible spectrophotometric method 

19 Uranium, ultraviolet fluorescence method 

21 Selected references . . . . . . . . . . . . . . 

ILLUSTRATIONS 

FIGURE 1. Schematic of modified Gutzeit apparatus 

2. Schematic of arsine generation assembly 

TABLES 

TABLE 1. Average abundance or range of trace elements in the Earth's crust, various rocks, soil, and river water, 

and limits of determination for six-step semiquantitative emission spectrography and analytical methods 

Page 

23 

25 

27 

29 

31 

33 

36 

39 

41 

42 

45 

Page 

7 

9 

Page 

of this compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2. Average abundance of major elements in the Earth's crust, basalt, and granite, and limits of determination 

for six-step semiquantitative emission spectrography and analytical methods of this compilation . . . 4 

III 

ANALYTICAL METHODS USED IN GEOCHEMICAL 

EXPLORATION, 1984 

By Richard M. O'Leary and Allen L. Meier 

INTRODUCTION 

Geochemical exploration involves the measurement of the concentration of an element or group 

of elements in naturally occurring materials. We 

use these measurements to recognize abnormal 

elemental patterns (geochemical anomalies) that 

may be related to mineralization. The analytical 

method used to differentiate between background 

and anomalous concentrations must have adequate 

sensitivity, accuracy, and precision. Most often 

the relative elemental concentration and not 

necessarily the absolute concentration is sufficient 

to allow recognition of geochemical anomalies. 

Thus, semiquantitative methods that are rapid, 

precise, sensitive, and easily applied to a variety 

of naturally occurring materials are most useful 

in geochemical exploration. 

The USGS (U.S. Geological Survey) uses a variety of analytical methods for geochemical exploration. These include arc and inductively coupled 

plasma emission spectrometry, atomic absorption 

spectrometry, X-ray fluorescence spectrometry, 

colorimetry, ion chromatography, ultraviolet 

fluorimetry, and specific ion electrodes. Arc emission spectrography has proven to be one of the 

most useful analytical techniques. Thirty or more 

elements can be determined simultaneously in 

each sample with little sample preparation other 

than pulverizing. The technique lacks sensitivity 

for some of the more useful elements, and a highly 

trained analyst must interpret the photographic 

spectra. Therefore, the other analytical methods 

are used in place of or to complement the spectrographic technique. In this report, we present 

some of these methods found to be most useful 

for this purpose. 

The purpose of this report is to bring together 

1 

previously published analytical methods currently 

being used by the Branch of Exploration 

Geochemistry of the USGS for geochemical exploration, to make them more available to interested 

persons within and outside the USGS. We present 

these methods in "cookbook" style and have incorporated slight changes in procedure made since 

the original publication to provide procedures 

more easily followed by nontechnical as well as 

technical persons. Read the original publications 

if a more comprehensive description of the method 

is desired. 

Statistical data obtained by replicate analysis of 

six GXR (geochemical exploration reference) samples define the expected precision of each method 

(Allcott and Lakin, 1975). 

Sample 

GXR-1 

GXR-2 

GXR-3 

GXR-4 

GXR-5 

GXR-6 

Jasperoid 

Soil 

Description 

Fe-Mn-W-rich hot spring deposit 

Porphyry copper mill heads 

B-horizon soil 

B-horizon soil 

These reference materials may be used as qualitycontrol check samples. Observed values for these 

reference materials are given to indicate the sensitivity and range expected for each method. 

In selecting an analytical method, it is useful 

to know the expected abundance of the element 

of interest and the sensitivity of methods available. Table 1 gives the average abundance or 

range of trace elements in the Earth's crust, various rocks, soil, and river water (Levinson, 1974), 

and table 2 gives the average abundance of major 

elements in the Earth's crust, basalt, and granite 

(Taylor, 1964). Both tables include a comparison 
TABLE !.-Average abundance or range oftrace elements in the Earth's crust, various rocks, soil, and river water, and limits of determination for six-step 

semiq1.tantitative emission spectrography and analytical methods of this compilation 

[Average abundance data from Levinson (1974). All values in ppm, except river water in ppb. Leaders(-), no data available] 

Average abundance or range Limits of determination 

Emission Other 

Element Earth's Ultra- Basalt Grano- Granite Shale Lime- Soil River 

crust mafic diorite stone water spectro- analytical 

graphy methods 

Ag 0.07 0.06 0.1 0.07 0.04 0.05 1 0.1 0.3 0.5 o.osa 

As 1.8 1 2 2 1.5 15 2.5 1-50 2 200 1b 

Au .004 .005 .004 .004 .004 .004 .005 -- .002 10 .sf, .oo 2c 

B 10 5 5 20 15 100 10 2-100 10 10 

Ba 425 2 250 500 600 700 100 100-3,000 10 20 

Be 2.8 -- .5 2 5 3 1 6 -- 1 

Bi .17 .o 2 .15 -- • 1 .18 -- -- -- 10 1a 

Br 2.5 1 3.6 -- 2.9 4 6.2 -- 20 

Cd .2 -- .2 .2 • 2 .2 • 1 1 -- 20 .sa 

Ce 60 8 35 40 46 50 10 -- .06 

~ Cl 130 85 60 -- 165 180 150 -- 7,800 

Co 25 150 50 10 1 20 4 1-40 .2 5 sa 

Cr 100 2,000 200 20 4 100 10 5-1,000 1 10 

Cs 3 -- 1 2 5 5 -- 6 .02 - Cu 55 10 100 30 10 50 15 2-100 7 5 1a,1b,sf 

Dy 3 .59 3 3.2 .5 5 .4 -- .05 

Er 2.8 .36 1.69 4.8 .2 2 .5 -- .05 

Eu 1.2 .16 1. 27 1.2 -- 1 -- -- .07 

F 625 100 400 -- 735 740 330 -- 100 -- 100d 

Ga 15 1 12 18 18 20 .06 15 .09 

Gd 5.4 .65 4.7 7.4 2 6 .6 -- .04 

Ge 1.5 1 1.5 1 1.5 1. 5 • 1 

Hf 3 .5 2 2 4 3 .5 

Hg .08 -- .08 .08 .08 .5 .05 .03 .007 -- .0 1c 

Ho 1.2 .14 .64 1.6 .07 1 • 1 -- .o 1 

I .5 .5 .5 -- .5 2.2 1.2 -- 7 

In .1 .o 1 • 1 • 1 • 1 • 1 .02 -- -- -- • 2f 

Ir .0004 

La 30 3.3 10.5 36 25 20 6 -- .2 20 

Li 20 -- 10 25 30 60 20 5-200 3 -- sf 
Lu .so .064 • 20 -- .o 1 .s -- -- .008 

Mn 950 1,300 2,200 1,200 500 850 1,100 850 7 10 

Mo 1. 5 .3 1 1 2 3 1 2 1 5 1f 

Nb 20 15 20 20 20 20 -- -- -- 20 

Nd 28 3.4 17.8 26 18 24 3 -- .2 

Ni 75 2,000 150 20 .5 70 12 5-500 .3 5 sa 

Os .0004 -- -- -- -- -- -- -- -- -- - Pb 12.5 • 1 5 15 20 20 8 2-200 3 10 sf' 1a 

Pd .004 .o 2 .02 -- .002 

Pr 8.2 1.0 2 3.9 8.5 4.6 6 1 -- .03 

Pt .002 .02 .02 -- .008 

Rb 90 -- 30 120 150 140 5 20-500 

Re .0005 -- .ooos -- .0005 

Rh .0004 

Ru .0004 

Sb .2 • 1 .2 .2 .2 1 -- 5 1 100 1f 

Sc 16 10 38 10 5 15 5 -- .004 5 

Se .OS -- .os -- .5 .6 .08 .2 .2 -- .osc 

Sm 6 .57 4.2 6.8 3 6 .8 -- .03 -- Sn 2 .s 1 2 3 4 4 10 -- 10 2f 

~ Sr 375 1 465 450 285 300 500 50-1,000 50 100 

Ta 2 1 .5 2 3.5 2 

Tb .9 .088 .63 1.3 .OS 1 -- -- .008 

Te .001 .001 .oo 1 .oo 1 .oo 1 .o 1 -- -- -- -- .1 f, .004c 

Th 10 .003 2.2 10 17 12 2 13 • 1 100 

Ti 5,700 3,000 9,000 8,000 2,300 4,600 400 5,000 3 20 

Tl .45 .05 • 1 .5 • 7 5 .3 -- • 1 -- -- • 2f 

Tm .48 .053 • 21 .5 -- • 2 • 1 -- .009 

u 2.7 .oo 1 .6 3 4.8 4 2 1 .4 -- .ose 

v 135 so 250 100 20 130 15 20-500 .9 10 

w 1. 5 .s 1 2 2 2 .5 -- .03 so 1b 

y 30 -- 25 30 40 25 15 -- .7 10 

Yb 3 .43 1.11 3.6 .06 3 • 1 -- .os - Zn 70 so 100 60 40 100 25 10-300 20 200 1a,1b,sf 

Zr 165 so 150 140 180 160 20 300 -- 10 

aorganic extraction by atomic absorption. dspecific ion electrode. 

bcolorimetry. eFluorimetry. 

cFlameless atomic absorption. fAqueous extraction by atomic absorption. 
TABLE 2.-Average abundance of major elements in the Earth's crust, basalt, and grar:ite, and limits 

of determination for six-step semiquantitative emission spectrography and analytwal methods of 

this compilation 

[Average abundance data from Taylor (1964). All values in percent. Leaders(-), no data available] 

Average abundance 

Element Earth's Basalt Granite 

crust 

Al 8.23 8.76 7.70 

Ca 4.15 6.72 1.58 

Fe 5.63 8.56 2.70 

K 2.09 .83 3.34 

Mg 2.33 4.5 .16 

Na 2.36 1.94 2. 77 

Si 28.15 24.0 32.3 

of the limits of determination of the analytical 

methods of this compilation and those of a commonly used semiquantitative emission spectrographic technique (Grimes and Marranzino, 1968). 

References to other analytical methods, sample 

preparation and treatment, comparison of 

methods, separations before analysis, special analysis, and basic information and discussion of chemical methodology published since 1963 by members 

of our group in the USGS that may be of interest 

to those involved in analysis of geologic materials 

are given in the "Selected references" section. 

Any use of trade names in this report is for 

descriptive purposes only and does not imply endorsement by the USGS. 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K. eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Grimes, D. J., and Marranzino, A. P., 1968, Direct-current 

arc and alternating-current spark emission spectrographic 

field methods for semiquantitative analysis of geologic materials: U.S. Geological Survey Circular 591, 6 p. 

Levinson, A. A., 1974, Introduction to exploration geochemistry: Maywood, Ill., Applied Publishing Ltd, p. 43-44. 

Taylor, S. R., 1964, Abundance of chemical elements in the 

continental crust-A new table: Geochimica et Cosmochimica Acta, v. 28, p. 1273-1284. 

ANTIMONY, ATOMIC ABSORPTION 

METHOD 

The method described here, excepting minor 

changes, was developed by Welsch and Chao 

(1975). 

4 

Limits of determination 

Emission Other analytical 

spectrography methods 

0.05 0.05 

.05 

.1 

.02 .001 

.1 

SUMMARY 

Traces of antimony in rock, soil, and streamsediment samples are determined using an atomic 

absorption method. A dried mixture of the sample 

and ammonium iodide is heated to volatilize antimony tri-iodide, which condenses on the walls of 

the test tube. The condensate is put into solution 

with 10 percent hydrochloric acid, which fails to 

attack most interfering ions remaining in the sample. When a sample of extremely high iron content 

is analyzed, ascorbic acid keeps the iron in the 

reduced form and thus prevents its extraction. 

The antimony tri-iodide is extracted by trioctylphospine oxide and methyl isobutyl ketone solution in the presence of an excess of potassium 

iodide. The extract is aspirated into an atomic absorption spectrophotometer for estimation. Fifty 

samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Ammonium iodide: Pulverize to the consistency of 

extra-fine granulated sugar with a mortar and 

pestle and store in a desiccator. 

Hydrochloric acid, 10 percent: Dilute 100 mL concentrated hydrochloric acid to 1 L with water. 

Iodide reagent: Dissolve 30 g potassium iodide and 

5 g ascorbic acid in 100 mL of 10 percent hydrochloric acid (prepare daily). 

TOPO-MIBK reagent: Dissolve 4 g TOPO (trioctylphosphine oxide) in 100 mL MIBK (methyl 

isobutyl ketone). 
APPARATUS 

Atomic absorption spectrophotometer 

Centrifuge 

Electric oven 

Horizontal shaking machine 

Pyropot (Pyroco Products, Margat, Queensland, 

Australia 4019): Heating device accommodating 

10 test tubes, provides a controlled heat source 

for the iodide fusion. When the test tubes are 

inserted into the holders, only the bottom 3-4 

em is exposed to the desired temperature, 

thereby leaving the upper portions of the tubes 

cool enough to allow condensation of antimony 

tri-iodide vapors. 

Test tube, disposable screw-cap, 16x 150-mm 

Vortex mixer 

Water bath 

STANDARDS 

Stock antimony solution, 1,000 J.Lg/mL: Dissolve 

1.370 g potassium antimony tartrate in 500 mL 

of 6 M hydrochloric acid. 

Dilute antimony solution, 100 J.Lg/mL: Combine 10 

mL of 1,000 J.Lg/mL stock antimony solution with 

5 mL concentrated hydrochloric acid and dilute 

to 100 mL with water. 

Standard antimony solutions in TOPO-MIBK, 0, 

10, and 50 J.Lg/mL: Add 0.0, 1.0, and 5.0 mL 

of 100 J.Lg/mL dilute antimony solution to three 

25 x 200-mm screw-cap test tubes containing 1 g 

potassium iodide and 1.5 g ascorbic acid. Bring 

the total volume to 50 mL with 10 percent hydrochloric acid and mix to dissolve. Add 10 mL 

TOPO-MIBK reagent and shake for 1 minute. 

PROCEDURE 

1. Weigh 0.5 g sample into a 16x 150-mm disposable screw-cap test tube. 

2. Add 2 g powdered ammonium iodide, mix 

thoroughly, and dry overnight at 110°C in 

an electric oven. 

3. Allow to cool and then heat the sample for 

10 minutes in the Pyropot, which has been 

preheated to 350°C. 

4. After sample has cooled, add 7 mL of 10 percent hydrochloric acid and 3 mL iodide reagent, and place in a boiling water bath for 

15 minutes. 

5. Mix the sample well while hot, then cool in 

a cold water bath. 

5 

6. When cool, add 2 mL TOPO-MIBK reagent, 

cap, shake for 1 minute, and centrifuge. 

7. Determine the antimony content by aspirating 

the organic layer into an atomic absorption 

spectrophotometer. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . 218.4 

Slit ................................ 0.2 nm 

Lamp current . . . . . use current recommended on lamp 

Source lamp . . . . . . . . antimony (hollow cathode lamp) 

Background correction . . . . . . . . . . . . . . . . . . . . . . on 

Integration time . . . . . . . . . . . . . . . . . . . . . 1 second 

Burner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boling 

Flame condition . . . . . . . . . . . . . oxidizing (lean blue) 

10 1-Lg/mL . . . . . . . . . . . . . . . . 0.310 absorbance units 

50 ~J.g/mL . . . . . . . . . . . . . . . . 1.240 absorbance units 

Lower limit of determination ............. 0.5 ppm 

DISCUSSION 

The reaction of antimony present in the sample 

as Sb203 with ammonium iodide is as follows: 

S~03 + 6 NH I~2 Sbi3 + 6 NH3 + 3 H20. 

Any excess of water would tend to reverse the 

reaction because water is a product of the reaction, therefore, the sample and ammonium iodide 

mixture must be as dry as possible for maximum 

efficiency. The temperature and duration of heating in the Pyropot are critical and must be controlled to obtain reproducible results. 

Interferences are minimized in three ways. (1) 

Heating at 350°C isolates the antimony from the 

sample matrix. The released antimony is dissolved 

by the 10 percent hydrochloric acid solution, 

whereas potential interfering ions are only slightly 

dissolved. (2) Ascorbic acid in the iodide reagent 

reduces iron (III) to iron (II), which is not extracted by the TOPO-MIBK, thus iron interference is eliminated. (3) The extraction of antimony 

into TOPO-MIBK further separates the antimony 

from other potential interferences. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 
1975), in replicas of five, by the described procedure. 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 89-113 101 8.7 

II 36--38 37.2 2.94 

III 17-19 17.8 4.7 

IV 6-7 6.4 8.6 

v 1 1 0 

VI 2-3 2.6 21 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Welsch, E. P., and Chao, T. T., 1975, Determination of trace 

amounts of antimony in geological materials by atomic absorption spectrometry: Analytica Chimica Acta, v. 76, p. 

65--69. 

ARSENIC, CONFINED-SPOT METHOD 

The method described here, excepting minor 

changes, was developed by Almond (1953), and 

described by Ward and others (1963). 

SUMMARY 

Arsenic is determined in rock, soil, and streamsediment samples by a confined-spot method using 

a modified Gutzeit apparatus. The sample is partly 

dissolved by molten potassium hydroxide in a test 

tube. The melt is dissolved with 30 percent hydrochloric acid and the arsenic is reduced by the addition of stannous chloride. Through the action of 

metallic zinc in the acid solution, arsenic III is 

reduced to arsine, and the evolved arsine gas is 

reacted with mercuric chloride paper to form a 

yellow- to orange-colored spot. The colored spot 

is compared against colored standards for estimation of arsenic content. Small amounts of hydrogen sulfide evolved with the arsine are removed 

by reaction with lead acetate on glass wool 

through which the gases are passed. The black 

lead sulfide formed by the reaction serves as an 

indicator for the need of renewing the lead acetate-coated glass wool. Samples with a high sulfide content must be treated with hydrogen 

peroxide because all hydrogen sulfide will not be 

6 

removed by the lead acetate. About 60 samples 

can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Hydrochloric etcid, 30 percent: Dilute 300 mL concentrated hydrochloric acid to 1 L with water. 

Hydrogen peroxide, 8 percent: Dilute 8.0 mL hydrogen peroxide, 50 percent strength, to 50 mL 

with water (prepare daily). 

Lead acetate-coated glass wool: Dissolve 15 g lead 

acetate [Pb(C2H302) 2·3H20] in 100 mL water 

and 3 mL glacial acetic acid. Saturate as much 

glass wool as can be conveniently covered with 

this solution for 15 minutes, remove, and allow 

to dry. Fill the lower section of the modified 

gutzeit apparatus with this dried glass wool. 

Store excess glass wool in stoppered bottle. 

Mercuric chloride paper: Dissolve about 25 g mercuric chloride (HgC12) in 100 mL ethyl alcohol. 

Place some filter paper (Black Ribbon, S and S 

No. 589) in the solution for 1 hour. Remove, and 

allow the paper to dry in the air. Cut into 1f2-

inch squares and store in a box. 

Potassium hydroxide, pellets. 

Stannous chloride, 10 percent: Dissolve 10 g 

stannous chloride (SnC12 ·2H20) in 100 mL concentrated hydrochloric acid. 

Zinc, mossy or shot. 

APPARATUS 

Burner 

Heating block; drilled to accommodate 25-mm test 

tubes 

Modified Gutzeit apparatus (fig. 1): To assemble 

the apparatus, insert the unflanged end of the 

glass pipe into the rubber stopper. Place a 

square of mercuric chloride paper on the flanged 

end. Attach a second glass pipe by clamping the 

flanged ends together with a joint clamp. Insert 

the rubber stopper into the test tube when the 

solution is prepared for arsine evolution. The apparatus is conveniently held upright by a funnel 

rack. For efficient operation in mass-production 

determinations, 24 units of the apparatus are required. Additional test tubes are required for fusion of the samples. Only two to four fusions 

may be made in each test tube before it must 

be discarded. When the black lead sulfide 
FIGURE 1.-Modified Gutzeit apparatus. A, two sections of glass pipe, 0.5-inch O.D.; B, glass wool 

coated with lead acetate; C, no. 4 rubber stopper 

with center hole 0.5 inch in diameter; D, test tube 

with rim, 25X150-mm; E, mercuric chloride paper; 

F, surgical tubing 0.2x0.5-inch O.D.; G, joint 

clamp. 

formed by the reaction has discolored the lower 

half of the glass wool, the glass wool should be 

removed from the apparatus and replaced by 

fresh glass wool. 

Test tubes, 25 x 150-mm; with rim 

Vortex mixer 

7 

STANDARDS 

Stock arsenic solution, 1,000 j.Lg/mL: Dissolve 0.13 

g arsenic trioxide in 2 mL of 1 M sodium hydroxide, dilute with water to about 50 mL, make 

slightly acid by adding 3 mL of 1 M hydrochloric 

acid, and dilute to 100 mL. 

Dilute arsenic solution, 100 j.Lg/ml: Dilute 10 mL 

of 1,000 j.Lg/mL stock arsenic solution to 100 mL 

with water. 

Standard arsenic solution, 10 j.Lg/mL: Dilute 10 

mL of 100 j.Lg/mL dilute arsenic solution to 100 

mL with water. 

Arsenic color-spot standards, 1, 2, 4, 8, 12, and 

16 j.Lg: Add 0.1, 0.2, 0.4, 0.8, 1.2, and 1.6 mL 

of 10 j.Lg/mL standard arsenic solution, respectively, to six 25x 150-mm test tubes. Prepare 

standards in triplicate. Add 10 mL of 30 percent 

hydrochloric acid and proceed with step 6 of procedure. Select the average (mid-color) spot of 

each spot standard representing the 1-lg of arsenic being considered. This will give corresponding 10-, 20-, 40-, 80-, 120-, and 160-ppm colorspot standards. Store color standards between 

pages of a book and expose to light as little as 

possible. Standards should be prepared weekly 

or more often if needed. 

PROCEDURE 

1. Weigh 0.1 g sample into a 25 x 150-mm test 

tube. 

2. Add 8 pellets (about 0.8 g) of potassium hydroxide. 

3. Heat the tube containing sample and potassium hydroxide over a burner for about 20 

seconds or until the potassium hydroxide is 

melted. A void continued heating of the 

tube after the flux has melted because potassium hydroxide attacks glass. Remove 

the tube from the flame and rotate the tube 

so that the melt will cool in a thin film on 

the side of the tube. 

4. When the tube is cool, add 10 mL of 30 percent (v/v) hydrochloric acid to the contents 

and let stand until the melt disintegrates 

(approximately 30 minutes). 

5. Mix on a vortex mixer. 

6. Add 0.5 mL of 10 percent stannous chloride 

and mix on a vortex mixer. If a yellow color 

is present, add additional 10 percent stannous chloride until the yellow disappears. 
This solution should not be left standing because any silica present will form a gel. 

7. Add 2-4 g zinc (mossy or shot), and immediately attach the test tube to the rubber stopper on the modified Gutzeit apparatus. 

8. After 30 minutes, remove the disk of mercuric 

chloride paper and compare as soon as possible with the arsenic color-spot standards. 

9. For samples with more than 160 ppm, it will 

be necessary to weigh and fuse the sample 

again. Add 10 mL of 30 percent (v/v) hydrochloric acid, then transfer a suitable aliquot 

to another 25 x 150-mm test tube containing 

10 mL of the 30 percent (v/v) hydrochloric 

acid and proceed to step 6 of procedure. 

SULFIDE PROCEDURE 

Sulfides can often be recognized in the sample 

or will form a cloudy, grey precipitate in the acid 

and give off hydrogen sulfide fumes. If high sulfide concentrations are taken through step 8 of 

the procedure, the glass wool will tum black. Use 

this sulfide procedure to prevent interference 

from high sulfide content. 

1. Add 0.3 mL of 8 percent hydrogen peroxide 

to a fused sample containing 10 mL of 30 

percent hydrochloric acid. Mix using a vortex mixer. 

2. Place in heating block and heat gently to not 

more than 65°C until bubbling has nearly 

ceased. 

3. Mix using a vortex mixer, cool, and proceed 

to step 6 of the previous procedure. If the 

sample still blackens the glass wool, reprepare sample and add 0.5 mL instead of 0.3 

mL of 8 percent hydrogen peroxide. 

DISCUSSION 

Many of the heavy metals in amounts of 50 mg 

or more interfere with the arsenic test, but their 

presence in soils in such high concentrations is 

most unlikely even in mineralized areas. Lower 

readings may result from arsenic that remains behind instead of being swept out of the test tube, 

and by arsine adsorption on the lead acetate. Although variation in the rate of hydrogen evolution 

causes errors in the readings of the conventional 

"strip test", they are much less serious for the 

confined spot method described. The difference in 

arsenic content of soil in a mineralized area and 

8 

that of soil in an unmineralized area is normally 

so great that there is little danger that the prospector will be misled by the small differences resulting from the errors implicit in this method. 

Leaks in the apparatus that allow arsine to escape 

without coming in contact with the mercuric 

chloride will cause lower values, but this can be 

avoided by proper assemblage. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[L( ), detected, but below limit of determination,or below value shown] 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 200--320 264 20.33 

II 10 10 0 

III 2,400--3,200 2,960 12.09 

IV 80-120 96 27.16 

v L(10) L(lO) 0 

VI 200-240 208 12.88 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 65~1. 

Almond, Hy, 1953, Field methods for the determination of 

traces of arsenic in soils--Confined-spot procedure using 

a modified Gutzeit apparatus: Analytical Chemistry, v. 25, 

no. 11, p. 1766. 

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. Geological Survey Bulletin 

1152, 100 p. 

ARSENIC, SPECTROPHOTOMETRIC 

METHOD 

The method described here was adapted by 

Welsch (1979) from Kopp (1973) and Marshall 

(1978). 

SUMMARY 

Geologic materials are decomposed by a mixture 

of aqua regia and hydrofluoric acid. Arsenic is reduced to the trivalent state with potassium iodide. 

Interferences are minimized by the addition of 
iron II and stannous chloride. Arsine, produced 

by the addition of zinc shot, is bubbled through 

a chloroform solution of silver diethyldithiocarba- -c 

mate and ephedrine. The resultant red complex 8 -

is used to determine trace amounts of arsenic by 

visible spectrophotometry. About 40 samples can 

be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

AgDDC reagent (silver diethyldithiocarbamate): 

Dissolve 3.4 g silver nitrate and 4.5 g NaDDC 

(sodium diethyldithiocarbamate) in separate 

beakers containing 200 mL water. Chill to soc 

or below. Combine the solutions while mixing 

vigorously. Vacuum filter and wash the resulting precipitate of AgDDC with two 100-mL portions of cold water. Dry the precipitate in a desiccator. Yield is about 4.5 g AgDDC. 

AgDDC-ephedrine solution: Dissolve 0.41 g of !-

ephedrine (formula weight 165.2, available from 

Aldrich Chemical Co., Milwaukee, Wis.) and 

0.625 g AgDDC reagent in 250 mL chloroform. 

Hydrochloric acid, concentrated. 

Hydrochloric acid, 1 M: Dilute 83 mL concentrated hydrochloric acid to 1 L with water. 

Hydrofluoric acid, 48 percent. 

Iron solution, 1 percent: Dissolve 7 g ferrous 

ammonium sulfate hexahydrate (Fe(NH4)2 

(S04)2·6H20) and 0.1 g ascorbic acid in 100 mL 

water. 

Nitric acid, concentrated. 

Potassium iodide, 40 percent: Dissolve 40 g potassium iodide in 100 mL water. 

Stannous chloride, 20 percent: Dissolve 20 g 

stannous chloride (SnCl2·2H20) in 100 mL concentrated hydrochloric acid. 

Zinc, shot. 

APPARATUS 

Arsine generation assembly (fig. 2) 

Centrifuge 

Hot plate 

Spectrophotometer 

Teflon beaker, 50-mL 

Teflon policeman 

Test tubes, 12 x 75-mm 

Test tubes, 16 x 150-mm 

Test tubes, 25 x 150-mm 

9 

-D 

-E 

A-

-F 

FIGURE 2.-Arsine generation assembly. A, test tube, 

25 x 150-mm with rim; B, no. 4 rubber stopper with center hole 5 mm diameter; C, glass tubing, 5-mm O.D.; 

D, tygon tubing, 5-mm I.D.; E, gas dispersion tube B 

porosity fritted glass tip, 5-mm O.D. x 10-mm, available 

from Ace Glass Inc., Vineland, N. J.; F, test tube, 

12x75-mm. 

STANDARDS 

Stock arsenic solution, 1,000 JJ..g/mL: Dissolve 

1.300 g arsenic trioxide in 20 mL of 1 M sodium 

hydroxide, add 500 mL water and 30 mL of 1 

M hydrochloric acid, and dilute to 1 L with 

water. 

Stock arsenic solution, 100 JJ..g/mL: Dilute 10 mL 

of 1,000 JJ..g/mL stock arsenic solution to 100 mL 

with water. 

Stock arsenic solution, 10 J.Lg/mL: Dilute 10 mL 

of 100 JJ..g/mL stock arsenic solution to 100 mL 

with water. 

Arsenic working standards: Pipette (in triplicate) 

0, 0.1, and 0.5 mL of 10 JJ..g/mL stock arsenic 

solution and 0.1, 0.25, and 0.5 mL of 100 J.Lg/mL 

stock arsenic solution, respectively, into six 
25 x 150-mm tubes and add 10 mL of 1 M hydrochloric acid to each tube, then proceed to step 

9 of the procedure. 

PROCEDURE 

1. Weigh 1.0 g sample into a 50-mL Teflon 

beaker. 

2. Add 2 mL concentrated nitric acid, 6 mL 

concentrated hydrochloric acid, and 15 mL 

of 48 percent hydrofluoric acid. 

3. Place beaker on a hot plate set at 110°C and 

evaporate to dryness. 

4. Add 5 mL of 1 M hydrochloric acid and warm 

to about 50°C. 

5. Loosen the residue with the aid of a Teflon 

policeman and transfer to a 16x 150-mm 

tube. 

6. Rinse the beaker with another 5 mL of 1 M 

hydrochloric acid and transfer to the 

16x 150-mm tube. 

7. Mix the contents of the tube and centrifuge. 

8. Transfer a 5-mL aliquot from the 16 x 150-

mm tube into a 25 x 150-mm tube. 

9. Add 5 mL of 1 M hydrochloric acid, 1 mL 

of 1 percent iron solution, 1 mL of 40 percent potassium iodide solution, and 1 mL 

of 20 percent stannous chloride solution to 

the 25 x 150-mm tubes containing samples 

and arsenic working standards. 

10. Mix the contents of the tubes and let stand 

for 15 minutes. 

11. Add 3 mL AgDDC-ephedrine solution to a 

12 x 75-mm tube for each sample and standard. 

12. . Add four pieces of zinc shot to the 25 x 150-

mm tube and immediately connect it to the 

arsine generation assembly as shown in figure 2. 

13. Allow the resulting hydrogen and arsine to 

bubble through the AgDDC-ephedrine solution for 30 minutes. 

14. Estimate the arsenic in the AgDDC-ephedrine solution by comparing the absorbance 

at 540 nm for sample and standards using 

a spectrophotometer. 

STANDARDIZATION OF INSTRUMENT 

The absorbances at a wavelength of 540 nm for 

the standard solutions are used to construct a calibration curve. Arsenic in the samples is estimated 

by comparing the absorbance of the sample to the 

calibration curve. The following absorbance values 

have been obtained using a Bausch and Lomb 

model spectronic 100 spectrophotometer. 

Standard 

(j.Lg) 

1.0 

5.0 

10.0 

25.0 

50.0 

[Lower limit of determination, 1 ppm] 

Absorbance 

units 

0.044 

.220 

.456 

1.100 

2.204 

DISCUSSION 

Corresponding sample 

concentration (ppm) 

2.0 

10.0 

20.0 

50.0 

100.0 

The digestion with aqua regia and hydrofluoric 

acid, in addition to rendering arsenic soluble, destroys organic material, dispels sulfur compounds, 

subdues frothing, and destroys nitrate ions that 

interfere with the evolution of arsine. Iron solution is added to insure a uniform rate of hydrogen 

evolution from sample to sample. The addition of 

potassium iodide and stannous chloride reduces 

arsenic to the trivalent state and minimizes interferences from copper, cobalt, nickel, chromium, 

and molybdenum. As much as 2,000 f.Lg of these 

elements may be present without seriously affecting the recovery of arsenic. Antimony, tellurium, 

mercury, and selenium also interfere; however, 

they are not normally present in concentrations 

that cause significant interference. The AgDDC 

reagent must be prepared as needed, otherwise 

it is difficult to dissolve and precipitates when the 

hydrogen is bubbled through it. The dispersion 

tubes should be rinsed with acetone after each use 

and should be cleaned in hot dichromate cleaning 

solution weekly. Samples that give absorbance 

readings higher than that of the highest standard 

may be rerun using a smaller aliquot of the sample 

solution to generate the arsine. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

10 
GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 360--490 432 12 

II 18-21 19 7 

III 3,600-4,400 4,020 7 

IV 110-130 123 7 

v 9-12 11 13 

VI 350-370 362 2 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Kopp, J. F., 1973, !-Ephedrine in chloroform as a solvent for 

silver diethyldithiocarbamate in the determination of arsenic: Analytical Chemistry, v. 46, no. 9, p. 1786-1787. 

Marshall, N. J., 1978, Colorimetric determination of arsenic 

in geochemical samples: Journal of Geochemical Exploration 10, p. 307-313. 

Welsch, E. P., 1979, Determination of arsenic in geologic materials with silver diethyldithiocarbamate: U.S. Geological 

Survey Open-File Report 79-1442, 10 p. 

BISMUTH, CADMIUM, COPPER, LEAD, 

SILVER, AND ZINC, 

ORGANIC EXTRACTION METHOD 

The method described here, excepting minor 

changes, was developed by Viets (1978). 

SUMMARY 

Interferences commonly encountered in the determination of bismuth, cadmium, copper, lead, 

silver, and zinc at crustal abundance levels are effectively eliminated using a rapid, sensitive, organic extraction method. A potassium chloratehydrochloric acid digestion solubilizes the metals 

not tightly bound in the silicate lattice of rock, 

soil, and stream-sediment samples. The six metals 

are selectively extracted into an organic phase of 

10 percent tricaprylylmethylammonium chloridemethyl isobutyl ketone in the presence of ascorbic 

acid and potassium iodide. 

Metals in the organic extract are determined by 

flame atomic absorption spectrometry to the 0.05 

ppm level for cadmium and silver and to the 1. 0 

ppm level for bismuth, copper, lead, and zinc. 

About 40 samples can be analyzed per day for the 

six metals. If only selected metals are determined, 

more samples can be analyzed. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free, unless otherwise 

indicated. 

Acetone, technical grade. 

Aliquat 336-MIBK, 10 percent v/v: Pour 300 mL 

Aliquat 336 (tricaprylylmethylammonium 

chloride) [obtainable from General Mills Chemical Division, Minneapolis, Minn.] into a clean 1-L 

graduated cylinder, dilute to 1 L with MIBK 

(methyl isobutyl ketone), and swirl to dissolve 

the Aliquat 336. Pour into a brown glass bottle, 

add additional 2,000 mL of MIBK, and shake. 

This solution is stable for at least 1 month. 

Ascorbic acid-potassium iodide solution, 20 percent-10 percent w/v: Weigh 200 g ascorbic acid 

(U.S.P. food grade or metal-free equivalent) and 

100 g potassium iodide into a brown glass bottle, 

add 1 L distilled water, and shake until dissolved (prepare weekly). Some potassium iodide 

has been found to contain cadmium and silver. 

However, if samples, standards, and blanks are 

prepared using the same bottle of reagent, small 

amounts of contaminants will not be a problem. 

Hydrochloric acid, concentrated. 

Potassium chlorate. 

APPARATUS 

Atomic absorption spectrophotometer 

Boiling chips 

11 

Centrifuge; 16-mm capacity head 

Heating block; drilled to accommodate 16-mm test 

tubes 

Horizontal shaking machine 

Powder funnel 

Scoop, 1-g plastic or aluminum 

Silicone rubber or polyethylene stoppers for 16-

mm test tubes 

Test tubes, disposable, 16x 150-mm 

STANDARDS 

Stock bismuth, cadmium, copper, lead, silver, and 

zinc solutions, 1,000 J,Lg/mL: Accurately weigh 

the following amounts of pure metal oxides into 

separate 100-mL class-A volumetric flasks. 

bismuth-0.1115 g bismuth oxide (Bi20 3) 

cadmium-0.1142 g cadmium oxide (CdO) 

copper-0.1252 g copper oxide (CuO) 

lead-0.1077 g lead oxide (PbO) 
silver-0.1074 g silver oxide (Ag20) 

zinc-0.1245 g zinc oxide (ZnO) 

Add two 1-g scoops of potassium chlorate and 

10 mL concentrated hydrochloric acid to all six 

flasks and allow to react for 1 hour. Place the 

flasks in a boiling water bath for 2 hours or until 

the solutions become colorless and the oxides are 

completely dissolved. The copper solution will be 

yellow but the other five standards should be 

nearly colorless, indicating that all yellow reaction products have been volatilized. For silver 

it may be necessary to add about 80 mL of 10 

N hydrochloric acid and heat on the water bath 

for several more hours to effect solution. Once 

dissolved, allow standards to cool and dilute to 

100 mL with 10 N hydrochloric acid. 

Combined standard solutions: Prepare three combined standards in 10 N hydrochloric acid from 

the individual1,000 f.Lg/mL stock solution to contain the following amounts of the six metals. 

Metal Std. 1 Std. 2 Std. 3 

(IJ.g/mL) (IJ.g/mL) (IJ.g/mL) 

Bi~muth 20.0 50.0 100 

Cadmium 2.00 5.00 10.0 

Copper 20.0 50.0 100 

Lead 20.0 50.0 100 

Silver 2.00 5.00 10.00 

Zinc 20.0 50.0 100 

Standard solutions in Aliquat 336-MIBK: Add 1.0 

mL each of Std. 1, Std. 2, and Std. 3 to three 

16x 150-mm test tubes containing boiling chips. 

Add 1 g potassium chlorate and slowly add 3 

mL concentrated hydrochloric acid and then 

treat as a normal sample continuing at step 4 

of the procedure section. Also prepare a blank 

with each set of standards. If all six metals are 

to be determined, two sets of standards should 

be prepared for every 50 samples. 

Working organic blank: Shake 300 mL of 10 percent Aliquat 336-MIBK solution with 100 mL of 

3 N hydrochloric acid in a 500-mL reagent bottle. 

PROCEDURE 

1. Weigh 1.00 g sample into a 16x 150-mm disposable test tube containing a clean boiling 

chip. Include a tube with no sample as a 

blank with each set of samples. 

12 

2. Add approximately 1 g potassium chlorate 

using a scoop and mix thoroughly. 

3. Slowly add four 1-mL portions of concentrated 

hydrochloric acid, allowing ample time for 

the reaction to subside between additions. 

Mix the contents of the tubes after the first 

and last addition of acid. Samples containing 

appreciable amounts of carbonate or organic 

materials may require a powder funnel to 

contain the foaming digestion solution. The 

residue in the funnel is rinsed into the tube 

after step 4 with the ascorbic acid-potassium iodide solution. 

4. After 30 minutes or longer, place the tubes 

in a heating block and bring to a moderate 

boil for 2 minutes or until all yellow fumes 

have evolved and the blank is colorless. 

5. Allow samples to cool, add 8 mL ascorbic acidpotassium iodide solution, and mix thoroughly. 

6. Accurately add 5. 0 mL of the Aliquat 336-

MIBK solution to the tube, which is then 

capped with a silicone rubber or 

polyethylene stopper and shaken on a horizontal shaker for 5 minutes. 

7. Centrifuge samples to separate all the organic 

phase. Estimate the metal content of the organic phase by flame atomic absorption 

spectrometry. If the metal content of a sample exceeds that of the standards, make a 

one-in-ten dilution using the Aliquat 336-

MIBK solution. If necessary, make further 

dilutions until the sample's metal content is 

below that of the highest standard. 

STANDARDIZATION OF INSTRUMENT 

The working organic blank is aspirated between 

samples and used to zero the instrument. If the 

reagent blanks read a higher absorbance than the 

working organic blank, they are used for zeroing 

the instrument as they will "blank out" any reagent contamination. The working organic blank 

should be used between samples to purge the 

burner system of the previous samples and maintain the burner's thermal stability. 

Typical absorbance values are given in the following table to check instrument performance for 

mid-range (Std. 2) and upper standards (Std. 3). 

Std. 1 should be within the linear range of most 

instruments, whereas Std. 2 and Std. 3 will require curve correction. Copper may be run with 
the burner perpendicular or parallel to light path. 

Zinc must be run with the burner perpendicular 

for proper calibration, as Std. 1 will not be in the 

linear range if the burner is parallel. 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

These conditions are the same for all six metals: 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Source lamp . . . . . . . . . . . . . . . . . . hollow cathode lamp 

Integration time . . . . . . . . . . . . . . . . . . . . . . . 1 second 

Flame conditions ............... oxidizing (lean blue) 

These conditions differ: 

[Star ("'), burner parallel to light path; dagger (t), burner perpendicular to light 

path] 

Wavelength (nm) ..... 

Slit (nm) ...••.•.... 

Lamp current (rnA) .. . 

Burner (em) ....... . 

Background correction 

X J.Lg/mL= Y absorbance 

units ........... . 

Lower limit of 

determination . . . . . . 

Wavelength (nm) .... . 

Slit (nm) .......... . 

Lamp current (rnA) .. . 

Burner (em) ....... . 

Background correction 

X J.Lg/mL= Y absorbance 

units ........... . 

Lower limit of 

determination . . . . . . 

Bismuth 

223.0 

0.2 

8 

10* 

Cadmium 

228.8 

0.7 

8 

10* 

Copper 

324.7 

0.7 

8 

lOt 

yes no no 

10=0.280 1=0.440 10=0.105 

20=0.540 2=0.680 20=0.200 

1.0 

Lead 

383.3 

0.7 

8 

10* 

0.05 

Silver 

328.1 

0.7 

12 

10* 

1.0 

Zinc 

213.9 

0.7 

8 

lOt no no no 

10=0.215 2=0.230 10=0.290 

20=0.380 2=0.440 20=0.520 

1.0 0.05 1.0 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

13 

[N( ), not detected at limit of determination, or at value shown. Data were recorded 

in multiples of lower reporting limits, rounded down, and reported at two significant digits] 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

Range 

(ppm) 

1,600-1,800 

N(l) 

N(l) 

20-22 

N(l) 

N(l) 

2.4-2.7 

3.5-3.9 

0.35-0.40 

0.3lH>.40 

0.10-0.15 

0.10-0.10 

1,000-1,100 

6S-72 

10-12 

6,800-7,000 

360-390 

55-60 

570-590 

610-660 

11-13 

43-44 

14-16 

79-84 

32--34 

17-19 

0.10 

3.6-3.8 

0.70-0.80 

0.2lH>.30 

610-630 

420-450 

180-200 

6S-72 

36-40 

78-88 

Mean 

(ppm) 

Bismuth 

1,700 

N(l) 

N(l) 

21.2 

N(l) 

N(l) 

Cadmium 

2.56 

3.76 

0.37 

0.36 

0.14 

0.10 

Copper 

1,080 

70.4 

11 

6,880 

370 

58 

Lead 

580 

630 

12 

43.4 

15 

81.2 

Silver 

32.8 

17.8 

0.10 

3.7 

0.76 

0.29 

Zinc 

622 

432 

188 

70.8 

37.6 

82.6 

Percent relative 

standard deviation 

4.1 

0 

0 

3.9 

0 

0 

4.4 

4.5 

7.5 

6.2 

15.9 

0 

4.1 

2.4 

6.4 

1.2 

3.3 

4.7 

1.2 

3.4 

5.9 

1.3 

2.2 

2.7 

2.6 

4.7 

0 

1.9 

5.5 

7.7 

1.3 

3.0 

4.4 

2.5 

4.5 

4.8 
DISCUSSION 

The potassium chlorate-hydrochloric acid digestion dissolves most metals not bound or occluded 

in insoluble silicates, desirable for most geochemical exploration purposes. However, a hydrofluoric 

acid digestion may be used to obtain a "total" digestion (Viets, 1978). 

Because the Aliquat 336-MIBK solution is 

sticky, disposable test tubes are used to avoid 

tedious clean up. Stoppers and glassware, however, may be easily cleaned with acetone or other 

water-miscible organics such as ethanol or 

methanol. 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Viets, J. G., 1978, Determination of silver, bismuth, cadmium, 

copper, lead, and zinc in geological materials by atomic 

absorption spectrometry with tricaprylylmethylammonium 

chloride: Analytical Chemistry, v. 50, no. 8, p. 1097-1101. 

CALCIUM, LITHIUM, MAGNESIUM, 

POTASSIUM, AND SODIUM, 

ATOMIC ABSORPTION METHOD 

The method described here was developed by 

Meier (1980). 

SUMMARY 

Calcium, lithium, magnesium, potassium, and 

sodium are determined by atomic absorption spectrophotometry in rock, soil, and stream-sediment 

samples. The sample is decomposed by heating in 

hydrofluoric acid. The residue formed upon decomposition is dissolved by hydrochloric acid with 

the addition of boric acid solution to complex 

fluorides that may lead to incomplete dissolution 

of calcium, lithium, and magnesium. Lanthanum 

chloride is added to mask interference in the determination of calcium and magnesium. 

The following concentration ranges can be determined using the method: calcium, 0.1 to 50 percent; lithium, 5 to 2,500 ppm; magnesium, 0.1 to 

5.0 percent; potassium, 0.005 to 6.0 percent; sodium, 0.005 to 3.0 percent. The sensitivity of atomic 

absorption for these elements allows considerable 

latitude in sample dilution to adjust the concentration of the element of interest to a level suitable 

for measurement. About 50 samples can be 

analyzed per day. 

14 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Boric acid solution: Dissolve 50 g boric acid in 1 

L water. Warm to aid dissolution. 

Hydrochloric acid, 6 N: Dilute 500 mL concentrated hydrochloric acid to 1 L with water. 

Hydrofluoric acid, 48 percent. 

Lanthanum chloride solution, 10 percent lanthanum: Weigh 58.7 g lanthanum oxide into a 1-L 

beaker and wet with water. Cautiously, add 100 

mL concentrated hydrochloric acid in small portions. When the oxide is completely dissolved, 

slowly dilute to 500 mL with water while stirring to avoid overheating. 

Lanthanum chloride solution, 1.1 percent lanthanum: Dilute 110 mL of the lanthanum chloride 

solution (10 percent lanthanum) to 1 L with 

water. 

APPARATUS 

Atomic absorption spectrophotometer 

Hot plate 

Teflon beaker, 50-mL; molded of TFE Teflon 

Volumetric flask, 100-mL 

STANDARDS 

Stock calcium solution, 1,000 f.Lg/mL: Dissolve 

2. 497 g dried calcium carbonate (overnight at 

100°C) in a mixture of 50 mL water and 10 mL 

of 6 N hydrochloric acid. Dilute to 1 L with 

water. 

Stock calcium solution, 100 f,Lg/mL: Dilute 10 mL 

of 1,000 f,Lg/mL stock calcium solution to 100 mL 

with water. 

Calcium working standard solution, 0, 2, 5, 10, 

50, and 100 f.Lg/mL: Pipette 0, 2, 5, and 10 mL 

of 100 f.Lg/mL stock calcium solution and 5 and 

10 mL of 1,000 f,Lg/mL stock calcium solution 

into separate 100-mL volumetric flasks containing 10 mL lanthanum chloride solution (10 percent lanthanum), and dilute to 100 mL with 

water. 

Stock lithium solution, 1,000 f,Lg/mL: Dissolve 

9. 935 g lithium nitrate in water and dilute to 

1 L with water. 

Stock lithium solution, 100 f.Lg/mL: Dilute 10 mL 

of 1,000 f,Lg/mL stock lithium solution to 100 mL 

with water. 
Lithium working standard solution, 0, 1, 3, and PROCEDURE 

1. Weigh 0.20 g sample into a 50-mL Teflon 

beaker. 

5 IJ.g/mL: Pipette 0, 1, 3, and 5 mL of 100 IJ.g/mL 

stock lithium solution into separate 100-mL volumet~c ~asks containing 5 mL of 6 N hydrochione acid and 5 mL boric acid solution and 2· 

dilute to 100 mL with water. ' 

Stock magnesium solution, 1,000 fJ.g/mL: Dissolve 3· 

1. 000 g pure metallic magnesium in a minimum 

amount of 6 N hydrochloric acid (about 15 mL) 

and dilute to 1 L with water. 

Stock magnesium solution, 100 fJ.g/mL: Dilute 10 4· 

mL of 1,000 !J.g/mL stock magnesium solution 

to 100 mL with water. 5. 

Stock magnesium solution, 10 fJ.g/mL: Dilute 10 

mL of 100 fJ.g/mL stock magnesium solution to 6· 

100 mL with water. 

Magnesium working standard solution, 0, 0.2, 0.5, 

1.0, 5.0, and 10.0 fJ.g/mL: Pipette 0, 2, 5, and 

10 mL of 10 fJ.g/mL stock magnesium solution 7· 

and 5 and 10 mL of 100 fJ.g/mL stock magnesium 

solution into separate 100-mL volumetric flasks 

containing 10 mL lanthanum chloride solution 8· 

(10 percent lanthanum), and dilute to 100 mL 

with water. 

Stock potassium solution, 1,000 fJ.g/mL: Dissolve 

1. 907 g potassium chloride in water and dilute 

to 1 L with water. 

Stock potassium solution, 100 fJ.g/mL: Dilute 10 

mL of 1,000 J.Lg/mL stock potassium solution to 

100 mL with water. 

Potassium working standard solution 0 2 6 12 ' ' ' ' ' 

Add 15 mL of 48 percent hydrofluoric acid to 

the beaker. 

Place the beaker on a hot plate set at high 

(about 250°C) and heat until all of the acid 

has been boiled off and all fumes have subsided. 

Add 5 mL of 6 N hydrochloric acid and heat 

until acid begins to boil. 

Add 5 mL boric acid solution and heat until 

the solution comes to a slight boil. 

Remove the beaker from the hot plate and 

transfer quantitatively, with water, to a 

100-mL volumetric flask. Dilute to volume 

with water. Mix well by shaking. 

Dilute a 1-mL aliquot of the solution to 10 mL 

with lanthanum chloride solution (1.1 percent lanthanun1). 

Lithium, potassium, and sodium concentrations are estimated by using the dilution in 

step 6 of the procedure. Calcium and magnesium concentrations are estimated by 

using the dilution with lanthanum chloride 

added in step 7 of the procedure. Estimations are performed on an atomic absorption 

spectrophotometer. 

STANDARDIZATION OF INSTRUMENT 

20, 60, and 120 fJ.g/mL: Pipette 0, 2, 6, and 12 

mL of 100 J.Lg/mL stock potassium solution and 

2, 6, and 12 mL of 1,000 IJ.g/mL stock potassium 

solution into separate 100-mL volumetric flasks 

containing 5 mL of 6 N hydrochloric acid and 

5 mL boric acid solution, and dilute to 100 mL 

with water. 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

Stock sodium solution, 1,000 J.Lg/mL: Dissolve 

2. 542 g sodium chloride in water and dilute to 

1 L with water. 

Stock sodium solution, 100 fJ.g/mL: Dilute 10 mL 

of 1,000 J.Lg/mL stock sodium solution to 100 mL 

with water. 

Sodium working standard solution 0 1 3 6 10 

30, and 60 J.Lg/mL: Pipette 0, i, 3, 6, 'and 10 

mL of 100 fJ.g/mL stock sodium solution and 3.0 

and 6.0 mL of 1,000 J.Lg/mL stock sodium solution into separate 100-mL volumetric flasks containing 5 mL of 6 N hydrochloric acid and 5 mL 

boric acid solution, and dilute to 100 mL with 

water. 

15 

These conditions are the same for all five elements: 

Lamp current . . . . . . . . . . . . . . . . . . . . . . . . . 10 rnA 

Integration time . . . . . . . . . . . . . . . . . . . . . 3 seconds 

Burner ............................... 10 em 

Fu~l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . acetylene 

OXIdant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . air 

Flame condition .............. oxidizing (lean blue) 

These conditions differ: 

Calcium 

Grating . . . . . . . . . . . visible 

Wavelength (nm) . . . . 211 

Slit (nm) . . . . . . . . . . 4 

Source lamp . . . . . . . calcium 

Filter . . . . . . . . . . . . out 

Lithium 

visible 

335 

3 

lithium 

in 

Magnesium 

ultraviolet 

285 

4 

magnesium 

out 
Grating .......... . 

Wavelength (nm) ... . 

Slit (nm) ......... . 

Source lamp ...... . 

Filter ........... . 

Potassium Sodium 

visible 

383 

3 

potassium 

in 

visible 

295 

4 

sodium 

out 

By using the instrument parameters described 

in the preceding, the instrument is calibrated to 

read sample concentration directly by using the 

following standards and settings with burners 

parallel or perpendicular to source beam. 

Element, 

and burner 

orientation 

Calcium 

Parallel 

Perpendicular 

Lithium 

Parallel 

Magnesium 

Parallel 

Perpendicular 

Potassium 

Parallel 

Perpendicular 

Sodium 

Parallel 

Perpendicular 

Standard Absorbance 

(f.i.g/mL) units 

2.00 0.190 

5.00 .424 

10.00 .835 

50.0 .146 

100.0 . 292 

1.00 .170 

3.00 .488 

5.00 .766 

.5 .410 

1.0 .753 

5.0 .149 

10.0 . 291 

2.0 .242 

6.0 .585 

12.0 .946 

20.0 .105 

60.0 .311 

120.0 .543 

1.0 .285 

3.0 .709 

6.0 1.062 

10.0 .114 

30.0 .302 

60.0 .509 

DISCUSSION 

Set concentration 

(percent, lithium 

in ppm) 

1.0 

2.5 

5.0 

25.0 

50.0 

500 

1,500 

2,500 

.25 

.50 

2.50 

5.00 

.100 

.300 

.600 

1.00 

3.00 

6.00 

.05 

.150 

.300 

.50 

1.50 

3.00 

The procedure as presented is useful for decomposition and dissolution of geologic material for 

the determination of calcium, lithium, magnesium, 

potassium, and sodium by atomic absorption spectrophotometry. The sample weight used can be 

decreased or increased to accommodate lower or 

16 

higher concentrations of the elements of interest. 

Hydrofluoric acid effectively removes silicon as 

the volatile tetrafluoride and renders most mineral components soluble in hydrochloric acid and 

boric acid. If organic material is present in the 

sample, a black residue will be observed. However, this residue has little effect on the solubility 

of the elements of interest. The 6 N hydrochloric 

acid and boric acid solution can be conveniently 

added to the Teflon beakers without removing 

them from the hot plate. The hydrochloric acid 

and boric acid solution additions should come to 

a slight boil but should not be allowed to boil excessively as the acid concentration of the final solution can affect the estimation. The final dilution 

can be increased or decreased to accommodate the 

range of concentrations expected in the sample. 

The dilution for the determination of calcium and 

magnesium can also be increased or decreased to 

accommodate the range of concentration expected 

in the sample, but should contain 0.1 percent 

lanthanum in the final solution . 

INTERFERENCES 

Calcium: Phosphate, sulfate, silicate, and aluminum can interfere with the estimation of calcium, but are masked by the addition of lanthanum chloride. Boric acid solution is added to 

dissolve slightly soluble or complex fluorides of 

calcium that form in the decomposition with hydrofluoric acid . 

Lithium: Brown and others (1970) reported the 

following elements to interfere with the estimation of lithium when concentrations were in excess of: sodium, 1,000 fJ.g/mL; potassium, 100 

fJ.g/mL; magnesium, 200 tJ.g/mL; calcium, 200 

fJ.g/mL; chloride, 1,000 tJ.g/mL; sulfate, 2,000 tJ.g/ 

mL; nitrate, 100 tJ.g/mL; and strontium, 5 tJ.g/ 

mL. Boric acid solution is added to dissolve 

slightly soluble or complex fluorides of lithium 

that form in the decomposition with hydrofluoric 

acid. 

Magnesium: Silicate and aluminum can interfere 

with the estimation of magnesium but are 

masked by the addition of lanthanum chloride. 

Boric acid solution is added to dissolve slightly 

soluble or complex fluorides of magnesium that 

form in the decomposition with hydrofluoric 

acid. 

Potassium: Interferences are not common to potassium determination. 

Sodium: Interferences are not common to sodium 

determination, but contamination from reagents, 
the environment, and apparatus can be a problem. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

Range Mean 

(percent, lithium in ppm) 

0.88-0.92 

0.94--1.0 

11-12 

0.9S-1.1 

O.SS-1.0 

0.44--0.47 

Calcium 

0.90 

0.98 

11 

1.1 

0.90 

0.45 

Lithium 

6--8 7 

62-68 64 

134-138 135 

11-12 11 

45-48 47 

aS-39 38 

Magnesium 

0.23-0.25 0.24 

0.82--0.87 0.84 

0. 7S-0.86 0.82 

1.4-1.7 1.6 

1.0--1.2 1.1 

0.5S-0.64 0.62 

0.03-0.04 

1.2-1.3 

0.61--0.64 

3.7--3.9 

0.69--0.73 

1.5--1.6 

0.039--0.041 

0.50--0.53 

0.69--0.71 

0.46--0.48 

0.63-0.68 

0.09--0.10 

Potassium 

0.03 

1.3 

0.62 

3.8 

0.71 

1.5 

Sodium 

0.040 

0.51 

0.70 

0.47 

0.66 

0.10 

Percent relative 

standard deviation 

2.0 

2.3 

4.8 

5.4 

8.2 

3.6 

10.1 

3.6 

1.3 

4.0 

3.2 

1.4 

3.5 

2.1 

4.0 

7.8 

8.2 

4.0 

3.2 

2.1 

1.9 

1.8 

2.5 

1.7 

1.8 

2.8 

1.4 

1.5 

2.5 

1.8 

17 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-{)81. 

Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, 

Methods for collection and analysis of water samples for 

dissolved minerals and gases: Techniques of Water-Resources Investigation of the U.S. Geological Survey, book 

5, chapter A1, p. 107. 

Meier, A. L., 1980, A technique for the decomposition and 

dissolution of rocks and the determination of lithium, calcium, and magnesium using atomic absorption spectroscopy, 

in Shorter contributions to geochemistry, 1979: U.S. 

Geological Survey Professional Paper 1129 A-I, p. 11-5. 

COBALT, COPPER, LEAD, 

NICKEL, AND ZINC, 

ATOMIC ABSORPTION METHOD 

The method described here, excepting minor 

changes, was developed by Ward and others 

(1969). 

SUMMARY 

Cobalt, copper, lead, nickel, and zinc are determined in samples of rock, soil, stream sediment, 

and heavy-mineral concentrates by atomic absorption spectrophotometry. The sample is treated 

with hot concentrated nitric acid, which solubilizes 

all adsorbed ions and most common sulfides and 

oxides of these elements. Although this method 

is not a quantitative analysis, as silicates are only 

sparingly attacked on surface exposures, secondary dispersions and economic sulfide minerals are 

readily detected. The acid solution is diluted and 

aspirated into an atomic absorption spectrophotometer. About 70 samples can be analyzed 

per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Nitric acid, concentrated. 

Nitric acid, 8N: Dilute 500 mL concentrated nitric 

acid to 1 L with water. 

APPARATUS 

Atomic absorption spectrophotometer 

Boiling granules, metal-free 

Centrifuge 

Hotplate; fitted with aluminum heating block drilled to hold 16-mm test tubes 

Test tubes, disposable, 16x 150-mm 
STANDARDS 

Stock cobalt, copper, lead, nickel, and zinc solutions, 1,000 ,_.,g/mL: Dissolve the indicated 

weight of the dried metal or metal salt in a 500-

mL volumetric flask with 8 N nitric acid. 

cobalt--1.970 g cobalt chloride (CoC12·6H20) 

copper-1.964 g clear-copper sulfate 

(CuS04 ·5H20) 

lead--0.800 g lead nitrate (Pb(N03)2) 

nickel-2.48 g nickel nitrate 

(Ni(N 0 3)2 ·6H20) 

zinc--0.500 g zinc metal 

Dilute copper, lead, and zinc combined solution, 

100 t-tg/mL: Combine 10 mL of each 1,000 t-tg/mL 

stock copper, lead, and zinc solution and dilute 

to 100 mL with 8 N nitric acid. 

Standard copper, lead, and zinc combined solution, 20, 10, and 5 t-tg/mL: Dilute 20, 10, and 

5 mL of 100 t-tg/mL dilute copper, lead, and zinc 

solution to 100 mL with 8 N nitric acid. 

Dilute cobalt and nickel combined solution, 100 

t-tg/mL: Combine 10 mL of each 1,000 t-tg/mL 

stock cobalt and nickel solution and dilute to 100 

mL with 8 N nitric acid. 

Standard cobalt and nickel combined solution, 20, 

10, and 5 t-tg/mL: Dilute 20, 10, and 5 mL of 

100 ,_.,g/mL dilute cobalt and nickel solution to 

100 mL with 8 N nitric acid. 

PROCEDURE 

1. Weigh 1.0 g sample into a 16 x 150-mm disposable test tube containing a metal-free boiling granule. (Samples being analyzed for 

nickel should be roasted for 1 hour at 

450°C.) 

2. Carefully add 5 mL concentrated nitric acid 

and place tube in the heating block on a hot 

plate and boil for 30 minutes. 

3. Dilute the sample to 10 mL with water and 

bring back to boil. 

4. Remove tube from block, cool, and centrifuge. 

5. Estimate the metal content by atomizing the 

solution in an atomic absorption spectrophotometer. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

These conditions are the same for all five metals: 

18 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Lamp current . . . . . use current recommended on lamp 

Source lamp . . . . . . . . . . . . . . . . . hollow cathode lamp 

Integration time . . . . . . . . . . . . . . . . . . . . . . 1 second 

Burner . . . . . . . . 10 em (perpendicular to source beam) 

Flame condition : . . . . . . . . . . . . . oxidizing (lean blue) 

Lower limit of determination . . . . . . . . . . . . . 5.0 ppm 

These conditions differ: 

Cobalt Copper Lead 

Wavelength (nm) ..... 240.7 324.8 283.3 

Slit (nm) ........... .2 .7 .7 

5 .,._g/mL (absorbance 

units) ............ .152 .279 .043 

10 .,._g/mL (absorbance 

units) ............ .288 .523 .085 

Nickel Zinc 

Wavelength (nm) ..... 232.0 213.9 

Slit (nm) ........... .2 .7 

5 .,._g/mL (absorbance 

units) ............ .157 .075 

10 .,._g/mL (absorbance 

units) ............ .287 .144 

DISCUSSION 

Copper is often used to check instrument performance because it is one of the easiest of all 

metals to determine by atomic absorption spectrophotometry and is free from interferences. 

The determination of lead is generally free of 

interferences, but is affected by molecular absorption of the dissolved calcium in carbonates. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

Range 

(ppm) 

21-22 

13 

36--38 

21-22 

38--39 

22-23 

Mean 

(ppm) 

Cobalt 

21.2 

13 

36.6 

21.6 

38.6 

22.4 

Percent relative 

standard deviation 

2.11 

0 

2.44 

2.54 

1.42 

3.99 
I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

1,051-1,075 

78-79 

13 

6,480-6,740 

349--357 

62--64 

613--647 

676-717 

42-44 

52-56 

~24 

95-99 

33-34 

19 

45-48 

44-46 

77-80 

26-28 

711-745 

513--551 

170-190 

80-82 

45-47 

105-109 

Copper 

1,064 

78.2 

13 

6,624 

351 

63.2 

Lead 

623 

694 

43 

53.8 

23.2 

96.4 

Nickel 

33.8 

19 

46 

45 

78.2 

27 

Zinc 

724 

533 

182 

80.6 

45.8 

107.4 

REFERENCES CITED 

1.12 

.57 

0 

1.56 

1.07 

1.32 

2.14 

2.27 

1.64 

2.76 

1.93 

1.74 

1.32 

0 

2.66 

2.22 

1.67 

3.7 

1.92 

3.23 

4.60 

1.11 

1.83 

1.69 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659---{)81. 

Ward, F. N., Nakagawa, H. M., Harms, T. F., and VanSickle, 

G. H., 1969, Atomic absorption methods useful in 

geochemical exploration: U.S. Geological Survey Bulletin 

1289, 45 p. 

COPPER, ACID-EXTRACTABLE METHOD 

The method described here, excepting minor 

changes, was developed by Canney and Hawkins 

(1958). 

SUMMARY 

The amount of copper that is extracted from a 

soil or sediment by cold hydrochloric acid is estimated by comparing the intensity of the color of 

19 

the copper-2, 2' -biquinoline complex in an isoamyl 

alcohol extract against colored standards. Although the ambient temperature extraction yields 

only about 10-30 percent of the total copper in 

most samples, it has been found that the geochemical pattern of analytical data revealed by its use 

is often more significant than the geochemical pattern of total copper; and because the method is 

rapid and requires only simple, portable items of 

equipment, it is ideally suited for use under rough 

field conditions-at the sample site if desired. The 

method has a limiting sensitivity of about 1 ppm, 

which means that a positive test can usually be 

obtained on most samples. The method is not recommended for use on samples containing appreciable quantities of decaying organic material because soluble organic matter may dissolve in the 

organic solvent to yield a yellow solution that 

masks the pink or purple color of the copperbiquinoline complex. Much data can be collected 

in a short time because a single determination requires about 2 minutes. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Biquinoline reagent solution, 0.02 percent: Add 

0.2 g of 2, 2' -biquinoline to about 900 mL 

isoamyl alcohol in a beaker. Warm on a steam 

bath in a well-ventilated area away from open 

flame until the biquinoline is dissolved. Allow 

the solution to cool and dilute to 1 L with 

isoamyl alcohol. The solution should be colorless. 

If it is yellow, pink, or purple, the reagent is 

impure and should not be used. The biquinoline 

reagent is stable. 

Buffer solution: Dissolve 400 g sodium acetate 

trihydrate (NaC2H30 2·3H20), 100 g sodium tartrate (N~C ·2H 0), and 20 g hydroxylamine hydrochloride (NH20H·HCI) in 1 L 

metal-free water. If necessary, adjust the pH 

of this solution to between 6 and 7 with hydrochloric acid or sodium hydroxide solutions. To 

check for copper contamination, place 10 mL of 

buffer in a culture tube, add 1 mL of the 

biquinoline reagent, cap, and shake for 2 minutes. Absence of a pink coloration in the organic 

layer indicates that the solution is satisfactory 

for use. If contaminated, the buffer may be 

cleaned as follows. Dissolve 0.005 g dithizone 
(diphenylthiocarbazone) in 500 mL carbon tetrachloride. Shake the mixture intermittently 

over a period of about 1 hour to effect solution. 

Transfer the contaminated buffer solution and 50 

mL of the dithizone solution to a large separatory funnel, shake, allow the carbon tet1. 

the isoamyl alcohol, and stored in a dark place 

to prevent sunlight from fading the standards. 

PROCEDURE 

Place an 0.2-g scoop of the fine fraction of a 

soil or stream-sediment sample in a 22 x 175-

mm culture tube. 

Add about 1 mL of 6 M hydrochloric acid and 

slosh the sample in the acid for about 30 

seconds. 

Immediately add 10 mL buffer solution and 

2 mL biquinoline reagent. Cap the tube 

with a polyethylene stopper, and shake the 

tube vigorously for about 30 seconds. 

Allow the organic layer to separate, centrifuge if necessary. 

rachloride phase to collect in the bottom of the 

funnel, and discard. Continue this process with 2. 

additional 50-mL increments until the discarded 

organic phase is green. Some dithizone is dissolved in the buffer solution, imparting to it a 3. 

yellow color. This is removed by successive extractions with 50-mL portion of chloroform. 

Chloroform dissolved in the buffer solution is removed by shaking the buffer solution with 100 4. 

mL carbon tetrachloride and discarding the organic phase. 5. Estimate the copper concentration by comparing the color of the upper layer with that 

of the standards, using an illuminator when 

in the laboratory or a white card when in 

the field. Tip the tube 45° if film on glass 

obscures color. 

Hydrochloric acid, 6M: Dilute 500 mL concentrated hydrochloric acid to 1 L with water. 

APPARATUS 

Centrifuge 

Culture tube, 22 x 175-mm 

Illuminator or white card 

Scoop, 0.2-g 

Stopper, polyethylene 

STANDARDS 

Stock copper solution, 100 J.Lg/mL: Dissolve 0.2 g 

copper sulphate (CuS04 ·5H20) in 500 mL of 0.1 

M hydrochloric acid. 

Dilute copper solution, 10 J.Lg/mL: Dilute 10 mL 

of 100 J.Lg/mL stock copper solution to 100 mL 

with 0.1 M hydrochloric acid. 

Dilute copper solution, 1 J.Lg/mL: Dilute 10 mL of 

10 J.Lg/mL dilute copper solution to 100 mL with 

0.1 M hydrochloric acid. 

Standard copper solutions, 0, 1, 5, 10, 20, and 50 

ppm: Add 0.0, 0.2, and 1.0 mL of 1.0 J.Lg/mL 

dilute copper solution and 0.2, 0.4, and 1.0 mL 

of 10 J.Lg/mL dilute copper solution to six 

22 x 175-mm culture tubes, respectively. Add 1 

mL of 6 M hydrochloric acid to each tube and 

proceed with step 3 of the following procedure. 

These are equivalent to 0, 1, 5, 10, 20, and 50 

ppm copper contents in an 0.2-g sample. Midpoint concentrations may be estimated giving reporting intervals of 0, 1, 3, 5, 7, 10, 15, 20, 35, 

and 50 ppm. One set of standards should remain 

stable for several months provided the tubes are 

kept tightly capped to prevent evaporation of 

20 

DISCUSSION 

The acid-extractable copper test is specific for 

copper, no other metal will react with 2, 2'-

biquinoline. Copper must be in the form of Cu + 

to react in this test. If the reducing agent in the 

buffer deteriorates, low values will result. A standard sample should be analyzed at the start of 

each day as a check on the reagents and standards, and to insure comparable results from day 

to day. 

The complex formed with the biquinoline reagent is not a strong complex. Ions that form 

strong complexes with copper (for example, the 

ammonium ion) will prevent the copperbiquinoline complex from forming and therefore 

should not be present during the test. 

Coarse organic matter in some samples may interfere by accumulating at the water-alcohol interface. If this happens, the debris can usually be 

removed from the interface and dispersed 

throughout the buffer by manually or mechanically centrifuging the tube. Sometimes samples 

with appreciable organic material will give a yellow color, but this does not normally completely 

mask the pink or purple color given by copper. 

The sample size can be increased if the amounts 

of acid and buffer are increased proportionately 

to achieve a lower limit of determination. Con-
versely, the upper limit of the method can be extended by decreasing the sample size or by increasing the volume of biquinoline reagent. In 

either case, appropriate factors are required for 

conversion from micrograms of copper in the standards to parts per million in the sample. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[N.A., method not applicable to this sample medium] 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I N.A. N.A. N.A. 

II 15-20 18.0 15.2 

III 1-3 1.4 63.9 

IV N.A. N.A. N.A. 

v 150-200 190 11.8 

VI 10-15 11 20.3 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Canney, F. C., and Hawkins, D. B., 1958, Cold acid extraction 

of copper from soils and sediments-A proposed field 

method: Economic Geology, v. 53, no. 7, p. 877-886. 

FLUORINE, ION-SELECTIVE 

ELECTRODE METHOD 

The method described here, excepting minor 

changes, was developed by Hopkins (1977). 

SUMMARY 

Fluorine is determined in rock, soil, and streamsediment samples by an ion-selective electrode 

method. Samples are fused with a sodium 

carbonate-potassium carbonate-potassium nitrate 

flux and the fused sample is dissolved with citric 

acid. Sodium citrate buffer, which also serves as 

an ionic strength adjustor, is then added to this 

solution prior to determining the fluorine concentration by standard-additions technique. About 50 

samples can be analyzed per day. 

REAGENTS 

All reagents should be of reagent grade and all 

water should be metal free. 

Citric acid, 1 M: Dissolve 210.15 g citric acid 

monohydrate (C6H80 7• H20) in 1 L water. 

Sodium carbonate-potassium carbonate-potassium 

nitrate flux: Separately grind anhydrous sodium 

carbonate (N ~C0 ) and anhydrous potassium 

carbonate (K2C03) (previously oven dried at 

100°C overnight) and potassium nitrate to pass 

an 80-mesh sieve, and prepare a 4:4:2 mixture, 

by weight. 

Sodium citrate buffer, 1M: Dissolve 294 g sodium 

citrate dihydrate (Na3C6H50 7·2H20) in 800 mL 

water and adjust the pH to 6.0 with 6 N hydrochloric acid. Dilute this solution to 1 L. 

21 

APPARATUS 

Beaker, 100-mL, graduated polypropylene 

Magnetic stirrer (with stirring mechanism extended from motor to prevent heating of sample 

solution) 

Orion reference electrode; model90-01-00 

Orion specific ion electrode; model94-09A 

Orion specific ion meter; model 407 

Pyropot (Pyroco Products, Margat, Queensland, 

Australia 4019): Heating device accommodating 

10 test tubes, provides a controlled heat source 

for the fusion. 

Stirring bar, Teflon-coated magnetic 

Test tube, disposable, 25 x 150-mm 

STANDARDS 

Stock fluorine solution, 10,000 tJ.g/mL: Dissolve 

2.21 g sodium fluoride (previously dried at 110°C 

overnight) in 100 mL water. 

Standard fluorine solution, 1,000 tJ.g/mL: Dilute 10 

mL of 10,000 tJ.g/mL stock fluorine solution to 

100 mL with water (prepare monthly). 

Standard fluorine solution, 100 j.Lg/mL: Dilute 10 

mL of 1, 000 j.Lg/mL standard fluorine solution 

to 100 mL with water (prepare monthly). 

Standard fluorine solution, 10 tJ.g/mL: Dilute 10 

mL of 100 tJ.g/mL standard fluorine solution to 

100 mL with water (prepare monthly). 

Store all fluoride standards in polyethylene bottles. 
PROCEDURE 

1. Weigh 0.05 g sample and mix with 2 g sod~um 

carbonate-potassium carbonate-potassiUm 

nitrate flux in a disposable, 25 x 150-mm test 

tube. 

2. Fuse at 700°C for 10 minutes in a Pyropot. 

Remove fused sample and roll it on heat-resistant surface. 

3. Allow test tube to cool and add 15 mL of 1 

M citric acid. 

4. Allow the mixture to digest until carbon 

dioxide evolution is no longer detected (15 

minutes). 

5. Transfer to a 100-mL polypropylene beaker 

containing 25 mL sodium citrate buffer. 

6. Rinse the test tube with water, and add the 

rinse to the beaker. 

7. Dilute the sample solution to 100 mL with 

water and add a Teflon-coated magnetic 

stirring bar. 

STANDARDIZATION OF INSTRUMENT 

1. Add 1 mL of 1,000- or 100-J.Lg/mL sodium 

fluoride working standard to a 100-mL 

polypropylene beaker containing 15 mL of 

1 M citric acid and 2 g of the sodium carbonate-potassium carbonate-potassium nitrate 

flux. 

2. Then add 25 mL of 1 M sodium citrate buffer 

solution, dilute to 100 mL with water, and 

add a Teflon-coated magnetic stirring bar. 

3. Place the electrodes of the specific ion meter 

in the solution while it is gently stirred on 

a magnetic stirrer. 

4. Turn the meter's calibration control for a 

center-scale reading, and then add 1 mL of 

the 1 000- or 100-J.Lg/mL sodium fluoride 

workfug standard that was used initially. 

5. Adjust the temperature control to give a reading of 1 on the known-addition scale. 

6. Move the slope control until the sample temperature is read on the temperature scale. 

The percent of theoretical slope can be read 

on the slope scale. 

7. The instrument is now calibrated and ready 

to determine fluorine content. 

ESTIMATION 

1. Using a calibrated specific ion meter for estimation, rinse electrodes with water, blot 

them with absorbent tissue, and place them 

in the sample solution, which is gently being 

stirred. 

2. Turn the meter calibration control knob until 

the needle points to midscale. 

3. Pipette an appropriate aliquot (approximately 

the same total J.Lg fluorine as the sample) 

from one of the working sodium fluoride 

standard solutions. 

4. Allow approximately 5 minutes before taking 

a reading if the standard addition totals less 

than 100 J.Lg fluorine. Allow 3 minutes for 

100- to 500-J.Lg additions and 1-3 minutes for 

higher additions. 

5. Calculate the fluorine concentration in the 

sample by using the following formula: 

. (.,..g/mL fluorine in standard)(mL standard added)(meter reading)_ tluonneppm 

gofsample 

6. Reagent blanks are run with the samples to 

determine an average value of fluoride contamination. The average blank value is subtracted from the sample readings. 

7. If the sample is estimated to contain 100,000 

ppm or more fluorine, the solution containing citric acid and fused sample should be 

diluted to 100 mL with water. A suitable 

aliquot should be added to 25 mL of sodium 

citrate buffer, diluted to 100 mL, and estimated. 

DISCUSSION 

The fluoride-selective electrode responds to and 

develops a potential to the activity of free, unbound, uncomplexed fluoride ions in solution. The 

potential difference forms between the faces of a 

single lanthanum fluoride crystal, which separates 

an internal filling solution from the sample solution. The measured potential, E, is given by the 

simplified N ernst equation: 

E RT (1) 

where E is a constant potential characteristic of 

the elect~ode, R is the gas constant, F is the 

Faraday constant, T is the ab~olu~e te~perat~re, 

and aF is the activity of fluonde Ions In solutiOn. 

The relationship between the free fluoride ion concentration and the free fluoride ion activity is: 

(2) 

22 
where -y is the activity coefficient of the fluoride 

ion, f is the fraction of the total fluoride ion concentration that is free, and Ct is ~he total concentration of fluoride ion in all its forms-free com- ' plexed, or bound. 

The success of standard additions depends on 

making the addition in such a way that -y and f 

remain constant. The fluoride ion concentration 

can then be substituted for the activity in equation 

(1) and a linear relationship between the logarithm 

of the fluoride ion concentration and the measured 

potential is obtained. The activity coefficient, -y, 

remains constant if a large amount of noninterfering salt is added to maintain a high constant ionic 

strength. The fraction of the total concentration 

of fluoride ions that is free, f, is held constant 

by adding a large excess of a reagent, such as 

sodium citrate, which releases fluoride ions from 

any complexes formed with Al3 +, Fe3 +, Si4 +, and 

Be2 + ions. Sodium citrate buffer solution also provides enough noninterfering ions to maintain a 

high constant ionic strength background in the 

method described. The buffer adjusts the pH between 5 and 6 to dissociate any HF formed when 

the pH is less than 5 and to prevent hydroxide 

ion interference at pH values above 7. Citric acid 

dissolution of the fused sample gives additional sodium, potassium, and citrate ions for ionic 

strength and decomplexing. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 1,025-1,075 1,053 1.83 

II 238-276 257 5.51 

III 89' 950-96,850 93,370 2.72 

IV 2,305-2,365 2,333 .98 

v 142-159 153 4.62 

VI 151-161 156 2.50 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Hopkins, D. M., 1977, An improved ion-selective electrode 

method for the rapid determination of fluorine in rocks 

and soils: U.S. Geological Survey Journal of Research, v. 

5, no. 5, p. 589-593. 

GOLD, FLAME ATOMIC ABSORPTION 

METHOD 

The method described here, excepting minor 

changes, was developed by Thompson and others 

(1968) and described by Ward and others (1969). 

SUMMARY 

Gold is determined in samples of rock, soil, 

stream sediment, and heavy-mineral concentrates 

by atomic absorption spectrophotometry. The 

sample is treated with a hydrobromic acid-0.5 percent bromine solution. The gold-bromide complex 

that is formed is extracted from the acid solution 

with MIBK (methyl isobutyl ketone). The interference due to iron is removed by washing the 

organic solvent with dilute hydrobromic acid. The 

MIBK is atomized in an atomic absorption spectrophotometer for estimation of gold content. 

About 50 samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Hydrobromic acid-0.5 percent bromine solution: 

Add 5 mL bromine per liter of concentrated hydrobromic acid. 

Hydrobromic acid 0.1 N, equilibrated: Dilute 12 

mL concentrated hydrobromic acid to 1 L with 

water. Add an excess of MIBK, shake thoroughly, allow phases to separate, and discard MIBK 

layer. 

MIBK (methyl isobutyl ketone), 4-methyl-2--pentanone. 

APPARATUS 

Atomic-absorption spectrophotometer 

Centrifuge 

Culture tubes, 25 x 150-mm, screw-cap 

Horizontal shaking machine 

Hot plate 

Muffle furnace 

Porcelain evaporating dish, 60-mm diameter 

23 

STANDARDS 

Stock gold solution, 1,000 ,_.,g/mL: Dissolve exactly 

1.000 g gold in 100 mL hydrobromic acid-0.5 

percent bromine solution. Heat gently to expel 
excess bromine. Cool and dilute to 1 L with concentrated hydrobromic acid. 

Dilute gold solution, 100 JJ.g/mL: Dilute 10 mL of 

1,000 JJ.g/mL stock gold solution to 100 mL with 

concentrated hydrobromic acid. 

Standard gold solution in MIBK, 0.5, 1.0, and 2.0 

tJ.g/mL: Add 1.0, 2.0, and 4.0 mL of 100 tJ.g/mL 

dilute gold solution to three 200-mL volumetric 

flasks containing 200 mL MIBK. Shake vigorously. Transfer contents to three 500-mL reagent bottles containing 100 mL equilibrated 0.1 

N hydrobromic acid and shake. 

PROCEDURE 

1. Weigh 10 g sample into a porcelain evaporating dish. 

2. Ignite sample at 700°C in a muffle furnace for 

1 hour or until sulfides and organic matter 

are completely oxidized. 

3. Transfer sample to a 25 x 150-mm screw-cap 

culture tube containing 10 mL hydrobromic 

acid-0.5 percent bromine solution. 

4. Mix sample thoroughly and gently heat on a 

hot plate or in a water bath. 

5. When cool, add 10 mL MIBK and 10 mL 

water, cap, and shake in a horizontal shaking machine for 3 minutes. 

6. Centrifuge until layers separate and transfer 

the MIBK layer to a 25 x 150-mm culture 

tube containing 40 mL equilibrated 0.1 N 

hydrobromic acid. Cap, shake for 2 minutes, 

and allow phases to separate. 

7. Estimate the gold content by atomizing the 

organic layer into an atomic absorption 

spectrophotometer. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Wavelength ........................... 242.8 nm 

Slit · · · · · · · · · · · · · ..................... 0.7 nm 

Lamp current . . . . . . . . . . . . . . . . . . . . . . . . . . 10 rnA 

Source ................. gold (hollow cathode lamp) 

Integration time . . . . . . . . . . . . . . . . . . . . . . . 1 second 

Burner . . . . . . . . . . . . . . . . . . . . . . . Boling (three slot) 

Flame condition . . . . . . . . . . . . . . . . oxidizing (lean blue) 

1 J.Lg/mL ................... 0.084 absorbance units 

2 J.Lg/mL ................... 0.165 absorbance units 

Lower limit of determination ............ 0.05 J.Lg/mL 

24 

DISCUSSION 

Care must be taken when adding a sample containing carbonate materials to the hydrobromic 

acid-0.5 percent bromine solution. Rapid effervescence may cause loss of sample from the culture 

tube. Add sample slowly, mix, and add extra hydrobromic acid-0.5 percent bromine solution to insure a high acid concentration. For samples that 

contain little organic materials or sulfides, it is not 

necessary to include the ignition step before the 

digestion. 

Lakin and Nakagawa (1965) stated that any 

gold minerals completely enclosed in insoluble particles such as quartz would not be brought into 

solution and that the resulting gold values would 

be low. The sample must be ground to a powder 

to insure reasonable attack on the gold. However, 

if the sample is ground too fine, it is difficult to 

get a separation of the ketone layer. Fine grinding 

may also result in loss of gold by plating onto the 

grinding equipment. Thus, a compromise must be 

reached. We have found, when a 10-g split of sample is used, that good reproducibility can be generally obtained if the sample is ground to pass a 

100-mesh screen but is mostly held on a 150-mesh 

screen. 

The only interference in the estimation is 

caused by the presence of an excessive amount 

of iron coextracted into the MIBK. The iron is 

presumed to interfere by causing light scattering. 

The iron is removed by washing the MIBK extract 

with the 0.1 N hydrobromic acid solution; the gold 

remains quantitatively in the ketone. 

The solubility of MIBK in hydrobromic acid increases with increasing concentration of the latterif the decrease in volume of the ketone is appreci~ able during extraction, additional ketone is added 

to correct for apparent loss. To avoid MIBK loss 

in the wash procedure, equilibrated 0.1 N hydrobromic acid (MIBK-saturated 0.1 N hydrobromic 

acid) is used. It has been observed, using radioactive gold as a tracer, that all the gold was in the 

separated ketone and none was in the solubilized 

ketone. 

If a detection level at or below the crustal abundance of gold is desired, refer to the flameless 

atomic absorption method for gold. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 
1975), in replicas of five, by the described procedure. 

GXR 

sample 

Range 

(ppm) 

Mean 

(ppm) 

Percent relative 

standard deviation 

I 2.6--3.5 3.0 9.3 

II 0.02-0.14 0.07 42.5 

III Less than 0.05 limit of determination 

IV 0.28--0.40 0.35 11.9 

V Less than 0.05limit of determination 

VI 0.04--0.07 0.06 19.5 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Lakin, H. W., and Nakagawa, H. M., 1965, A spectrophotometric method for the determination of traces of 

gold in geologic materials, in Geological Survey Research 

1965: U.S. Geological Survey Professional Paper 525-C, 

p. C168--C171. 

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. Geological Survey 

Professional Paper 600-B, p. B130-B132. 

Ward, F. N., Nakagawa, H. M., Harms, T. F., and VanSickle, 

G. H., 1969, Atomic absorption methods useful in 

geochemical exploration: U.S. Geological Survey Bulletin 

1289,45 p. 

GOLD, FLAMELESS ATOMIC 

ABSORPTION METHOD 

The method described here was developed by 

Meier (1980). 

SUMMARY 

Gold is determined in rock, soil, and streamsediment samples by flameless atomic absorption 

spectrophotometry and a graphite furnace 

atomizer. A solution of hydrobromic acid and 

bromine is used to dissolve the gold. MIBK 

(methyl isobutyl ketone) is used to extract the 

gold from the acid solution to eliminate interference. Gold can be determined by the method as 

described in the concentration range of 0.002 to 

0.05 ppm. About 50 samples can be analyzed per 

day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Hydrobromic acid-0.5 percent bromine solution: 

Dissolve 5 mL bromine in 1 L concentrated hydrobromic acid. 

Hydrobromic acid, 0.1 N, MIBK-saturated: Dilute 

23 mL concentrated hydrobromic acid to 2 L 

with water. Add an excess of MIBK (approximately 50 mL), shake thoroughly, allow phases 

to separate, and discard MIBK layer. 

MIBK (methyl isobutyl ketone), 4-methyl-2-

pentanone. 

APPARATUS 

Atomic absorption spectrophotometer; equipped 

with graphite furnace 

Centrifuge 

25 

Evaporating dish, porcelain; size 0000 

Horizontal shaking machine 

Hot plate 

Metal rack; to accommodate 25-mm diameter 

tubes 

Muffle furnace 

Pipette, automatic, 5-mL 

Test tubes, 25 x 150-mm, screw-cap 

Vortex mixer 

STANDARDS 

Stock gold solution, 1,000 J.Lg/mL: Dissolve exactly 

1.000 g gold in 100 mL hydrobromic acid-0.5 

percent bromine solution. Heat the solution 

gently to expel excess bromine. Cool and dilute 

to 1, 000 mL with concentrated hydrobromic 

acid. 

Stock gold solution, 100 J,Lg/mL: Dilute 10 mL of 

1,000 J.Lg/mL stock gold solution to 100 mL with 

concentrated hydrobromic acid. 

Stock gold solution, 10 J.Lg/mL: Dilute 10 mL of 

100 J.Lg/mL stock gold solution to 100 mL with 

concentrated hydrobromic acid. 

Stock gold solution, 1 J.Lg/mL: Dilute 10 mL of 10 

J.Lg/mL stock gold solution to 100 mL with concentrated hydrobromic acid. 

Dilute gold solutions, 0.002, 0.005, 0.01, 0.02, and 

0.05 J.Lg/mL: Dilute 0.2, 0.5, 1.0, 2.0, and 5.0 

mL of 1 J.Lg/mL stock gold solution to 100 mL 

with concentrated hydrobromic acid. 

Standard gold solutions in MIBK, 0.002, 0.005, 

0.01, 0.02, and 0.05 J.Lg/mL: Transfer 10.0 mL 

of each of the dilute gold solutions (0.002, 0.005, 

0.01, 0.02, and 0.05 J.Lg/mL) and a blank solution 

to a series of six 25 x 150-mm screw-cap tubes. 

Add 10.0 mL MIBK and 10 mL water to each 
tube and shake for 5 minutes. Transfer the 

MIBK layer, with an automatic pipette, to 

another tube containing 40 mL MIBK-saturated 

0.1 N hydrobromic acid. Shake the tubes for 2 

minutes and use the MIBK layer for calibration 

of the atomic absorption spectrophotometer. 

PROCEDURE 

1. Transfer 10 g pulverized sample to an 0000 

porcelain evaporating dish. 

2. Roast the sample in a muffle furnace at 

700°C for 1 hour. 

3. After the sample is cool, transfer to a 

25 x 150-mm, screw-cap test tube containing 10 mL hydrobromic acid-0.5 percent 

bromine solution. 

4. Mix sample and acid thoroughly using a vortex mixer. 

5. Heat the tubes by placing them in a metal 

rack on a hot plate set at its maximum setting until the acid begins to boil at the bottom of the tube. Remove the tubes from 

the hot plate when the tubes begin to 

bump the rack and chatter, and allow them 

to cool. 

6. Add 10 mL water and 10 mL MIBK to each 

tube. 

7. Cap the tubes and shake on a horizontal 

shaking machine for 5 minutes. 

8. Centrifuge the samples to separate the 

MIBK layer from the acid layer. 

9. Using a 5-mL automatic pipette, transfer the 

MIBK layer to another 25 x 150-mm screwcap tube containing 40 mL MIBK-saturated 0.1 N hydrobromic acid. 

10. Cap the tubes and shake on the shaking 

machine for 2 minutes. 

11. Allow the phases to separate, and estimate 

the gold in the MIBK layer using an atomic 

absorption spectrophotometer equipped 

with a graphite furnace. 

STANDARDIZATION OF INSTRUMENT 

A 20-j.LL aliquot of the MIBK layer is pipetted 

into the graphite furnace using either a micropipette or an automatic sampler. The peak absorbance observed is compared with absorbance values 

for standards prepared and atomized using the 

same conditions to determine the gold concentration of the sample. The following operating conditions are for a Perkin-Elmer model360 atomic absorption spectrophotometer equipped with a Perkin-Elmer model HGA 2200 graphite furnace. 

Wavelength ........ . 

Slit .............. . 

242.8 nm 

0. 7 nm, alternate (alternate setting reduces slit height for furnace use) 

Mode . . . . . . . . . . . . . absorbance (readout is in unexpanded absorbance units) 

Signal ............ . 

Lamp current ...... . 

Source lamp ....... . 

Purge gas ........ .. 

Purge gas flow . . . . . . 

Flow time ......... . 

Drying time ....... . 

Ramp drying time ... . 

Drying temperature .. . 

Charring time . . . . . . . 

Charring temperature . . 

Atomizing time . . . . . . 

Atomizing temperature . 

Graphite tube . . . . . . . 

peak (peak height is measured 

and is displayed on readout) 

lOrnA 

gold (hollow cathode lamp) 

nitrogen 

20 divisions on flow meter (approximately 35 cclminute) 

normal, 3 seconds (internal gas 

flow through the graphite tube 

is reduced to approximately 35 

cc/minute during the first 3 

seconds of atomization) 

20 seconds 

15 seconds 

120°C 

20 seconds 

500°C 

8 seconds 

2,700°C 

pyrolitic coated 

Using the preceding instrument parameters, 

the following absorbance readings were observed 

for five replicate analyses of the gold working 

standard solutions. 

[Lower limit of detennination, 0.002 ppm] 

Gold concentration Mean Standard Percent relative 

(j.tg/mL) absorbance deviation standard deviation 

0.000 0.000 0.000 0.0 

.002 .018 .003 16.7 

.005 .055 .003 5.5 

.010 .120 .002 1.7 

.020 .250 .002 .8 

.050 .630 .003 .5 

DISCUSSION 

Roasting of geologic material that contains sulfides or organic carbon is necessary to remove 

these materials which consume a large amount of 

bromine. Excess bromine is essential to ensure 

dissolution of gold. Although metallic gold and 

gold-bearing minerals are soluble in acid solutions 

of bromine even at room temperature (Thompson 

and others, 1968), observations of gold particles 

in some samples showed that the reaction was 

26 
slow or incomplete in the hydrobromic acid-0. 5 

percent bromine solution unless the solution was 

heated. The heating step ensures a more rapid 

and complete dissolution of gold. 

GOld is efficiently extracted by MIBK from hydrobromic acid solutions over a wide range of acid 

concentrations (Morrison and Freiser, 1957). Iron, 

a source of interference in the estimation of gold, 

is also extracted into MIBK. Iron is removed from 

the MIBK extract by washing with 0.1 N hydrobromic acid, the gold remains quantitatively in the 

MIBK (Thompson and others, 1968). The 0.1 N 

hydrobromic acid used for washing is saturated 

with MIBK to prevent loss of volume of the MIBK 

extract due to its solubility in acid solutions. Standard solutions are carried through the same extraction and washing to closely match standards 

to samples. 

The calibration curve for gold in the extract by 

flameless atomic absorption is linear to about 0.05 

J.Lg/mL under the conditions described. Samples 

with concentrations greater than 0.05 ppm must 

be diluted or analyzed by conventional flame 

atomic absorption. Ramp drying provides the best 

precision, although step drying produces reasonable precision. Sensitivity is reduced if greater flow 

rates are used. Sensitivity also decreases as the 

graphite tube deteriorates with use; however, the 

decrease is small over several hundred atomizations. The graphite tube should be changed when 

sensitivity decreases significantly if maximum sensitivity is to be maintained. No background absorption was observed on standards or samples 

used in this study, so the use of background correction is unnecessary. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 2.86-3.14 2.98 4.3 

II 0. 020--0.026 0.022 11.4 

III 0.002-0.003 0.003 21.1 

IV 0.317~.372 0.353 6.0 

v 0.005--0.008 0.007 17.3 

VI 0.061~.066 0.063 3.7 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Meier, A. L., 1980, Flameless atomic-absorption determination 

of gold in geological materials: Journal of Geochemical Exploration, v. 13, p. 77-85. 

Morrison, G. H., and Freiser, Henry, 1957, Solvent extraction 

in analytical chemistry: New York, John Wiley and Sons, 

p. 131-133. 

Thompson, C. E., Nakagawa, H. M., and VanSickle, G. H., 

1968, Rapid analysis for gold in geologic material: U.S. 

Geological Survey Professional Paper 600-B, p. B130-

B132. 

HEAVY METALS, AMMONIUM 

CITRATE-EXTRACTABLE METHOD 

The method described here, excepting minor 

changes, was developed by Bloom (1955) and described by Ward and others (1963). 

SUMMARY 

An aqueous ammonium citrate solution is used 

at ambient temperature to extract heavy metals 

from a sample of soil or sediment, and a xylene 

or toluene solution of dithizone is used to estimate 

the concentration of heavy-metal ions. Although 

this method measures only a small fraction of the 

total metal content of a sample, geochemical dispersion patterns related to base-metal deposits 

are sometimes revealed more clearly than by 

using the total metal content. 

The method is extremely rapid; a determination 

can be made in less than 1 minute. As only a few 

simple items of equipment are needed, the method 

is well adapted to field use because samples can 

be analyzed as rapidly as they are collected. Thus, 

a geologist's efficiency in finding and outlining 

economically important dispersion patterns of 

heavy metals is increased. The method has been 

widely used in geochemical prospecting. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Ammonium citrate solution: Dissolve 50 g ammonium citrate ((NH4)2HC6H50 7) and 8 g hydroxylamine hydrochloride (NH20H · HCl) in 1 L 

water. Adjust the pH of the solution to 8.5 by 

adding metal-free ammonium hydroxide. This 

27 
final adjustment of the pH should be made carefully because the rate of extraction of the metal 

dithizonates is dependent on pH. If the color of 

1 mL of 0.0015 dithizone does not remain green 

when shaken for 30 seconds with 5 mL of the 

ammonium citrate solution, clean the ammonium 

citrate buffer solution as follows. Heavy metals 

are removed from the ammonium citrate solution by shaking the solution with 15-mL portions 

of 0.01 percent dithizone solution using a 

separatory funnel until the organic layer remains green. Extract the dissolved dithizone by 

shaking the aqueous solution with 25-mL portions of chloroform until the latter is colorless. 

Then wash twice with carbon tetrachloride to remove dissolved chloroform. 

Dithizone solution, 0.1 percent in chloroform: Dissolve 0.1 g dithizone (diphenylthiocarbazone) in 

100 mL reagent-grade chloroform approximately 

2 hours before using. Store the solution in a 

glass-stoppered Pyrex bottle that has been 

wrapped with paper or aluminum foil, and keep 

it cool. 

Dithizone solution, 0.0015 percent in xylene: Dilute 1.5 mL of 0.1 percent dithizone solution to 

100 mL with xylene. This solution can be 

conveniently dispersed in the field from an aluminum-foil-wrapped polyethylene wash bottle. 

As this dilute solution is less stable than the 0.1 

percent dithizone solution, only about 100 mL 

should be made up at one time, and care should 

be taken in the field to keep the container away 

from direct sunlight. 

Xylene: Occasionally a manufacturer's best grade 

of xylene may be unsatisfactory, as indicated 

when fresh dithizone solutions rapidly take on 

yellow hues caused by oxidation of dithizone. 

Such xylene can sometimes be made satisfactory 

for use by distillation or by shaking it with small 

amounts of concentrated sulfuric acid, and freezing out water remaining after the acid wash. 

APPARATUS 

Graduated cylinders, glass-stoppered, 25-mL 

Scoop, 0.2-g 

PROCEDURE 

1. Weigh or scoop 0.2 g of the fine fraction of 

a soil or sediment sample into a 25-mL 

glass-stoppered cylinder and add 5 mL ammonium citrate solution and 1 mL of 0.0015 

percent dithizone solution. 

28 

2. Shake the cylinder vigorously for 30 seconds. 

3. Allow the organic and aqueous phases to separate, and observe the color of the xylene 

(upper) layer. If it is green, green-blue, or 

blue, record as 0, 0.5, or 1 mL, respectively. 

4. If the color is blue-purple to red, add 0.0015 

percent dithizone solution in 1-mL increments and shake for a few seconds after 

each addition until a blue color is obtained. 

5. Record the volume of dithizone solution used 

and convert to parts per million using the 

standardization chart. 

STANDARDIZATION CHART 

Chart was prepared by titrating a series of solutions that contain known amounts of zinc. 

Volume in mL of 

0.0015 percent dithizone solution 

added to reach blue endpoint 

0 

1.0 

1.5 

2.0 

2.5 

3.0 

4.0 

5.0 

6.0 

7.0 

8.0 

9.0 

10.0 

11.0 

12.0 

13.0 

14.0 

15.0 

16.0 

17.0 

18.0 

19.0 

20.0 

DISCUSSION 

Heavy-metal content 

(ppm) 

0 

1 

2 

3 

4 

5 

7 

9 

11 

14 

17 

20 

22 

25 

27 

30 

32 

35 

38 

40 

40 

45 

45 

To ensure comparable results in day-to-day use 

of this method, the frequent determination of a 

standard sample whose exchangeable-metal content is known is highly recommended. 

In areas where the exchangeable-metal content 

of most samples is unusually high, the strength 
of the work solution can be advantageously increased to 0.003 percent, or the sample size reduced. 

The 0.1 percent dithizone solution may be prepared in either xylene or chloroform. Chloroform 

was chosen because of its more rapid dissolution 

of dithizone. If desired, other lighter-than-water 

solvents may be used; toluene is widely used. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[N.A., method not applicable to this sample medium. For sample GXR II, an 

0.04-g sample was titrated with 0.003 percent dithizone solution] 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I N.A. N.A. N.A. 

II 200-250 218 9.4 

III 17-22 19.2 11.3 

IV N.A. N.A. N.A. 

v 14-20 17.0 12.5 

VI 1-2 1.4 39.1 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Bloom, Harold, 1955, A field method for the determination of 

ammonium citrate-soluble heavy metals in soil and alluvium: Economic Geology, v. 50, no. 5, p. 533--541. 

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. Geological Survey Bulletin 

1152, p. 27-29. 

INDIUM AND THALLIUM, ATOMIC 

ABSORPTION METHOD 

The method described here, excepting minor 

changes, was developed by Hubert and Lakin 

(1972). 

SUMMARY 

Rock, soil, and stream-sediment samples are decomposed by hydrofluoric acid, and indium and 

thallium are determined by atomic absorption 

spectrophotometry. Organic-rich samples are 

ashed over a Meeker burner before the hydrofluoric-acid attack. The hydrofluoric acid is evaporated to dryness and the sample residue is dissolved in a solution of bromine in hydrobromic acid, 

heated to dispel the excess bromine, and diluted 

with water. The cool solution is then shaken with 

methyl isobutyl ketone to extract indium and thallium. Interference of iron is minimized by extracting the ketone with 1. 5 N hydrobromic acid. Indium and thallium are measured in the samples 

at concentrations of 0.2 ppm or more by atomic 

absorption spectrophotometry using an airacetylene flame. Fifty samples can be analyzed 

per day for the two elements. 

Special digestion procedures for iron and manganese oxides, sulfides, organic materials, and a 

more drastic attack using perchloric, nitric, and 

hydrofluoric acids are described by Hubert and 

Lakin (1972). 

REAGENTS 

All chemicals should be of reagent grade, and 

all water should be metal free. 

Bromine. 

Hydrobromic acid, 1.5 N: Dilute 172 mL concentrated hydrobromic acid to 1 L with water. 

Hydrobromic acid-1 percent bromine solution: Dissolve 10 mL liquid bromine in 1 L concentrated 

hydrobromic acid. 

Hydrofluoric acid, 48 percent. 

MIBK (methyl isobutyl ketone), 4-methyl-2-pentanone. 

Potassium hydroxide, pellets. 

Sodium bromate. 

APPARATUS 

Atomic absorption spectrophotometer 

Centrifuge 

Horizontal shaking machine 

Hot plate 

Meeker burner 

Porcelain evaporating dish, 105-mm diameter 

Teflon beakers, 100-mL capacity 

Test tubes, 25 x 150-mm screw-cap 

29 

STANDARDS 

Stock indium and (or) thallium solutions, 1,000 JLg/ 

mL: Dissolve 1.00 g pure indium and (or) 1.00 

g pure thallium in 200 mL hydrobromic acid-1 

percent bromine solution; add concentrated hydrobromic acid to make 1 L. 
Dilute indium and (or) thallium solutions, 100 JJ.g/ 

mL: Dilute 10 mL of 1,000 JJ.g/mL stock indium 

and (or) thallium solutions to 100 mL with concentrated hydrobromic acid. 

Standard indium and (or) thallium solutions in 

MIBK, 0, 0.1, 0.2, 1.0, and 2.0 JJ.g/mL: Add 0, 

0.1, 0.2, 1.0, and 2.0 mL of 100 JJ.g/mL dilute 

indium and (or) thallium solutions to 100 mL of 

1.5 N hydrobromic acid. Add 100 mL MIBK and 

shake for 1 minute to partition metals into organic phase. 

PROCEDURE 

1. Place 2 g pulverized rock or soil sample in 

a 100-mL Teflon beaker. (If the sample is 

organic rich, place 2 g in a porcelain 

evaporating dish and ash over a meeker 

burner. Then place the ashed sample in a 

100-mL Teflon beaker.) 

2. Wet the sample with water and add 15 mL 

of 48-percent hydrofluoric acid. 

3. Allow the solution to stand overnight, and 

then evaporate on a hot plate to dryness. 

4. Let the Teflon beaker cool, add 10 mL 

water, warm the beaker to effect solution, 

and add two potassium hydroxide pellets. 

5. Evaporate the resulting solution to dryness, 

cool the beaker, and add 25 mL hydrobromic acid-1 percent bromine solution. 

6. Let the beaker stand for 30 minutes and then 

warm to effect solution. 

7. Once the sample is in solution, add about 0.1 

g sodium bromate. 

8. Place the Teflon beaker on a hot plate and 

boil gently until the volume of the solution 

is reduced to approximately 15 mL, and 

then allow to cool. 

9. Transfer the entire contents (sample and 

acid) of the beaker to a 25 x 150-mm screwcap test tube, wash the beaker with approximately 15 mL water, and add to test 

tube. 

10. Add 10 mL MIBK, cap, and shake the tube 

using the horizontal shaking machine for 5 

minutes. 

11. Centrifuge the tube until the two layers separate. 

12. Transfer all of the MIBK layer to another 

screw-cap test tube containing 30 mL of 

1.5 N hydrobromic acid. 

30 

13. Shake the tube using the horizontal shaking 

machine for 5 minutes and then centrifuge 

to separate the two layers. 

14. The indium and thallium remaining in the 

MIBK phase are ready for determination 

by atomic absorption spectrophotometry. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

Indium Thallium 

Grating ................. ultraviolet ultraviolet 

Wavelength (run) . . . . . . . . . . 304.0 276.8 

Slit (run) . . . . . . . . . . . . . . . . 0.7 0.7 

Lamp current . . . . . . . . . . . . . use current recommended on lamp 

Source lamp .............. indium (hollow thallium (hollow 

cathode lamp) cathode lamp) 

Integration time (seconds) . . . . 1 1 

Burner. . . . . . . . . . . . . . . . . . three-slot three-slot 

Flame condition . . . . . . . . . . . oxidizing oxidizing 

(lean blue) (lean blue) 

0.1 ~g/mL (absorbance 

units) ................ . 0.004 0.003 

0.2 ~g/mL (absorbance 

units) ................ . 0.008 0.006 

1.0 ~g/mL (absorbance 

units) ................ . 0.036 0.028 

Lower limit of determination (~g/mL) ........ . 0.2 0.2 

DISCUSSION 

A radioactive isotope of thallium was used to 

determine that thallium bromide can be extracted 

from a hydrobromic acid solution with MIBK. If 

the normality of the hydrobromic acid used in the 

extraction is between 0.5 and 6.0 N, the percentage extraction into MIBK is at least 98. Because 

there is no suitable radioisotope for indium for 

study of extraction methods, the amount of indium 

in both phases of the extraction was determined 

by atomic absorption. At a concentration of 0.5 

N hydrobromic acid, approximately 75 percent of 

the indium bromide is extracted into MIBK, and 

with a 1 N to 4 N hydrobromic acid solution (and 

increasing concentrations) the extraction of indium 

is greater than 98 percent. 

If enough iron is present in the sample to color 

the MIBK layer brown, it will interfere with the 

indium determination, possibly by a process called 

"light scattering'', as described by Billings (1965). 

Most of the iron is normally extracted from the 

MIBK in the 1.5 N hydrobromic acid wash step. 
If the MIBK layer remains brown in color from 

iron, it must be washed a second time with 1. 5 

N hydrobromic acid. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[N( ), not detected at limit of detennination, or at value shown; L( ), detected, 

but below limit of detennination, or beiow value shown. Data were recorded 

in multiples of lower reporting limits, rounded down, and reported at two significant digits] 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

I 

II 

III 

IV 

v 

VI 

Range 

(ppm) 

0.9-1.3 

0.2-0.4 

N(0.2) 

L(0.2}-0.2 

0.~.7 

L(0.2) 

0.4-0.6 

1.4-1.7 

4.0-4.7 

3.4-3.6 

0.9-1.1 

2.6-2.8 

Mean 

(ppm) 

Indium 

1.08 

0.34 

N(0.2) 

L(0.2) 

0.56 

L(0.2) 

Thallium 

0.5 

1.54 

4.34 

3.48 

1.04 

2.7 

REFERENCES CITED 

Percent relative 

standard deviation 

13.73 

26.3 

0 

0 

23.96 

0 

14.14 

7.4 

5.78 

2.4 

8.6 

3.7 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Billings, G. K., 1965, Light scattering in trace-element analysis by atomic absorption: Atomic Absorption Newsletter, 

v. 4, p. 357-62. 

Hubert, A. E., and Lakin, H. W., 1972, Atomic absorption 

determination of thallium and indium in geologic materials: 

Geochemical exploration 1972, Amsterdam, Elsevier, p. 

38:h387. 

MOLYBDENUM, ATOMIC ABSORPTION 

METHOD 

The method described here was modified from 

a procedure described by Rao (1971). 

31 

SUMMARY 

Rock, soil, and stream-sediment samples are decomposed by fusion with potassium pyrosulfate. 

Molybdenum is converted to phosphomolybdate 

with phosphoric acid and hydrogen peroxide, and 

extracted into a solution of methyl isobutyl ketone 

and 3 percent tricaprylylmethylammonium 

chloride. Molybdenum in the extract is determined by atomic absorption spectrophotometry 

using a nitrous oxide-acetylene flame. Molybdenum concentrations of 1. 0 ppm or more can be 

determined by this method. Approximately 50 

samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Aliquat 336-MIBK solution, 3 percent: Dilute 30 

mL Aliquat 336 (tricaprylylmethylammonium 

chloride), available from General Mills Inc., Minneapolis, Minn., to 1 L with MIBK (methyl 

isobutyl ketone). 

Phosphoric acid-hydrogen peroxide solution: Add 

200 mL phosphoric acid to 134 mL hydrogen 

peroxide and dilute to 2 L with distilled water. 

This solution should be prepared weekly. The 

hydrogen peroxide should be checked by shaking 

0.1 mL hydrogen peroxide with 10 mL water 

to which 0.2 g ferrous ammonium sulfate has 

been added. A rust brown color should appear, 

if not the hydrogen peroxide has decomposed 

and should not be used. 

Potassium pyrosulfate: Molybdenum contamination is common in reagent potassium pyrosulfate. Each bottle of reagent should be tested to 

insure low molybdenum contamination prior to 

use. Reagent-grade potassium pyrosulfate, marketed by Mallinckrodt Inc., St. Louis, Mo., has 

proven to be the most molybdenum free. 

APPARATUS 

Atomic-absorption spectrophotometer 

Centrifuge 

Horizontal shaking machine 

Meeker burner 

Polyethylene or silicone rubber stoppers; to fit 16-

mm tubes 

Test tube, 16x 150-mm disposable 

Test tube, 25X200-mm screw-cap 

Vortex mixer 

Water bath 
STANDARDS 

Stock molybdenum solution, 1,000 f..Lg/mL: Fuse 

0.3751 g molybdenum trioxide (Mo03) with 3.0 

g potassium pyrosulfate over a burner. Cool and 

dissolve the melt in approximately 20 mL phosphoric acid-hydrogen peroxide solution. Bring 

the volume to 250 mL with additional phosphoric 

acid-hydrogen peroxide solution. 

Stock molybdenum solution, 100 f..Lg/mL: Dilute 10 

mL of 1,000 JLg/mL stock molybdenum solution 

to 100 mL with phosphoric acid-hydrogen 

peroxide solution. 

Stock molybdenum solution, 10 JLg/mL: Dilute 10 

mL of 100 f..Lg/mL stock molybdenum solution to 

100 mL with phosphoric acid-hydrogen peroxide 

solution. 

Stock molybdenum solution, 1 f..Lg/mL: Dilute 10 

mL of 10 IJ.g/mL stock molybdenum solution to 

100 mL with phosphoric acid-hydrogen peroxide 

solution. 

Organic working standard solutions, 0, 0.25, 0.5, 

1.0, 2.5, and 5.0 f..Lg/mL: Pipette 0, 5, and 10 

mL of 1 f..Lg/mL stock molybdenum solution and 

2, 5, and 10 mL of 10 JLg/mL stock molybdenum 

solution into six 25x200-mm screw-cap test 

tubes. Bring the volume of each tube to 50 mL 

with phosphoric acid-hydrogen peroxide solution 

and add 20 mL Aliquat 336-MIBK solution. Cap 

and shake the tubes for 45 seconds. These standards should be prepared daily and are equivalent to 0, 5, 10, 20, 50, and 100 ppm in the sample. 

PROCEDURE 

1. Weigh 0.2 g sample into a 16X150-mm disposable test tube. 

2. Add 1.5 g potassium pyrosulfate to each tube 

containing the sample and to three tubes 

containing no sample to be used as blanks. 

3. Mix sample and potassium pyrosulfate thoroughly with a vortex mixer. 

4. Heat the sample and potassium pyrosulfate 

over a Meeker burner until a uniform melt 

is achieved. 

5. Cool the tubes in a horizontal position while 

rotating them to deposit the melt uniformly 

on the sides of the tubes. 

6. Add 10 mL phosphoric acid-hydrogen 

peroxide solution. 

7. Heat the tubes in a boiling water bath until 

32 

the melt is dissolved. Vortex mixing aids 

in breaking up the melt. 

8. Allow the tubes to cool and add 4 mL Aliquat 

336-MIBK solution. 

9. Cap the tubes with polyethylene stoppers 

and shake on a horizontal shaking machine 

for 45 seconds. 

10. Centrifuge to separate the organic layer. 

11. Determine the molybdenum in the organic 

layer, within the same day, by atomic absorption using a nitrous oxide-acetylene 

flame. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic-absorption spectrophotometer. 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Wavelength ........................... 313.3 nm 

Slit .................................. 0.7 nm 

Lamp current . . . . . . . . . . . . . . . . . . . . . . . . . . 15 rnA 

Source lamp . . . . . . . . . . . . . . . . . molybdenum (hollow 

cathode lamp) 

Integration time . . . . . . . . . . . . . . . . . . . . . . . 1 second 

Burner . . . . . . . . . . . . . . . . . . . . . . 5 em, nitrous oxide 

Fuel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . acetylene 

Oxidant . . . . . . . . . . . . . . . . . . . . . . . . . . . nitrous oxide 

Flame condition . . . . . . . . . . . . . . . . rich, but not yellow 

INSTRUMENT CALIBRATION 

Using the preceding instrument parameters, 

the instrument is calibrated to read sample concentration directly by using the following standards and settings. 

[Lower limit of detennination, 1 ppm] 

Standard Absorbance Set concentration 

(j.~.glmL) units (ppm) 

0.25 0.013 5 

.5 .026 10 

1.0 .054 20 

2.5 .135 50 

5.0 .270 100 

DISCUSSION 

Molybdenum is rendered soluble by fusion with 

potassium pyrosulfate. Blanks of potassium 

pyrosulfate are run with samples to determine an 

average blank value of the pyrosulfate used. The 

average blank is subtracted from sample readings. 
The phosphoric acid-hydrogen peroxide solution 

dissolves the melt and oxidizes molybdenum to 

hexavalent state forming molybdenum (VI) phosphomolybdate. It is this species that is extracted 

into the Aliquat 336-MIBK solution. Iron and 

other elements that might possibly interfere in the 

estimation are not extracted. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 14-15 14 1.4 

II 0.8--1.1 1.0 17.5 

III not detected 

IV 310-330 310 2.7 

v 23-25 24 3.8 

VI 1.7-2.1 1.8 9.8 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Rao, P. D., 1971, Determination of molybdenum in geological 

materials: Atomic Absorption Newsletter, v. 10, no. 5, p. 

118--119. 

SELENIUM, FLAMELESS ATOMIC 

ABSORPTION METHOD 

The method described here, excepting minor 

changes, was developed by Sanzolone and Chao 

(1981). 

SUMMARY 

Selenium is determined in geological material 

by flameless atomic absorption spectrophotometry 

and a carbon-rod atomizer. The sample is decomposed by a mixture of nitric, perchloric, and hydrofluoric acids. Selenium is reduced to Se + 4 by 

heating with hydrochloric acid. Selenium is then 

extracted into toluene from a hydrochloric and hydrobromic acid solution that contains iron. 

33 

Selenium is determined directly in the toluene extract using a nickel solution as a matrix modifier 

for samples that contain from 0.2 to 200 ppm 

selenium. For samples that contain less than 0.2 

ppm and have a concentration range of 0.05 to 

100 ppm, a back-extraction of the selenium into 

dilute hydrochloric acid is used. The only observed 

interferences are from arsenic in concentrations 

greater than 1,000 ppm and from nitric acid if it 

is not removed in the sample digestion procedure. 

About 30 samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Hydrobromic acid, concentrated. 

Hydrochloric acid, concentrated. 

Hydrochloric acid, 3. 0 M: Dilute 250 mL concentrated hydrochloric acid to 1 L with water. 

Hydrochloric acid-nickel solution, 0.01 M: Dissolve 

4.95 g nickel nitrate (Ni(N03) 2·6H20) in 50 mL 

water, add 0.83 mL concentrated hydrochloric 

acid, and dilute to 1 L with water. 

Hydrofluoric acid, 48 percent. 

Iron solution, 10 percent (w/v) in 1M hydrochloric 

acid: Dissolve 10 g Specpure iron powder in 53 

mL concentrated hydrochloric acid while heating 

on a hot plate. After dissolution, add 5 mL of 

50 percent hydrogen peroxide to oxidize the 

iron. Heat to remove residual peroxide and dilute to 100 mL with water. 

Nickel solution, 0.1 percent (w/v): Dissolve 0.49 

g nickel nitrate (Ni(N03) 2·6H20) in 100 mL 

water. 

Nitric acid, concentrated. 

Perchloric acid (HC104), 70 percent. 

Toluene. 

APPARATUS 

Aluminum heating block; drilled to accommodate 

25-mm test tubes 

Atomic absorption spectrophotometor; equipped 

with a carbon rod atomizer 

Centrifuge 

Horizontal shaking machine 

Hot plate 

Teflon beakers, 50-mL 

Test tubes, 16 x 150-mm disposable 

Test tubes, 25 x 150-mm screw-cap 
STANDARDS 

Stock selenium solution, 1,000 J,Lg/mL: Dissolve 

2.190 g anhydrous sodium selenite (N 3QSe03) in 

50 mL water, add 10 mL concentrated hydrochloric acid, and dilute to 1 L with water. 

Stock selenium solution, 100 J,Lg/mL: Dilute 10 mL 

of 1,000 J,Lg/mL stock selenium solution to 100 

mL with 0.1 M hydrochloric acid. 

Stock selenium solution, 10 J,Lg/mL: Dilute 10 mL 

of 100 J.Lg/mL stock selenium solution to 100 mL 

with 0.1 M hydrochloric acid. 

Stock selenium solution, 1 J,Lg/mL: Dilute 10 mL 

of 10 J,Lg/mL stock selenium solution to 100 mL 

with 0.1 M hydrochloric acid. 

Selenium working standards, 0, 0.025, 0.1, 0.5, 

1.0, 3.0, 5.0, and 10 J,Lg: Pipette 0 mL, 0.25 mL, 

and 1.0 mL of 0.1 J,Lg/mL stock selenium solution; 0.5 mL and 1.0 mL of 1.0 J,Lg/mL stock 

selenium solution; and 0.3 mL, 0.5 mL, and 1.0 

mL of 10 J,Lg/mL stock selenium solution into 

eight 25x 150-mm test tubes, respectively, containing 5 mL of 3 M hydrochloric acid. These 

are to be carried through the procedure with the 

samples starting at step 8. 

PROCEDURE 

with extraction B if samples contain less 

than 0.2 ppm selenium. 

Extraction procedure A (for samples containing 

0.2 ppm or more selenium): 

1. Add 2 mL toluene and shake the tubes on a 

horizontal shaking machine for 5 minutes. 

2. Centrifuge the tubes. 

3. Pipette 1 mL of the toluene extract into a 

16 x 150-mm disposable tube and cap. 

4. Carefully remove the remaining toluene layer 

with a pipette and discard. 

5. Add another 2 mL toluene to the 25 x 150-mm 

tubes containing samples and standards and 

shake the tubes for 5 minutes. 

6. Pipette 1 mL of the toluene extract into the 

16 x 150-mm tube containing the first extract and mix. 

7. Transfer the organic extract into the automatic sample dispenser of the atomic absorption spectrophotometer for selenium determination using the carbon-rod atomizer. 

Extraction procedure B (for samples containing 

less than 0.2 ppm selenium): 

1. Add 2 mL toluene and shake the tube on a 

horizontal shaking machine for 5 minutes. 

1. Weigh 0.5 

beaker. 

2. Centrifuge the tubes. 

g sample into a 50-mL Teflon 3. Transfer the entire toluene layer into a 

2. Moisten with 1 to 2 mL water. 

3. Add 10 mL each of concentrated nitric, 70 

percent perchloric, and 48 percent hydrofluoric acids. 

4. Heat the beaker on a hot plate preset at 

150°C under a perchloric-acid fume hood. 

5. Gradually raise the temperature of the hot 

plate to 220°C over a 1.5-hour period. 

6. Remove the beaker from the hot plate when 

the volume has been reduced to between 

1 and 2 mL. 

7. Transfer the contents of the beaker into a 

25x 150-mm screw-cap tube using two 2.5-

mL portions of 3 M hydrochloric acid. 

8. Heat the tubes containing sample and standards in an aluminum heating block preset 

at 100°C for 5 minutes. 

9. Cool tubes to room temperature, add 10 J.LL 

iron solution to each, and mix. 

10. Add 10 mL concentrated hydrobromic acid 

and mix. 

11. Proceed with extraction A if samples contain 

more than 0.2 ppm selenium or proceed 

16x 150-mm screw-cap tube. 

4. Add another 2 mL toluene to the 25x150-mm 

tubes containing samples and standards and 

shake the tubes for 5 minutes. 

5. Centrifuge the tubes. 

6. Transfer the entire toluene layer into the 

16 x 150-mm tube containing the frrst extract. 

7. Pipette 1 mL of the 0.01 M hydrochloric acidnickel solution into the 16 x 150-mm tube 

containing the combined extracts. 

8. Shake the tube for 5 minutes and centrifuge. 

9. Transfer the aqueous back-extract, with a 

34 

pipette, into the automatic sample dispenser of the atomic absorption spectrophotometer for selenium determination 

using the carbon-rod atomizer. 

STANDARDIZATION OF INSTRUMENT 

Five J.LL of the nickel solution is pipetted manually into the carbon-rod atomizer just prior to the 

injection of a 2- or 5-J.LL aliquot of the toluene 

extract by the automatic sample dispenser from 

extraction procedure A. Two or ten J.LL of the 
aqueous back-extract from extraction procedure B 

is injected by the automatic sample dispenser 

without the addition of nickel solution. The peak 

absorbance observed is compared with absorbance 

values for standards using the same conditions. 

The following operating conditions are used for a 

Varian model AA-6 atomic absorption spectrophotometer equipped with a model 63 carbonrod atomizer and a model 53 automatic sample dispenser. 

Wavelength ........................... 196.0 nm 

Slit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0 nm 

Background correction . . . . . . . . . . . . . . . . . . . . . . . . on 

Source lamp . . . . . . . . . . . . . . . . selenium (electrodeless 

discharge lamp) 

Lamp current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 W 

Readout mode . . . . . . . . . . . . . . . . . . . . . . . peak height 

Purge gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . nitrogen 

Purge gas-flow setting . . . . . . . . . . . . . . . . . . . . . . . 8.0 

Drying voltage setting . . . . . . . . . 6.0 (90°C) for toluene, 

6.5 (100°C) for aqueous 

Drying time . . . . . . . . . . . . . . . . 25 seconds for toluene, 

30 seconds for aqueous 

Ashing voltage setting .................. 6.0 (400°C) 

Ashing time ......................... 10 seconds 

Atomizing voltage setting . . . . . . . . . . . . . . 8.5 (2,500oC) 

Atomizing time . . . . . . . . . . . . . . . . . . . . . . . 2 seconds 

Using the preceding instrument parameters, the 

following absorbance values for standards have 

been observed and correspond to the indicated 

selenium concentration in samples. 

Toluene from extraction procedure A 

[Lower limit of determination, 0.2 ppm] 

2-tJ.L aliquot 5-fJ.L aliquot 

Selenium Absorbance Sample Selenium Absorbance Sample 

(j.~.g) units (ppm Se) (tJ.g) units (ppm Se) 

3.0 0.085 6.0 0.1 0.010 0.2 

5.0 .190 10.0 1.0 .100 2.0 

7.5 .317 15.0 3.0 .290 6.0 

10.0 .450 20.0 5.0 .440 10.0 

Aqueous back-extract from extraction procedure B 

[Lower limit of determination, 0.05 ppm] 

2-tJ.L aliquot 10-tJ.L aliquot 

Selenium Absorbance Sample Selenium Absorbance Sample 

(fJ.g) units (ppmSe) (fJ.g) units (ppm Se) 

1.0 0.086 2.0 0.025 0.009 0.05 

3.0 .253 6.0 .1 .036 . 2 

5.0 .420 10.0 .5 .172 1.0 

1.0 .352 2.0 

DISCUSSION 

The use of hydrofluoric acid in the sample digestion is necessary to decompose· selenium occluded 

in or coated by siliceous materials. The digest 

should not be taken to dryness, as a loss of 

selenium is possible through volatilization. 

The procedure may be interrupted after the 

sample is transferred with 3 M hydrochloric acid 

into 25x 150-mm tubes. However, once the sample 

is heated with the hydrochloric acid, the procedure should be completed within the same day. 

The sample is heated with hydrochloric acid to insure that the selenium is in the + 4 oxidation 

state, this is the only valence that will be extracted by toluene from the hydrochloric acidhydrobromic acid solution. Iron is added before 

extraction to exert a synergistic effect on the extraction of selenium. The iron must be added before hydrobromic acid is added or the extraction 

of selenium will be incomplete. Two extractions 

are necessary to achieve 95 percent or more extraction efficiency. 

Selenium is effectively back-extracted into 0.01 

to 0.1 M hydrochloric acid. The selenium present 

may thereby be concentrated into a smaller volume, allowing a lower concentration to be determined. 

The addition of nickel solution along with 

selenium to the carbon-rod atomizer greatly enhances the absorbance signals of selenium. This 

effect may be due to the formation of a stable 

nickel selenide compound, which reduces the loss 

of selenium during the ashing stage and increases 

the efficiency of the atomization. A similar enhancement of absorbance is obtained for the aqueous back-extracted selenium when the nickel is 

present in the 0.01 M hydrochloric acid solution. 

The sensitivity (1 percent absorption) of the carbon rod for selenium in either toluene or aqueous 

solution has been determined to be 6x 10-12 g. 

When using the selenium standards in toluene, a 

linear relationship exists between absorbance and 

J.Lg selenium: 0.1 to 5 J.Lg selenium using 5-J.LL 

aliquots and 3 to 10 J.Lg selenium using 2-J.LL 

aliquots. These selenium amounts correspond to 

0.2 to 20 ppm in the actual sample. For selenium 

values greater than 20 ppm and as much as 200 

ppm, a one-to-ten dilution of the organic layer 

may be made. For values higher than 200 ppm, 

a sample size smaller than 0.50 g should be used . 

To determine selenium values lower than 0.2 ppm, 

a back-extraction of the selenium into an aqueous 

35 
nickel-0.01 M hydrochloric acid solution is necessary. A linear relationship exists between absorbance and f.Lg selenium in the aqueous backextracted standards: 0.025 to 1 f.Lg selenium using 

10-f.LL aliquots and 0.25 to 5 J.Lg selenium using 

2-~J.L aliquots. These selenium amounts correspond to 0.05 to 10 ppm in the actual sample. 

Nitric acid prevents the extraction of selenium 

into toluene. Therefore, it must be removed by 

evaporation during digestion. Arsenic apparently 

hinders the extraction of selenium into toluene if 

its concentration is 1, 000 ppm or greater in the 

sample. Samples with greater than 1,000 ppm arsenic may be analyzed by using a smaller sample 

size to reduce the apparent arsenic concentration. 

Other major and trace elements commonly encountered in geologic samples do not cause interference. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[GXR III detennined by back-extraction procedure] 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 18.0-20.7 3.0 6.1 

II 0.54-0.62 0.07 6.4 

III 0.17-0.20 0.19 6.1 

IV 5.95-6.50 6.28 4.1 

v 0.96--1.08 1.03 5.9 

VI 0.86-0.97 0.91 4.7 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Sanzolone, R. F., and Chao, T. T., 1981, Determination of 

submicrogram amounts of selenium in geological materials 

by electrothermal atomic-absorption spectrophotometry 

after solvent extraction: The Analyst, v.106, p. 647--652. 

36 

TELLURIUM, FLAME AND FLAMELESS 

ATOMIC ABSORPTION METHODS 

The method described here, excepting minor 

changes, was developed by Chao and others 

(1978). 

SUMMARY 

Tellurium in rock, soil, and stream-sediment 

samples is determined by flame atomic absorption 

in the 0.1 to 20 ppm range and by flameless atomic 

absorption in the 0.004 to 0.2 ppm range. Decomposition is accomplished by heating with hydrobromic acid and bromine. Tellurium is extracted 

into MIBK (methyl isobutyl ketone) after the addition of ascorbic acid to reduce iron (III). The 

MIBK extract is washed with 0.5 M hydrobromic 

acid before flameless atomic absorption determination. About 20 samples can be analyzed per day 

by the flameless method, and about 40 by the 

flame method. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Ascorbic acid. 

Bromine. 

Hydrobromic acid-2 percent bromine solution: 

Pipette 20 mL bromine into a 1-L volumetric 

flask and bring to 1L with concentrated hydrobromic acid. 

Hydrobromic acid, 0.5 M: Dilute 60 mL concentrated hydrobromic acid to 1 L with water. 

Hydrobromic acid, 3 M: Dilute 360 mL concentrated hydrobromic acid to 1 L with water. 

Iron solution, 10 percent (w/v) in 3M hydrobromic 

acid: Dissolve 10 g of Specpure iron powder in 

concentrated hydrobromic acid, add 1 mL 

bromine, and evaporate to dryness. Add 34.2 

mL concentrated hydrobromic acid to the residue, heat gently to effect dissolution, and dilute 

to 100 mL with water. 

MIBK (methyl isobutyl ketone). 

APPARATUS 

Atomic-absorption spectrophotometer; with 

graphite atomizer 

Centrifuge 
Horizontal shaking machine 

Hot plate; with aluminum heating block for 25-

mm tubes 

Separatory funnel, 60-mL 

Shaker, separatory-funnel 

Test tubes, 25 x 150-mm screw-cap 

Vile, capped 

Vortex mixer 

STANDARDS 

Stock tellurium solution, 100 jl.g/mL: Dissolve 

0.125 g Specpure tellurium dioxide in 1 L of 3 

M hydrobromic acid. 

Stock tellurium solution, 10 fl.g/mL: Dilute 10 mL 

of 100 fl.g/mL stock tellurium solution to 100 mL 

with 3 M hydrobromic acid. 

Stock tellurium solution, 1 fl.g/mL: Dilute 10 mL 

of 10 fl.g/mL stock tellurium solution to 100 mL 

with 3 M hydrobromic acid. 

Stock tellurium solution, 0.1 jl.g/mL: Dilute 10 mL 

of 1.0 fl.g/mL stock tellurium solution to 100 mL 

with 3 M hydrobromic acid (prepare daily). 

Standard tellurium solutions in MIBK for flame 

atomic absorption, 0, 0.25, 1, 5, 10, 20, and 50 

jl.g: Add 0, 0.25, 1.0, and 5.0 mL of 1 j.J.g/mL 

stock tellurium solution and 1.0, 2.0, and 5.0 mL 

of 10 fl.g/mL stock tellurium solution to seven 

25 x 150-mm tubes, respectively, and dilute to 43 

mL with 3M hydrobromic acid. Add 0.5 mL of 

the iron solution and 1 g ascorbic acid to each 

standard and mix. Add 3 mL MIBK, cap, and 

shake for 2 minutes on a horizontal shaking 

machine. Centrifuge to separate the phases and 

use to calibrate the atomic absorption spectrophotometer. 

Standard tellurium solution in MIBK for flameless 

atomic absorption, 0, 0.01, 0.025, 0.05, 0.1, 0.25, 

and 0.5 jl.g: Add 0, 0.1, 0.25, 0.5, 1.0, 2.5, and 

5.0 mL of the 0.1 JLg/mL stock tellurium solution 

to seven 25x 150-mm tubes, respectively, and 

dilute to 43 mL with 3M hydrobromic acid. Add 

0.5 mL of the iron solution and 1 g ascorbic acid 

to each standard and mix. Add 3 mL of MIBK, 

cap, and shake for 2 minutes on a shaker. Centrifuge to separate the phases and use to calibrate the atomic absorption spectrophotometer. 

The standards for flameless determination are 

also carried through the procedure for flameless 

determination. 

37 

PROCEDURE 

For flame and flameless determinations: 

1. Weigh 2.5 g pulverized sample into a 25x 150-

mm screw-cap test tube. 

2. Add 15 mL hydrobromic acid-2 percent 

bromine solution in small portions to prevent vigorous reactions that may be caused 

by carbonated and organic matter. If sample contains visible sulfides, add an additional 2 mL bromine. 

3. Allow the acid to react with the sample for 

1 hour with occasional vortex mixing. 

4. Heat the tube in an aluminum heating block 

on a hot plate set at 135°-140°C for 30 minutes to expel excess bromine. Sample containing sulfide must be heated longer. 

5. Add 28 mL water and mix. Solution is approximately 3 M hydrobromic acid. 

6. Centrifuge and decant the liquid phase into 

another 25 x 150-mm tube containing approximately 1 g ascorbic acid. 

7. Mix to dissolve ascorbic acid. The solution 

should have a light-yellow color. If the color 

does not appear, add , additional ascorbic 

acid. 

8. Add 3 mL of MIBK, cap, and shake the tube 

for 2 minutes on a horizontal shaking 

machine. 

9. Centrifuge to separate the phases and determine tellurium in the organic phase by 

flame atomic absorption, or use the contents 

of the tube in the following procedure for 

flameless determination. 

For flameless determination: 

1. For samples containing less than 0.1 ppm tellurium, transfer the contents of the tube in 

step 8 of the previous procedure to a 60-mL 

separatory funnel. 

2. Drain and discard the aqueous layer. 

3. Add 2 mL of 0.5 M hydrobromic acid and 

shake for exactly 5 minutes on a separatoryfunnel shaker. 

4. Allow the layers to separate completely, drain 

and discard the aqueous layer. 

5. Transfer the organic extract to a capped vial 

to be used for the flameless atomic absorption tellurium determination. 
STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a V arian model AA-6 atomic absorption spectrophotometer and a Varian model 63 carbon-rod 

atomizer. 

(Leaders(-), not used] 

Flame Flameless 

Background correction . . . . . on on 

Source lamp . . . . . . . . . . . . Tellurium 

(electrodeless discharge lamp) 

Lamp current (W) . . . . . . . . 8 8 

Wavelength (nm) . . . . . . . . . 214.3 214.3 

Slit (nm) . . . . . . . . . . . . . . . 0.5 0.5 

Readout mode . . . . . . . . . . . peak height 

Burner . . . . . . . . . . . . . . . . air-acetylene 

Fuel flow meter setting .... acetylene=0.2 

Oxidant flow meter setting · · · air=4 

Flame conditions . . . . . . . . . oxidizing 

Purge gas and flow meter . . 

Dry voltage setting . . . . . . . 

Dry time (seconds) ...... . 

Ash voltage setting . . . . . . . 

Ash time (seconds) ...... . 

Atomize voltage setting ... . 

Atomize time (seconds) .... . 

(lean blue) 

nitrogen 7. 5 

5.5=100°C 

35 

6.5=500°C 

15 

Ramp 8=2,100°C 

5 

INSTRUMENT CALIBRATION 

Using the preceding instrument parameters, a 

calibration curve of absorbance versus concentration for the standards is constructed. 

[Lower limits of detection: flame, 0.1 ppm; flameless, 0.004 ppb] 

Flame Flameless 

Sample Sample 

Standard Absorbance concen- Standard Absorbance concen-

(J.Lg) units tration (J.Lg) units tration 

(ppm) (ppm) 

0.25 0.006 0.1 0.01 0.002 4 

1.0 .023 .4 .025 .005 10 

5.0 .122 2.0 .05 .010 20 

10.0 .240 4.0 .1 .020 40 

20.0 .439 8.0 .25 .050 100 

50.0 .728 20.0 .5 .100 200 

38 

DISCUSSION 

The hydrobromic acid-bromine digestion is adequate to dissolve tellurium in most geologic samples. However, if tellurium minerals are occluded 

in, or protected by, silicates or siliceous materials, 

decomposition involving hydrofluoric acid may be 

beneficial. The optimum sample weight is 2.5 g; 

larger samples tend to reduce the efficiency of the 

digestion. 

Tellurium can be extracted from 2 to 4 M hydrobromic acid into MIBK in the presence of ascorbic 

acid. Iron suppresses the extraction. However, 

once 0.05 g iron (corresponding to 2 percent iron 

dissolved from the sample in the digestion) is 

present in the sample solution, additional amounts 

of iron do not further suppress the extraction of 

tellurium into MIBK, provided that sufficient ascorbic acid is added to reduce the iron. The 

amount of ascorbic acid added is not critical as 

long as it is sufficient to reduce the iron present. 

Add 0.5 mL of 10 percent iron solution to all the 

standards along with 1 g ascorbic acid to compensate for the variable amounts of iron encountered 

in diverse sample types. 

The procedure may be interrupted after decanting the sample solution from the dige3tion tube 

following centrifugation. The solution is stable and 

can be saved for future analysis. Once ascorbic 

acid has been added, the extraction of tellurium 

into MIBK should proceed without delay and the 

analysis should be completed on the same day. 

Iron causes suppression of absorbance readings 

in both flame and flameless determinations of tellurium. Ascorbic acid removes this interference in 

the flame procedure. The tolerance to iron is much 

less with the carbon-rod atomizer, and ascorbic 

acid alone is not sufficient to prevent suppression 

of the absorbance. Washing the organic extract 

with 0.5 M hydrobromic acid is necessary to remove the residual iron and maintain the absorbance readings constant at iron concentrations 

greater than 0.05 gin the sample solutions. Without washing, the absorbance decreases with increasing concentrations of iron in the original sample solution and the sensitivity of the flameless 

procedure is greatly reduced. Absorbance readings obtained after washing are linearly related 

to concentrations of tellurium, with an absorbance 

of 0.10 corresponding to 0.50 g tellurium in standards (200 ppb in samples). 
For both flame and flameless procedures, the 

tolerance to the presence of iron is remarkable, 

amounting to 1 g iron in the sample solution, or 

40 percent iron in the sample; this is equivalent 

to 57 percent Fe203• The use of background correction is necessary to eliminate interference due 

to molecular absorption and (or) light scattering 

in the flame procedure. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

[GXR I, II, and IV, flame atomization; GXR III and VI, flameless atomization] 

GXR Range Mean Percent relative 

sample (ppm, GXR III and VI in ppb) standard deviation 

I 8.4-9.0 8.7 1.2 

II 0.85-0.90 0.88 1.4 

III 7-10 9 15.6 

IV 0.96--1.0 1.0 2.6 

v not determined 

VI 1&-16 16 2.8 

REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Chao, T. T., Sanzolone, R. F., and Hubert, A. E., 1978, Flame 

and flame less atomic-absorption determination of tellurium 

in geological materials: Analytica Chimica Acta, v. 96, p. 

251-257. 

TIN, ATOMIC ABSORPTION METHOD 

The method described here, excepting minor 

changes, was developed by Welsch and Chao 

(1976). 

SUMMARY 

Traces of tin in rock, soil, and stream-sediment 

samples are determined using an atomic absorption method. A dried mixture of the sample and 

ammonium iodide is heated to volatilize tin tetraiodide, which condenses on the walls of the test 

tube. The condensate is put into solution with 5 

percent hydrochloric acid, extracted into trioctylphosphine oxide-methyl isobutyl ketone, and aspirated into a nitrous oxide-acetylene flame of an 

atomic absorption spectrophotometer. About 50 

samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all·water should be metal free. 

Ammonium iodide: Granular (pulverize to a consistency of extra-fine granulated sugar with a mortar and pestle and store in a desiccator). 

Ascorbic acid solution: Dissolve 10 g ascorbic acid 

in 100 mL of 5 percent hydrochloric acid (prepare daily). 

Hydrochloric acid, 5 percent: Dilute 50 mL concentrated hydrochloric acid to 1 L with water. 

Hydrofluoric acid, 49 percent. 

Nitric acid, concentrated. 

TOPO (trioctylphosphine oxide)-MIBK (methyl 

isobutyl ketone) reagent: Dissolve 4 g TOPO in 

100 mL MIBK. 

APPARATUS 

Atomic-absorption spectrophotometer; with nitrous oxide burner 

Centrifuge 

Electric oven 

Evaporating dish, porcelain; size 0000 

Glass rod 

Horizontal shaking machine 

Hot plate 

Muffle furnace 

Pyropot (Pyroco Products, Margat, Queensland, 

Australia 4019): Heating device accommodating 

10 test tubes, provides a controlled heat source 

for the iodide fusion. When the test tubes are 

inserted into the holders, only the bottom 3-4 

em is exposed to the desired temperature, 

thereby leaving the upper portions of the tubes 

cool enough to allow condensation of tin vapors. 

Separatory funnel, 125-mL 

Teflon beakers, 100-mL 

Test tubes, 16 x 150-mm screw-cap, disposable 

Test tubes, 25 x 200-mm screw-cap 

Vortex mixer 

Water bath 

STANDARDS 

Stock tin solution, 1,000 J.Lg/mL: Dissolve 1.000 g 

reagent-grade tin metal in 1 L of 6 M hydrochloric acid. 

Dilute tin solution, 100 J.Lg/mL: Dilute 10 mL of 

39 
1,000 J.Lg/mL stock tin solution to 100 mL with 

5 percent hydrochloric acid. This solution is stable for at least 2 weeks. 

Standard tin solution in TOPO-MIBK, 0, 20, and 

50 J.Lg/mL: Add 0, 2, and 5 mL of 100 fJ.g/mL 

dilute tin solution to three 125-mL separatory 

funnels each containing 3 g of ascorbic acid. Dilute to a total volume of 90 mL with 5 percent 

hydrochloric acid and add 10 mL TOPO-MIBK 

reagent solution. Shake for 30 seconds, drain 

aqueous portion, and transfer organic portion to 

16 x 150-mm test tube. 

PROCEDURE 

Procedure for non-silicate-bound tin: 

1. Weigh 1.0 g sample into a 25x200-mm screwcap test tube containing 4 g of dry granular 

ammonium iodide and mix thoroughly. 

2. Dry overnight in an electric oven at 105°C. 

3. Allow tube to cool to room temperature and 

heat the sample for 15 minutes in a preheated Pyropot set at 500°C. 

4. After sample has cooled, add 12 mL of 5 percent hydrochloric acid and 6 mL ascorbic 

acid solution. 

5. Heat the mixture in boiling water bath for 15 

minutes. 

6. Mix while hot on a vortex mixer until the 

sample is finely dispersed. 

7. When thoroughly cooled, add 2 mL TOPOMIBK reagent, cap, and shake for 30 seconds. 

8. Decant the liquid into a 16 x 150-mm test tube, 

cap, centrifuge, and aspirate into an atomicabsorption spectrophotometer. 

Procedure for silicate-bound tin: 

1. Weigh 1 g sample into a 0000 porcelain 

evaporating dish and roast for 1 hour at 

700°C in muffle furnace. 

2. Transfer to a 100-mL Teflon beaker and add 

5 mL concentrated nitric acid and 20 mL 

of 49 percent hydrofluoric acid. 

3. Heat to dryness and transfer residue to a 

25 x 200-mm test tube. 

4. Reduce the residue to fine powder with aid 

of a vortex mixer and a glass rod. Proceed 

from step 1 of the preceding procedure. 

40 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Perkin-Elmer model 603 atomic absorption spectrophotometer. 

Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ultraviolet 

Wavelength ........................... 286.3 nm 

Slit .................................. 0.7 nm 

Lamp current . . . . . . use current recommended on lamp 

Source .................. tin (hollow cathode lamp) 

Integration time . . . . . . . . . . . . . . . . . . . . . . . 1 second 

Burner . . . . . . . . . . . . . . . Varian nitrous oxide-acetylene 

Flame condition . . . . . . . . . nitrous oxide-acetylene flame, 

reducing (rich, red) 

20 f.Lg/mL .................. 0.154 absorbance units 

50 f.Lg/mL . . . . . . . . . . . . . . . . . . 0.390 absorbance units 

Lower limit of determination . . . . . . . . . . . . . . . . 2 ppm 

DISCUSSION 

The reaction of cassiterite with ammonium 

iodide is as follows: 

As water is a product of the reaction, obviously 

the sample-ammonium iodide mixture must be as 

dry as possible for maximum efficiency. Any excess of water would tend to reverse the reaction. 

The temperature and duration of heating in the 

Pyropot are critical and must be controlled to obtain reproducible results. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

Range 

(ppm) 

42--46 

0.7-1.1 

0.7-1.2 

3.5--3.7 

0.&-0.7 

0.8-1.4 

Mean 

(ppm) 

43.4 

0.88 

1.02 

3.54 

0.64 

1.14 

Percent relative 

standard deviation 

3.7 

19 

19 

2.5 

8.6 

21 
REFERENCES CITED 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659--681. 

Welsch, E. P., and Chao, T. T., 1976, Determination of trace 

amounts of tin in geological materials by atomic-absorption 

spectrometry: Analytica Chimica Acta, v. 82, p. 337--342. 

TUNGSTEN, VISIBLE SPECTROPHOTOMETRIC METHOD 

The method described here, excepting minor 

changes, was developed by Welsch (1983). 

SUMMARY 

Tungsten is determined in rock, soil, and 

stream-sediment samples by visible spectrophotometry. The sample is decomposed by nitric acid and hydrofluoric acid and dissolved in 

concentrated hydrochloric acid. A clear aliquot of 

the solution is treated with stannous chloride to 

inhibit interferences, particularly from molybdenum. The blue tungsten dithiol complex is developed at 85°C within a half-hour period. The 

complex is extracted into 5 mL heptane and the 

concentration is determined by measuring the absorption at 630 nm with a spectrophotometer. 

About 50 samples can be analyzed per day. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Dithiol, 1.5 percent solution: Acidify 1.5 g zinc 

dithiol (toluene-3, 4-dithiol, zinc derivative) with 

2 mL concentrated hydrochloric acid and dissolve in 100 mL ethanol, preferably by immersion in a sonic bath, for 10 minutes or until a 

clear pink solution is obtained (prepare daily). 

Heptane. 

Hydrochloric acid, concentrated. 

Hydrofluoric acid, 49 percent. 

Nitric acid, concentrated. 

Potassium ferricyanide, 10 percent (w/v): Dissolve 

10 g potassium ferricyanide (K3Fe(CN)6) in 100 

mL water. 

Stannous chloride, 30 percent solution: Dissolve 30 

g stannous chloride (SnCl2 ·2H20) in 100 mL 

concentrated hydrochloric acid. 

Boiling chips 

Centrifuge 

APPARATUS 

Cuvettes, 12-mm; matched to within 0.002 absorbance unit 

Heating block; drilled to accommodate 16-mm test 

tubes 

Hot plate 

Spatula, Teflon 

Spectrophotometer 

Teflon beakers, 50-mL 

Test tubes, 16 x 150-mm, disposable 

STANDARDS 

Stock tungsten solution, 1,000 J,Lg/mL: Dissolve 

0.897 g sodium tungstate, dihydrate 

(NaW04 ·2H20) in 500 mL water. 

Dilute tungsten solution, 100 J,Lg/mL: Dilute 10 

mL of 1,000 J,Lg/mL stock tungsten solution to 

100 mL with water. 

Dilute tungsten solution, 10 J,Lg/mL: Dilute 10 mL 

of 100 J.Lg/mL dilute tungsten solution to 100 mL 

with water. 

Standard tungsten solution in heptane, 1, 10, and 

50 J,Lg: Add 0.1 mL of 10 J.Lg/mL and 100 J,Lg/mL 

dilute tungsten solutions and 0.50 mL of 100 J,Lg/ 

mL dilute tungsten solution to three 16 x 150-mm 

test tubes, each containing a boiling chip, 1 mL 

water, 4 mL concentrated hydrochloric acid, and 

5 mL of 30 percent stannous chloride solution. 

Prepare standards in triplicate and continue 

through the procedure starting at step 5. These 

solutions correspond to samples containing 2, 20, 

and 100 ppm tungsten. 

PROCEDURE 

1. Weigh 1 g sample into a 50-mL Teflon beaker, 

add 5 mL concentrated nitric acid and 15 

mL of 49 percent hydrofluoric acid, place on 

a hot plate, and heat at 125°C to dryness. 

2. Add 2 mL water and warm to effect solution, 

transfer to a 16 x 150-mm test tube with a 

total of 8 mL concentrated hydrochloric 

acid, and mix well. (If copper is in excess 

of 1,000 ppm in the sample, the sample 

should now be treated with 1 mL of 10 percent potassium ferricyanide. Mix, wait 10 

minutes, and proceed to step 3.) 

41 
3. Centrifuge and transfer a 5-mL aliquot to a 

second 16 x 150-mm test tube. 

4. Add 5 mL of 30 percent stannous chloride solution and a boiling chip, and mix solution. 

5. Place tube in a heating block set at 85°C for 

10 minutes, then add 1 mL of 1.5 percent 

dithiol solution, mix, and continue to heat 

for 30 minutes. (Dithiol solution should be 

injected down the center of the tube rather 

than the sides to avoid the formation of excessive amounts of tin dithiol.) 

6. Add 5 mL heptane to the cooled sample, cap, 

and shake for 1 minute. 

7. Allow the layers to separate, transfer 3 mL 

of the organic phase to matched cuvettes, 

and read absorbance on the spectrophotometer at 630 nm. Samples containing greater than 100 ppm tungsten should 

be reprocessed from step 3 using a smaller 

aliquot. 

STANDARDIZATION OF INSTRUMENT 

The following operating conditions are for a 

Bausch and Lamb spectronic 100 spectrophotometer. 

Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . 630 nm 

Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . absorbance 

The following absorbance readings were observed from the corresponding standards. · 

Standards 

(jl.g) 

1 

10 

Absorbance 

units 

0.020 

0.220 

Find the mean absorbance value for each standard concentration and plot the absorbance values 

versus the concentration on linear graph paper. 

DISCUSSION 

The digestion of the sample cannot be hurried 

as temperature above 125°C will result in the 

evolution of tungsten halides. The additional 2 mL 

of water helps to loosen the digested material 

prior to transfer with hydrochloric acid. 

Stannous chloride prevents formation of green 

molybdenum dithiol and eliminates a number of 

interfering elements that would otherwise compete with the tungsten for dithiol bonding sites. 

42 

Matched cuvettes differing by no more than 

0.002 absorbance units are required for accurate 

results at the limits of determination. 

No interference from lead, chromium, cobalt, 

nickel, manganese, molybdenum, and vanadium 

has been observed. However, copper and bismuth 

at 1,000-ppm concentrations cause a severe suppression of the absorbance reading of approximately 50 percent. The addition of potassium ferricyanide will reduce or eliminate this problem for 

samples with copper concentrations of 1,000 ppm 

and greater. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin, 

1975), in replicas of five, by the described procedure. 

GXR 

sample 

I 

II 

III 

IV 

v 

VI 

Range Mean 

(ppm) (ppm) 

160-180 172 

1.4-1.6 1.5 

12,500-14,000 12,900 

30--32 30.9 

1.~2.1 1.9 

2.1-2.4 2.3 

REFERENCES CITED 

Percent relative 

standard deviation 

4.9 

5.5 

5.0 

2.9 

6.9 

5.6 

Allcott, G. H., and Lakin, H. W., 1975, The homogeneity of 

six geochemical exploration reference samples, in Elliott, 

I. L., and Fletcher, W. K., eds., Geochemical exploration 

1974: Amsterdam, Elsevier, p. 659-681. 

Welsch, E. P., 1983, A rapid geochemical spectrophotometric 

determination of tungsten with dithiol: Talanta, v. 30, p. 

876--878. 

URANIUM, ULTRAVIOLET 

FLUORESCENCE METHOD 

The method described here, excepting minor 

changes, was developed by Centanni and others 

(1956). 

SUMMARY 

Uranium is determined in rock, soil, and 

stream-sediment samples by ultraviolet fluorescence. The sample is decomposed by means of the 

volatile nature of nitric, hydrofluoric, and hydrochloric acids or a partial leach by a hot nitric acid 
digestion. Uranium is taken up in dilute nitric 

acid, and a salting agent (aluminum nitrate) is 

added that also complexes fluoride, phosphate, 

and sulfate ions that otherwise seriously hinder 

the extraction of uranium. The uranyl nitrate is 

extracted by ethyl acetate, which separates ura..; 

nium from its quencher elements. A carbonate 

fluoride phosphor is obtained by fusing the extracted uranyl nitrate with sodium and potassium 

carbonate and sodium fluoride. The phosphor is 

introduced to a ultraviolet fluorometer and estimated for uranium content. About 20 to 40 samples can be analyzed per man day depending on 

digestion chosen. 

REAGENTS 

All chemicals should be of reagent grade and 

all water should be metal free. 

Aluminum nitrate solution: Dissolve 1,000 g aluminum nitrate, 9-hydrate (Al(N03)3 ·9H20) in 500 

mL of H20 by heating solution. 

Ethyl acetate. 

Hydrochloric acid, concentrated. 

Hydrochloric acid, 6 N: Dilute 500 mL concentrated hydrochloric acid to 1 L with water. 

Hydrofluoric acid, 48 percent. 

Nitric acid, concentrated. 

Nitric acid, 15 percent(v/v): Dilute 150 mL concentrated nitric acid to 1 L with water. 

Sodium carbonate-potassium carbonate-sodium 

fluoride flux: Oven dry anhydrous sodium carbonate, potassium carbonate, and sodium 

fluoride at 105°C overnight. Separately grind sodium carbonate and potassium carbonate to pass 

an 80-mesh sieve and prepare a 5:5:1 mixture, 

by weight. 

APPARATUS 

Beakers, Teflon, 50-ml 

Boiling chip 

Centrifuge 

Fisher burner,or automatic fusion rotary burner 

Heating block; drilled to accommodate 25 x 150-

mm culture tubes 

Horizontal shaking machine 

Hot plate 

Platinum crucibles; 10 x 35-mm inside diameter 

Quartz glass triangle 

Teflon stirring rod 

Test tubes, 25x150-mm, screw-cap 

Ultraviolet fluorimeter 

43 

STANDARDS 

Stock uranium solution, 1,000 J.Lg/mL: Dissolve 

exactly 0.211 g uranyl nitrate (U02(N03) 2·6H20) 

in 100 mL of 4N nitric acid. Prepare 100, 10, 

and 1 f.Lg/mL standard uranium solutions by successive 10-fold dilutions of 1,000 JJ.g/mL stock 

uranium solution with 4N nitric acid. 

Standard uranium phosphor: Pipette 0.2 mL of the 

1 f.Lg/mL standard uranium solution into a 

25 x 150-mm culture tube containing 6 mL of 15 

percent nitric acid and 10. mL aluminum nitrate 

and proceed from step 8 of procedure A. The 

standard phosphor contains 0.1 f.Lg uranium. A 

blank phosphor is prepared in the same manner, 

but with no uranium added. 

PROCEDURE 

A. Digestion with hydrochloric, hydrofluoric, 

and nitric acids: 

1. Weigh 0.5 g sample into a Teflon beaker 

and add 5 mL concentrated nitric acid 

and 20 mL hydrofluoric acid. 

2. Place on a hot plate at moderate heat 

and evaporate to dryness. Alternately 

heat overnight on warm-to-touch hot 

plate. 

3. Add 3 mL concentrated hydrochloric 

acid and evaporate to dryness. 

4. Add 3 mL concentrated nitric acid and 

evaporate to dryness. Rinse the sides 

with water and evaporate to dryness 

if needed. 

5. Add 10 mL aluminum nitrate solution to 

a 25 x 150-mm screw-cap test tube. 

6. Add 3 mL of 15 percent (v/v) nitric acid 

to the Teflon beaker and warm 

slightly. Break up residue with Teflon 

stirring rod if necessary. Transfer to 

the 25 x 150-mm test tube containing 

the aluminum nitrate solution. 

7. Add 3 mL of the 15 percent ( v /v) nitric 

acid to beaker, warm slightly, and 

transfer to the test tube. 

8. After mixture has cooled, add 10 mL of 

ethyl acetate, cap, shake for 2 minutes, and then centrifuge to obtain a 

clean layer separation. 

9. Pipette 5-mL aliquot of organic layer 

and transfer to a clean, dry platinum 

crucible. 
B. 

10. Ignite the ethyl acetate to evaporate the 

organic solvent. 

11. Place the crucible over a Fisher burner 

to red heat for 10-20 seconds. 

12. After cooling, add 2.0 g sodium 

carbonate-potassium carbonate-sodium 

fluoride flux and place the platinum 

crucible on a quartz-covered triangle 

that is mounted over a Fisher burner. 

The height should be adjusted so that 

most of the flux melts within the first 

45-60 seconds. Heat over burner for 

a total of 3 minutes. If an automatic 

fusion rotary burner is available, consult the operation manual for set-up 

conditions that best approximate these 

conditions. 

13. Cool the crucible and measure the fluorescence with an ultraviolet fluorimeter. 

14. Clean the platinum crucibles in hot 6 N 

hydrochloric acid. 

Hot nitric acid digestion: 

1. Weigh 0.5 g sample into a 25x150-mm 

screw-cap culture tube. 

2. Add three boiling chips and 5 mL concentrated nitric acid. 

3. Nestle each tube into a sand-filled heating 

block and boil moderately to dryness. 

4. Add 6 mL of 15 percent (v/v) nitric acid 

and boil for 5 minutes in heating block. 

Remove tube and add 10 mL of the 

aluminum nitrate solution. 

5. Allow tubes to cool and proceed to step 

8 of procedure A. 

STANDARDIZATION OF INSTRUMENT 

The estimation of uranium content is made with 

an ultraviolet flourimeter using the manufacturer's operating procedure. The instrument is 

standardized by setting the 0.1 J.Lg standard phosphor to read 50 divisions. Readings are taken for 

the blank phosphor and the sample. Uranium concentration in the sample is calculated using the 

following formula: 

Sample uranium concentration ppm = 0.4 x (sample reading-blank reading) . 

50-blank reading 

44 

DISCUSSION 

The digestion with hydrochloric, hydrofluoric, 

and nitric acids is effective for decomposing and 

releasing most silicate-bound uranium. Refractory 

minerals usually require fusion with fluxes, such 

as mixtures of sodium carbonate and sodium borate. 

The hot nitric acid digestion is a partial extraction because silicates are only sparingly attacked 

on surface exposures; however, the secondary dispersions are readily detected. 

Care must be taken when heating Teflon beakers. Teflon loses strength at 320°C and complete 

depolymerzation occurs at 400°-450°C. 

The aluminum nitrate solution, a salting out 

agent, also prevents the interference from anions. 

Uranium is separated from quenchers, elements 

that reduce the fluorescence, by extraction into 

ethyl acetate. 

High blank readings can originate from several 

of the following sources: contamination in the flux 

reagents or in the aluminum nitrate solution, or 

improperly cleaned platinum dishes. 

REFERENCE SAMPLES 

The following is a tabulation of statistical data 

obtained by analyzing the six GXR (geochemical 

exploration reference) samples (Allcott and Lakin 

1975), in replicas of five, by the described proce~ dures. 

PROCEDURE A. Hydrochloric, hydrofluoric, and nitric acids digestion 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 23-29 26 8.16 

II 1.9-2.7 2.2 12.9 

III 1.6-2.4 2.1 19.6 

IV 3.0-5.6 4.4 23.7 

v 1.4-2.0 1.6 15.1 

VI 0.89-1.4 1.1 20.5 

PROCEDURE B. Hot nitric acid digestion 

GXR Range Mean Percent relative 

sample (ppm) (ppm) standard deviation 

I 23-26 24.2 6.9 

II 1.2-1.7 1.5 14.2 

III 1. 7-2.4 2.0 13.8 

IV 3.7-5.1 4.3 12.5 

v 1.6-1.9 1.6 10.1 

VI 0.72-0.84 0.75 8.74 
REFERENCES CITED 

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Centanni, F. A., Ross, A. M., and DeSesa, M. A., 1956, 

Fluorometric determinations of uranium: Analytical 

Chemistry, v. 28, p. 1651. 

SELECTED REFERENCES 

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__ 1978, Tabulation of geochemical data furnished by 109 

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__ 1964b, Determination of ammonium-citrate soluble 

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45 

Chao, T. T., Jenne, E. A., and Heppting, L. M., 1968a, Adsorption of traces of silver on sample containers: U.S. 

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__ 1968b, Prevention of adsorption of trace amounts of gold 

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__ 1977, Chemical dissolution of sulfide minerals: U.S. 

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Chao, T. T., and Theobald, P. K., Jr., 1976, The significance 

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Crenshaw, G. L., and Ward, F. N., 1975, Determination of 

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Curry, K. J., Cooley, E. F., and Dietrich, J. A., 1975, An 

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Dietrich, J. A., Cooley, E. F., and Curry, K. J., 1977, A de-
vice for automatic photoelectric control of the analytical 

gap for emission spectrographs: U.S. Geological Survey 

Circular 748, 5 p. 

Ficklin, W. H., 1970, A rapid method for the determination 

of fluoride in rocks and soils, using an ion-selective electrode: U.S. Geological Survey Professional Paper 700-C, 

p. C18&-C188. 

___ 1975, Ion-selective electrode determination of iodine in 

rocks and soils: U.S. Geological Survey Journal of Research, v. 3, p. 753-755. 

___ 1982, The separation of tungsten and molybdate by ion 

chromatography and its application to natural waters: 

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__ 1983, Separation of arsenic (III) and arsenic (V) in 

ground waters by ion-exchange: Talanta, v. 30, p. 371-

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Ficklin, W. H., and Ward, F. N., 1976, Flameless atomic absorption determination of bismuth in soils and rocks: U.S. 

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Filipek, L. H., Chao, T. T., and Theobald, P. K., Jr., 1981, 

Comparison of hot hydroxylamine hydrochloride and oxalic 

acid leaching of stream sediment and coated rock as anomaly enhancement techniques: Journal of Geochemical Exploration, v. 17, p. 35-48. 

Filipek, L. H., and Theobald, P. K., Jr., 1981, Sequential extraction techniques applied to a porphyry copper deposit 

in the Basin and Range Province: Journal of Geochemical 

Exploration, v. 15, p. 15~174. 

Griffitts, W. R., Ward, F. N., and Alminas, H. V., 1976, A 

simple spot test for molybdenum minerals: Economic Geology, v. 71, p. 1595. 

Grimes, D. J., and Marranzino, A. P., 1968, Direct-current 

arc and alternating-current spark emission spectrographic 

field methods for the semiquantitative analysis of geologic 

materials: U.S. Geological Survey Circular 591, 6 p. 

Harms, T. F., and Ward, F. N., 1975a, Determination of arsenic in vegetation, in Ward, F. N., ed., New and refined 

methods of trace analysis useful in geochemical exploration: U.S. Geological Survey Bulletin 1408, p. 13-20. 

___ 1975b, Determination of selenium in vegetation, in 

Ward, F. N., ed., New and refined methods of trace analysis useful in geochemical exploration: U.S. Geological 

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__ 1975c, Atomic absorption determination of bismuth, 

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trace analysis useful in geochemical exploration: U.S. 

Geological Survey Bulletin 1408, p. 43-49. 

Harms, T. F., Ward, F. N., and Erdman, J. A., 1981, Laser 

fluorometric analysis of plants for uranium exploration: 

Journal of Geochemical Exploration, v. 15, p. 617--624. 

Hinkle, M. E., 1971, Determination of mercury in crude oils: 

U.S. Geological Survey Professional Paper 7~B. p. 

B171-B174. 

Hinkle, M. E., and Crenshaw, G. L., 1975, Determination of 

mercury in natural water, plants, and soils by cold-vapor 

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Geological Survey Bulletin 1408, p. 51--63. 

Hinkle, M. E., and Kantor, J. A., 1978, Collection and analysis 

of soil gases emanating from buried sulfide mineralization, 

46 

Johnson Camp Area, Cochise County, Arizona: Journal of 

Geochemical Exploration, v. 9, p. 209-216. 

Hinkle, M. E., and Learned, R. E., 1969, Determination of 

mercury in natural waters by collection on silver screens: 

U.S. Geological Survey Professional Paper 650-D, p. 

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Hopkins, D. M., 1977, An improved ion-selective electrode 

method for the rapid determination of fluorine in rocks 

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Hubert, A. E., 1971a, Determination of tellurium in geologic 

materials in the parts-per-billion range: U.S. Geological 

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__ 1971b, A sensitive method for the determination of tellurium in vegetation: U.S. Geological Survey Professional 

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___ 1983, Determination of arsenic in geological materials 

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Hubert, A. E., and Chao, T. T., 1977, X-ray fluorescence determination of titanium in geological materials: Analytica 

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__ 1979, Multi-element analysis of natural waters for hydrogeochemical prospecting by X-ray fluorescence following preconcentration and filter deposition: Economic Geology, v. 74, p. 1669-1672. 

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Lakin, H. W., and Thompson, C. E., 1963a, Effect of copper 

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__ 1963b, Tellurium-A new sensitive test: Science, v. 141, 

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materials: U.S. Geological Survey Journal of Research, v. 

6, p. 259-262. 

McCarthy, J. H., Jr., 1972, Mercury vapor and other volatile 

components in the air as guides to ore deposits: Journal 

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___ 1982, Determination of uranium in geologic materials by 

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__ 1983, Determination of gold in water in the nanogram 

range by electrothermal atomization after coprecipitation 

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___ 1984, Gold in natural water-A method of determination 
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McHugh, J. B., and Turner, R. L., 1975, Flameless atomic 

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__ 1979, Indirect determination of chloride in plants by 

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McHugh, J. B., and Welsch, E. P., 1975, Atomic absorption 

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__ 1979, A technique for decomposition and dissolution of 

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__ 1980, Flameless atomic-absorption determination of gold 

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___ 1981, Determination of lithium isotopes in geologic materials by atomic-absorption spectroscopy: Analytical 

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__ 1975, Use of emission spectroscopy for the semiquantitative analysis of trace elements and silver in native 

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Mosier, E. L., Antweiler, J. C., and Nishi, J. M., 1975, Spectrochemical determination of trace elements in galena: 

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Mosier, E. L., and Motooka, J. M., 1984, Induction coupled 

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Motooka, J. M., and Grimes, D. J., 1976, Analytical precision 

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Motooka, J. M., Mosier, E. L., Sutley, S. J., and Viets, J. 

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47 

Nakagawa, H. M., 1975, Atomic absorption determination of 

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Nakagawa, H. M., and Lakin, H. W., 1965, A field method 

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Nakagawa, H. M., and Ward, F. N., 1975, Determination of 

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__ 1970, A field method for the determination of cold-extractable nickel in stream sediments and soils: U.S. 

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___ , ed., 1975, New and refined methods of trace analysis 

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48 

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Watterson, J. R., Ficklin, W. H., and Turner, J. H., 1976, 

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Watterson, J. R., and Neuerburg, G. J., 1975, Analysis for 

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___ 1983, A rapid geochemical spectrophotometric determination of tungsten with dithiol: Talanta, v. 30, p. 876-878. 

Welsch, E. P., and Chao, T. T., 1975, Determination of trace 

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___ 1976, Determination of trace amounts of tin in geological 

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