GEOLOGICAL SURVEY CIRCULAR 864 

A Review of Classical Silicate-Rock 

Analysis and Recommended Modifications 

of Classical Methods of Analysis 

A Review of Classical Silicate-Rock 

Analysis and Recommended Modifications 

of Classical Methods of Analysis 

By Lillie B. Jenkins 

GEOLOGICAL SURVEY CIRCULAR 864 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Jenkins, Lillie B. 

A review of classical silicate-rock analysis and recommended modifications of classical methods of 

analysis. 

(Geological Survey circular ; 864) 

Bibliography: p. 

Supt. of Docs. no.: I 19.4/2:864 

1. Rocks, Siliceous-Analysis. I. Geological Survey (U.S.) II. Title. Ill. Series. 

QE75.C5 no. 864 557.3s [549' .6] 82-600109 

AACR2 

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
Abstract 

Introduction __________________________ _ 

Treatment of the sample _________________ _ 

Determination of total water ______________ _ 

The sample blank for the main portion _______ _ 

Determination of moisture ________________ _ 

Roasting the sample ____________________ _ 

Determination of silica in samples containing less than 2 

percent fluorine _____________________ _ 

Determination of the R20 3 group ___________ _ 

Removal of ammonium salts _______________ _ 

Removal of manganese by ammonium persulfate 

Gravimetric determination of calcium oxide 

Gravimetric determination of magnesium ______ _ 

Recovery of calcium from the magnesium pyrophosphate 

precipitate ________________________ _ 

CONTENTS 

Page 

1 

2 

3 

3 

6 

7 

7 

7 

11 

13 

13 

14 

16 

17 

Solution of the R20 3 precipitate and the determination 

of residual silica 

Determination of titanium oxide on the solution of the 

R203 group ________________________ _ 

Determination of total iron on the solution of the R20 3 

group ____________________________ _ 

Determination of ferrous iron ______________ _ 

Determination of Al20 3 __________________ _ 

Preparation of the solution for the determination of 

manganese and phosphorus _____________ _ 

Determination of manganese _______________ _ 

Determination of phosphorus ______________ _ 

Determination of Na20 and K20 ____________ _ 

Summation of analytical results _____________ _ 

Selected references 

ILLUSTRATIONS 

FIGURE 1. Diagram for analysis of the sample after total water is determined on a 1-g sample ____________ _ 

2. 

3--4. 

Sketch of a Penfield tube __________________________________________ - - - - - - - - - - - -

Photographs of: 

3. Penfield tube in tube support __________________________ - - - __ - - - - - - - - - - - - - - -

4. Silver reductor _____________________________________ - - __ - - - - - - - - - - - - - - -

TABLE 

TABLE 1. Comparison of results (in percent) obtained in this laboratory by the methods recommended in this work and 

Page 

18 

19 

21 

23 

24 

24 

25 

26 

30 

31 

33 

Page 

2 

4 

5 

21 

Page 

recommended averages from Flanagan (1976, p. 171) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 32 

III 

A Review of Classical Silicate-Rock Analysis 

and Recommended Modifications 

of Classical Methods of Analysis 

By Lillie B. Jenkins 

ABSTRACT 

This work was undertaken to review current classical chemical methods, to define existing problems, and to suggest modifications to improve the accuracy and precision of classical 

chemistry. Problems that may arise in the analysis of silicate 

rocks are discussed, and detailed procedures are given for the 

determination of 13 constituents commonly present in silicate 

rocks. 

The method for the determination of water is a simplification 

of the Penfield method and includes some modifications made 

by other authors. The detailed description of the method can 

easily be followed by inexperienced chemists who can thus obtain very accurate results. The equipment needed is simple and 

readily available. 

A detailed di8CU88ion of the sample blank is given. The blank 

and the standard sample are used as controls for the analysis. 

A known amount of iron is added to the blank to ensure a precipitate in the blank of approximately the same size as the precipitate obtained in the sample. 

Many possible errors that could be made in the determination of silica and that would lead to errors in the remaining determinations are presented, and precise directions are given on 

how to avoid these errors. If the methods recommended in 

many textbooks are followed as outlined, serious errors can be 

made. For example, heating the residue from the second dehydration of silica at l05°C to lUre is the procedure commonly 

recommended in the literature. However, the well-known reaction 

Pt0 + 4Fe3 + + 6Cl- ~ PtC~ + 4Fe2+ 

is greatly catalyzed when the residue is heated at 1050C, 

thereby introducing platinum into the analysis. Thus, the silica 

residue should not be heated above the temperature of the 

steam bath in order to avoid the introduction of platinum into 

the analysis. The complications encountered in the subsequent 

analysis when this precaution is not taken are discussed. 

The necessity of removing platinum from solution with hydrogen sulfide prior to the determination of the ammonium hydroxide group has always been questionable. This step is elimi1 

nated in this work because the introduction of platinum into 

the analysis is avoided when the steps outlined are followed. 

The techniques outlined here eliminate the difficulties encountered in the precipitation of excess iron with ammonium 

hydroxide. 

An excessive amount of ammonium salts can interfere with 

the precipitation of calcium and magnesium. These salts are 

easily and efticiently removed by the addition of nitric acid. 

Reprecipitating calcium oxide after dissolving the ignited calcium oxide precipitate ensures the removal of any magnesium 

that may have accompanied the calcium precipitate. 

L. C. Peck (1964, U.S. Geological Survey Bulletin 1170) recommended that the filtrate from the calcium determination 

stand several days after the addition of dibasic ammonium 

phosphate and that the beakers be scratched in order to precipitate the magnesium ammonium phosphate. The procedure 

as outlined here eliminates these steps. 

Calcium should be recovered from the magnesium pyrophosphate precipitate if the sample contains much magnesium and 

very little calcium. 

The small amount of Si~ that escapes recovery by two dehydrations is customarily recovered from the R:z()3 precipitate 

by fusion of the ignited oxides with K~ , dissolution of the 

cake in dilute H~ , and evaporation of the solution to fumes 

of 803• This procedure is recommended in this report with the 

following modifications: 

1. A temperature of SOOOC is set for the pyrosulfate fusion. 

2. Platinum dishes are recommended for the dehydration of 

the residual silica rather than glass beakers. 

3. The filtrate is made to a definite volume, and separate 

aliquots are taken for the determination of total iron and 

titanium. 

For the ferrous iron determination, the sample is decomposed in a 100-ml platinum crucible, and the method used for 

calculating the percentage of iron in the sample is simple and 

direct. 

The conditions for the precipitation of ammonium phosphomolybdate as reported in the literature are varied and indefinite. The method ''The Determination of Phosphorus in Rocks 

Containing Vanadium" by R. B. Randolph and F. S. Grimaldi 
(1953, U.S. Geological Survey Bulletin 992, p.49-55) was chosen for the precipitation of ammonium phosphomolybdate because it is a routine procedure for precipitating P205 in 

amounts from 0.1 mg to 2.0 mg with no changes in conditions 

or quantities or reagents. The precipitate is dissolved, and 

phosphorus is determined colorimetrically or gravimetrically as 

described. 

Tests were made to determine the suitability of the IL 443 

Flame Photometer for the quantitative determination of 

sodium and potassium in silicate rocks after determining a 

method of decomposition and a suitable solution for presentation of the sample to the flame. A 1-g sample is decomposed 

in order to ensure a representative sample. Complete decomposition is effected in a covered Teflon beaker with nitric, sulfuric, and hydrofluoric acid, and the sample is dissolved in hydrochloric acid. The standards are prepared with an internal 

standard, and the samples are closely bracketed with the standards when the readings are made. 

Tests were made to determine the effect of various levels of 

sodium on the potassium determinations and the effect of various levels of potassium on the sodium determinations. No efSilicate-rock sample 

I 

Heated at 1 05°C 

(Determine H20-) 

I 

Roasted 

I 

fects were observed. The accuracy of the data obtained and the 

simplicity of the method recommends its use even though the 

method is not classical. 

INTRODUCTION 

In order to detennine which methods would be 

used in a newly created classical-analysis project 

devoted exclusively to a highly accurate and precise analysis of silicate rocks, a study was made of 

the methods published by Kolthoff and Sandell 

(1943), Hillebrand and others (1953), Shell (in 

Kolthoff and Elving, 1962), Peck (1964), Maxwell 

(1968), Bennett and Reed (1971), and others listed 

in the "Selected References." The project was 

created (1) to train chemists in classical analytical 

methods because of the progressively worsening 

Fused with Na2C03: Na202 

I 

Evaporated with HCI (double dehydration of Si02) 

Residue 

(Si02) 

Precipitate 

(R203) 

I 

R203 precipitate ignited 

I 

Pyrosulphate fusion 

I 

Solution 

I 

Dehydration 

I 

Filtration 

I 

I 

Residue 

·(residual Si02) 

I 

Aliquot for 

determination of Ti02 

I 

HCI added 

Filtrate 

I 

NH40H 

I 

Precipitate 

(CaC204) 

I 

Precipitate 

(MgNH4P04) 

I 

Filtrate 

I 

I 

Aliquot for 

determination of Fe203 

I 

Filtrate 

I 

(NH4)2HP04 

I 

FIGURE 1.-Diagram for analysis of the sample after total water is determined on a 1-g sample. 

2 

I 

Filtrate 

(discard) 
shortage of chemists trained in applied analytical 

chemistry by educational institutions and (2) to 

satisfy the demand for more classical analyses at 

the U.S. Geological Survey(USGS). 

The procedures recommended are basically 

those described in the "Selected References," with 

some modifications. The discussions are given 

primarily to enable relatively inexperienced 

analysts to perform the analysis without difficulty 

and to eliminate some of the problems that more 

experienced analysts have encountered. 

Methods are described for determining Si02, 

Al20a, CaO, H20+, H2o-, and MgO by gravimetric analysis; Fe20 3 and FeO by volumetric analysis; N ~0 and K20 by flame photometric analysis; 

and Ti02, P 20 6, and MnO by spectrophotometric 

analysis. I recommend determining C02 by the 

widely accepted method of decomposing the sample with HCl and absorbing the carbon dioxide 

evolved into previously weighed absorption tubes. 

The C02 determination is not discussed further in 

this report as the special apparatus required and 

detailed procedures for the determination are 

given by Kolthoff and Sandell (1943, p. 385-389), 

Groves (1951, p. 110-114), Hillebrand and others 

(1953, p. 768-770), Peck (1964, p. 79-80), Maxwell 

(1968, p. 430-433), and Bennett and Reed (1971, 

p. 208-210). 

The diagram for analysis of the sample after 

total water is determined is shown in figure 1. 

TREATMENT OF THE SAMPLE 

Ten to 20 g of a representative rock sample 

ground to pass a 100-mesh screen is usually required. Although an 8-g representative sample is 

adequate for a single determination of each constituent, more material is desirable to have areserve for checking results and for use as a future 

reference material. A semiquantitative spectrographic analysis of the sample should be made in 

order to determine its approximate composition 

and the presence of any interfering constituents. 

The sample should be mixed well, then rolled and 

quartered on a piece of glazed paper with a demagnetized steel or plastic spatula. The quarters 

are then returned to the original sample bottle by 

use of the spatula. Pouring the sample causes a 

segregation of the particles and should be avoided. 

Four samples, a blank, and a standard sample can 

be analyzed conveniently and economically in each 

operation. 

3 

If sufficient sample is available, separate 1-g 

samples are used for the determination of total 

water (H20-H20+), P20 6 and MnO, N~O and 

K20, and FeO. Another 1-g sample is used for the 

determination of H2o-,Si02,Fe20a, Al20a, CaO, 

Ti02, and MgO. Exactly 1 g of sample is weighed 

so that results can be calculated easily. 

DETERMINATION OF TOTAL WATER 

The water content of a silicate rock is divided 

into two parts: that released at and below 105°C 

is reported as moisture, or· H2o-; that released 

above this temperature is reported as combined 

water, or H20 +. 

Percent total water= 

percent H20- +percent H20 + 

The method of Penfield (1894) with some modifications is the method generally used for the determination of water in silicates. The sample mixed 

with a flux is fused in the enlarged end of a Penfield tube in order to expel the water into the 

upper part of the tube, which is cooled in an ice 

bath (figs. 2, 3). ·11te fused bulb and sample are 

pulled off, and this end is sealed. A closely fitting 

glass rod is inserted into the tube, and the tube is 

stoppered and weighed. The tube is then dried 

and reweighed; the loss in weight is the weight of 

the total water in the sample. 

Other volatile substances except carbon dioxide 

are retained by the flux. If carbon dioxide remains 

in the tube, the value for total water is high. I use 

Sandell's (1951) technique of conveniently displacing carbon dioxide from the tube by inserting a 

closely fitting glass rod into the tube just before 

it is stoppered. This technique eliminates the need 

for special equipment to drain C02 from the tube 

(Peck, 1964). 

Maxwell (1968) used lead oxide as the flux. 

Shapiro (1975) used sodium tungstate. Peck (1964) 

stated that neither sodium tungstate, lead oxide, 

nor lead chromate is a satisfactory flux and recommended that a mixture of two parts lead oxide and 

one part lead chromate be used. Peck stated that 

this mixture is sufficiently reactive to decompose 

hornblende and that intumescence is not troublesome when this mixture is used in the analysis of 

samples containing carbon or carbon dioxide. I 

have found sodium tungstate to be a very satisfactory flux for the decomposition of silicate rocks. 
5Ys inches 

2 inches 

FIGURE 2.-Penfield tube. 

4 

APPARATUS 

Penfield tube in tube support 

Funnel-A long-stemmed funnel, which, when 

supported, will reach nearly to the bottom of 

the Penfield tube. 

Capillary plug-A piece of Tygon1 tubing about 

1¥2 in long, one end of which fits snugly over 

the top of the Penfield tube and the other end 

of which has a capillary tube about 11/4 in long 

fitted into it. 

Meker burner, safety glasses, propane-oxygen 

torch, timer 

Displacement rod-Glass rod that fits snugly in 

the upper part of the Penfield tube. The 

length should be approximately the length of 

the tube minus the bulb. 

REAGENT 

Sodium tungstate-Heat several hundred grams 

of N~W04 overnight at 180°C in a drying 

oven. Allow to cool, grind to a fine powder, 

and store in an open container in the drying 

oven. 

PROCEDURE 

1. Determination of reagent blank. 

2. Transfer 3 g of oven-dried N~W04 to a Penfield tube with the aid of a long-stemmed funnel. The funnel should extend well into the 

bulb of the tube. 

3. Stopper the tube with a capillary plug and 

transfer the tube to the tube support in such 

a manner that the bulb end extends out of the 

tube support and the enlarged part of the 

tube is just inside. 

4. Put on safety glasses. 

5. Cover the upper part of the tube well with 

ice, and light the Meker burner. 

6. Set the timer for 10 min and brush the flame 

around the tube for about 1 min. 

7. Set the burner under the bulb of the Penfield 

tube and turn the tube slowly as the sample 

fuses. 

8. Light the propane part of the torch; turn on 

the oxygen and adjust the flame to a fine 

point. 

'Any Wle of trade names in tbia report ia for deecriptive purpoees only and does 

not c:oiUititute endonement by the U.S. Geologjeal Survey. 
FIGURE 3.-Penfield tube in tube support. 

9. Continue to heat the sample with the Meker 

burner until the 10 min have passed. 

10. Remove the Meker burner and place the 

torch in the left hand and a pair of tongs in 

the right hand. 

11. Place the tip of the torch in the middle of the 

extended tube, and pull off the bulb as the 

tube melts. Touch the torch to the end of the 

tube and seal it. 

12. Turn off the oxygen. 

13. Turn off the propane. 

14. Carefully remove the tube from the ice bath; 

grasp with a piece of filter paper and gently 

blot it dry. DO NOT RUB. The development 

by friction of a charge of static electricity on 

the surface of the tube can cause a serious 

error. 

15. Quickly remove the capillary plug from the 

tube and insert a displacement rod. Close the 

tube with a small cork. 

5 

16. Place the tube upright in a small beaker and 

allow it to stand in the balance room 30 min 

before weighing. 

17. Weigh a 150-ml beaker and record its weight. 

Place the tube with the cork removed in the 

weighed beaker and record this weight. Replace the cork in the tube and let it stand 

another 30 min. 

18. Repeat step 17 every 30 min until constant 

weight is obtained. 

19. Remove the cork and reserve. 

20. Set the tube upright in a beaker and place in 

an oven at a temperature of 105°-l10°C overnight. 

21. Replace the cork in the tube and set in the 

balance room for 30 min. 

22. Remove the cork and weigh the tube. Repeat 

the weighings until constant weight is obtained. 

23. The difference in the weight of the tube be-
fore and after drying is the weight of total 

water in the blank (a). 

24. Repeat this procedure with a 1-g sample and 

the 3 g of N~W0 • The weight loss of the 

tube in grams is the weight of total water in 

the sample (b). 

25. Calculate the percentage of water in the sample as follows: 

Percent total water (H20 + plus H20-) 

= 100 x (b- a)/sample weight in grams 

THE SAMPLE BLANK FOR THE MAIN PORTION 

Although running blank and standard determinations is considered standard practice, analysts 

may not follow this practice unless asked to do so. 

No determination can be considered accurate unless a reliable blank has been determined and the 

results obtained in the analysis have been corrected accordingly (Bennett and Reed, 1971, p. 7; 

American Society for Testing Materials, 1950a,b). 

Hillebrand and others (1953, p. 926) stated that 

a hot water bottle even of the best glass will afford weighable amounts of alkali to the water it 

holds. As Kolthoff and Sandell (1943, p. 287) 

stated: 

Errors arising from the impurities through the reagents 

and vessels, and sometimes from other sources, can be allowed for by running what is called a blank determination. 

In making a blank, the sample is omitted; otherwise the 

details of the procedure are followed exactly as far as possible. The same amount of reagent is added, the same volume of solution is employed, the same temperature is used 

in the operations, the same period of time is allowed for 

each step, and so on. The precipitate finally obtained is 

weighed, if the determination is a gravimetric one, and the 

amount found is deducted from the weight of precipitate 

given by the sample. The blank is best carried along simultaneously with the sample. Although the errors arising 

from the sources mentioned above are often greatly reduced by running a careful blank, they are not always entirely eliminated or even minimized, because the conditions in the determination itself cannot be exactly reproduced in the blank. 

Because conditions in the determination cannot 

always be duplicated in the blank, the analyst 

must be able to evaluate a reliable blank. For example, a sodium carbonate-sodium peroxide flux, 

if used to decompose the sample, will attack the 

platinum crucible at a lower temperature than will 

the mixture of the flux and the sample. Therefore, 

the fusion of the flux used to prepare the blank 

6 

should be made at a temperature no higher than 

900°C or large amounts of platinum may be unnecessarily introduced into the analysis. An example of another problem related to blanks was given 

by Hillebrand and others (1953, p. 19-21): "a large 

iron or aluminum hydroxide precipitate will carry 

down all silica, arsenic, and antimony derived 

from the glassware, whereas the very much smaller precipitate obtained in the blank experiment 

may not." 

Hillebrand and others (1953, p. 503) pointed out 

that one of the chief errors in a determination of 

Al20 3 in materials is that made by "misleading 

corrections for impurities derived from the reagents or through attack on the vessels used." 

They also said (Hillebrand and others, 1953, p. 

682): 

Corrections for impurities derived from the reagents or by 

attack of the containing vessels are apt to be entirely misleading if made as in the usual fashion by carrying the reagents alone through the analysis. The reason for this lies 

chiefly in the size of the precipitate obtained in the analysis, as compared with that obtained with the reagents. 

The former carries down practically all of the silica that is 

introduced by the reagents, attack on glassware, or other 

causes; the latter carries down very little. This then leads 

to high results for silica and can be avoided by adding to 

the solution of the reagents approximately as much iron 

and aluminum as are contained in the sample under analysis. 

For example, in analyses of 0.5-g portions of bauxite, an 

average correction of less than 0.1 mg of silica was indicated by the reagents alone, and 0.9 mg when 0.25 g of 

alumina as pure aluminum chloride was added to the reagents. Corrections based on the analysis of the reagents 

alone would therefore have caused a plus error of 0.18 percent of silica. 

In the procedure outlined in this report, a 

known amount of iron is added to the blank determination in order to increase the size of the precipitate obtained with the reagents to approximately the size of the precipitate obtained in the 

analysis of the samples. 

REAGENTS 

Standard iron solution (see page 22) 

Hydrofluoric acid (48 percent) 

PROCEDURE 

Transfer a suitable aliquot of standard iron solution to a 300-ml platinum dish, add 10 ml of HF, 

and evaporate the solution to dryness. 
Reserve this blank and use it in step 1 of "Removing the Melt from the Crucible" in the section 

"Determination of Silica in Samples Containing 

Less Than 2 Percent Fluorine." 

DETERMINATION Of MOISTURE 

A 1. 000-g portion of the sample is added to a 

weighed platinum crucible; the crucible and its 

contents are weighed, heated in an oven having a 

temperature of 105°-110°C, cooled in a desiccator, 

and reweighed. The heating and weighing are repeated until constant weight is obtained. The sample is reserved for the determination of silica. 

PROCEDURE 

1. Obtain the weight of a covered 30-ml 

platinum crucible that has been cleaned by 

being fused with potassium pyrosulfate, 

washed with water, boiled in 1 + 1 HCl, 

washed with water again, and then heated at 

1,200°C in an electric furnace until its weight 

is constant. 

2. Transfer a 1.0000-g sample to this crucible, 

and record the weight of the covered crucible 

plus the sample. 

3. Remove the cover from the crucible; replace 

it with a small piece of filter paper and place 

the platinum cover and the crucible and its 

contents in an· oven at 105°-110°C for 1 h. 

4. Remove crucible from oven. Replace the filter paper with the platinum lid, cool in desiccator, and weigh. 

5. Repeat the heating at 105°-110°C as before 

until constant weight is obtained. A loss in 

weight of more than 5 mg indicates an hygroscopic sample, of which all portions to be 

used in the analysis of the major constituents 

should be weighed at one time. 

6. Reserve this sample for the preparation of 

the main portion of the analysis as directed in 

the following procedures. 

7. Calculate the percentage of moisture (H20-) 

by multiplying the loss in weight in grams by 

100. 

ROASTING THE SAMPLE 

Roasting the sample as described below oxidizes 

any reducing substances present in silicates that 

would attack the crucible during the fusion. 

7 

PROCEDURE 

1. Support the covered platinum crucible, which 

contains the sample used for the moisture determination, on a clay or silica triangle over a 

burner and heat the crucible with a low flame 

to drive off combined water. 

2. Increase the height of the flame until the bottom part of the crucible is dull red. 

3. Move back the lid to permit the entrance of 

air and maintain the bottom of the crucible at 

a dull red for 5 min. 

4. Cover the crucible and allow it to cool. 

5. Reserve for fusion of the sample. 

DETERMINATION OF SILICA IN SAMPLES 

CONTAINING LESS THAN 2 PERCENT FLUORINE 

For determination of silica in samples containing less than 2 percent fluorine, platinum crucibles 

are generally preferred to porcelain crucibles because removing the separated silica from porcelain 

crucibles is difficult. However, special care must 

be taken to prevent the introduction of an unnecessary amount of platinum into the analysis as 

it precipitates fractionally from solution when the 

solution is treated with ammonium hydroxide. The 

precipitation of platinum can cause serious difficulties in the subsequent steps of the analysis (Maxwell, 1968, p. 132, 365-366; Hillebrand and others, 

1953, p. 341-343; Groves, 1951, p. 226). Platinum 

may be introduced by ferric chloride attacking the 

crucible. 

The removal of platinum by precipitation with 

hydrogen sulfide is not a pleasant operation and 

can lead to other complications. Elements other 

than those of the hydrogen sulfide group may be 

precipitated by hydrogen sulfide in strong acid 

solution through the formation of mixed sulfides. 

Iron may be carried down to some extent, and, 

unless the precipitate is very small (which indicates the presence of platinum alone), the precipitate should be redissolved and the hydrogen sulfide precipitation should be repeated. 

Rather than remove the platinum, the analyst 

should avoid introducing it by using a sodium carbonate-sodium peroxide fusion made in the electric furnace at 950°C for 20 min. This fusion effectively decomposes silicate rocks and does not attack the platinum crucible. The crucible should be 

weighed before and after fusions to verify that the 

loss of weight of the crucible is negligible. When 
this procedure is followed, the platinum crucible 

can be used without worries about introducing 

platinum into the sample even if the sample is iron 

rich. 

Hillebrand and others (1953), Shell (in Kolthoff 

and Elving, 1962), and Maxwell (1968) suggested 

that the residue from the second dehydration of 

silica be heated at 105°-110°C for 1 h. However, 

analysts in this laboratory have found that several 

milligrams of platinum are introduced into the 

analysis from the platinum dish if the residue from 

the HCl dehydration of silica in the platinum dish 

is heated at 105°-110°C, even if the residue contains little iron, and that the attack on the 

platinum dish increases as the iron content of the 

sample increases. Heating at 105°-110°C greatly 

catalyzes the reaction shown in the following equation: 

Pt0 +4Fes+ +6Cl- ~ PtCJ.s2- +4Fe2+ 

Once the solution in which platinum is present 

has been treated with ammonium hydroxide, it is 

apt to precipitate fractionally in subsequent operations involved in a complete analysis. It may show 

up as a yellowish- to greenish-gray deposit on the 

bottom of the beaker during evaporation following 

the separation of the R20s group or it may contaminate other precipitates. No problems were 

encountered when the residue from the silica dehydration was heated only to the temperature of 

the steam bath as recommended by Peck (1964, p. 

23), and silica was completely recovered. 

Potassium peroxide is added to the N ~COs 

used in the main fusion to ensure that iron in the 

sample is not reduced to the metallic state during 

the fusion. 

Alcohol is added to the platinum dish containing 

the fused cake before HCl is added in order toreduce Mn (VI), which would react with HCl to 

form chlorine. The presence of manganese is indicated if the carbonate melt is green; the presence 

of chromium is indicated if it is yellow. 

If the HF residue from the silica fluoridation 

weighs less than 5 mg, the R20s precipitate 

should be added to the crucible containing the 

small residue, as the introduction of platinum from 

a fusion of the residue could introduce a greater 

error than that made by weighing a small residue 

as R20s. If the weight of the residue exceeds 5 

mg, it must be added to the filtrate from the silica 

dehydrations before the R20s is precipitated. The 

solution of the HF residue can be effected for 

8 

many samples if a little warm 1 + 1 HCl is added 

to the crucible. 

The residual silica must be recovered from the 

R20s precipitate and this value added to the silica 

value obtained by double dehydration in order to 

obtain an accurate silica value. The procedure for 

the recovery of residual Si02 follows the procedure for solution of the R20s precipitate. 

N~COs 

N~02 

REAGENTS 

HCI (1 + 1, 5 + 95, and concentrated) 

H2S04 (1+ 1) 

HF (concentrated) 

Ethyl alcohol 

PROCEDURES 

FUSION OF SAMPLE 

1. Weigh 4 g of anhydrous N ~COs and carefully 

add about 2 g of it to the cooled roasted sample. 

2. Mix the sample and flux thoroughly with a 

short platinum stirring rod. 

3. Add approximately 0.2 g N~02 and again 

mix thoroughly. Do not attempt to brush off 

the stirring rod. Tap it against the sides of 

the crucible and "rinse" it in the remainder of 

the flux. 

4. Tap the crucible gently on the bench top to 

settle the fusion mixture and sprinkle the rest 

of the flux over the surface to provide a trap 

for decrepitating particles during the initial 

heating stages of the fusion step. 

5. Place the crucible in an electric oven at a 

temperature of 400° -500°C and gradually increase the temperature of the furnace to 

900°C. 

6. Remove the crucible and immediately grasp 

it with a pair of Blair tongs; swirl over a 

Meker burner for about 1 min. 

7. Remove the crucible cover and check the 

completeness of the fusion. 

8. Set the crucible containing the reagent blank 

aside and return the crucible containing the 

sample to the furnace. 

9. Increase the temperature of the furnace to 

950°C and maintain this temperature for 20 

min. 
10. Remove the crucible from the oven and immediately grasp it with a pair of Blair tongs. 

11. Remove the cover and swirl the crucible over 

a Meker burner for about 1 min. Do not allow 

the flame to envelop the crucible, and do not 

allow the crucible bottom ever to come into 

contact with the blue cone of the burner 

flame. 

12. Check for completeness of the fusion. 

13. Replace the cover of the crucible and return 

it to the furnace. 

14. Heat at 950°C for 10-15 min. If a preliminary 

semiquantitative spectrographic analysis indicated the presence of chromite or zircon, 

further heating at about 1, 100°C is necessary 

to insure the decomposition of these minerals. 

Some platinum attack occurs if heating is prolonged at this temperature and should be 

avoided if possible. 

15. Remove the crucible from the oven. 

16. Grasp the crucible with a pair of Blair tongs 

and immediately rotate the crucible as it cools 

so that the contents will solidify around its 

walls. 

17. Cover the crucible and allow it to cool. 

18. Grasp the crucible with the Blair tongs and 

again heat it over the Meker burner for no 

more than 15 s at a dull red heat; remove it 

from the flame and allow it to cool to room 

temperature. 

REMOVING THE MElT FROM THE CRUCIBlE 

1. Gently strike the bench top with the crucible 

to loosen the melt from the crucible, and 

transfer the melt to a 300-mi platinum dish. 

Should the cake not separate freely and easily, add water and place the water-filled crucible on the steam bath and heat to loosen the 

cake. 

2. Carefully transfer the contents of the crucible 

to the platinum dish and police the inside of 

the crucible to remove any adhering particles, 

adding all rinsings to the dish. If necessary, 

fill the crucible with water and digest overnight at room temperature. 

3. Rinse the crucible with a few milliliters of 

1 + 1 HCl and finally with water, transferring 

all washings to the platinum dish. 

4. Police the crucible and its cover. 

5. Heat the crucible at 1,200°C for 30 min. Cool 

in a desiccator and weigh. 

9 

6. Compare this weight with the weight of the 

crucible before the fusion was made in order 

to determine the attack of the fusion on the 

platinum. 

7. Repeat the heating and cooling until the crucible is at constant weight. 

8. Record this weight and reserve the crucible 

for the ignition of the dehydrated silica. 

9. Add enough water to the platinum dish tomake the volume about 75 mi. Add 10 mi of 

ethyl alcohol to the dish, cover it with a 

speedy-vap cover, and let the solution stand 

overnight or until cake has disintegrated. 

DEHYDRATION OF SiliCA 

1. Bend the lower 1¥2 in of a funnel at about a 

30° angle and support it in a filter stand. 

2. Stir contents of the platinum dish and verify 

that the cake has disintegrated. Crush any 

lumps that may be left by exerting light pressure with a platinum stirring rod. 

3. Set the dish under the funnel in such a manner that the end of the bent funnel stem is between the lip of the dish and the speedy-vap 

cover. 

4. Pour 50 mi of 1 + 1 HCl into a graduated cylinder and add through the funnel into the 

dish in portions small enough to avoid violent 

effervescence. 

5. Let the dish stand until most of the effervescence stops; then rinse the funnel with a little 

water, catching the rinsings in the dish. 

6. Remove the funnel. 

7. Wash the underside of the speedy-vap cover 

into the dish and replace it on the dish. 

8. Set the dish on the steam bath and let the 

solution evaporate to apparent dryness. 

9. Stir the residue as the solution goes dry. 

10. Without further heating, remove the dish 

from the heat, cool, and add 10 mi of concentrated HCI. 

11. Tilt the dish and allow the acid to wet all the 

residue and to run up the sides of the dish. 

After 1 or 2 min, add approximately 50 ml of 

water, washing down the sides of the dish and 

the stirring rod. 

12. Cover the dish and heat on the steam bath 

for 5-10 min, stirring to dissolve all soluble 

salts. 

13. Wash down the sides of the dish if necessary 

to keep the volume at approximately 50 mi. 
14. Decant the solution through an 11-cm #40 filter paper that has been washed once with 

HCl (5+95). 

15. Wash down the sides of the dish, stir the 

silica into the acid, and pour it quickly onto 

the filter. 

16. Wash any remaining silica onto the filter with 

a stream of hot HCl (5+95). 

17. Scrub the stirring rod and the inside of the 

dish with a rubber policeman; then wash with 

hot HCl (5+95) and pour the washings 

through the filter. 

18. Wipe out the inside of the dish and the underside of the lip with a small piece of filter 

paper, and add it to the filter. 

19. Finally wash the residue and paper 10 times 

with the hot HCl (5+95). Allow the solution 

to drain through each time before the next 

wash is added. 

20. When all traces of yellow color have been removed, lift the paper and drain the funnel 

stem into the beaker. 

21. Carefully fold the paper in the funnel with 

the aid of a small platinum stirring rod and 

place the paper in the weighed crucible reserved for the ignition of the silica. 

22. Place the crucible in an oven at 105°-110°C to 

dry. 

23. Rinse the funnel into the beaker; then quantitatively transfer the solution to the platinum 

dish. 

24. Reserve the beaker for the filtrate from the 

second dehydration. 

25. Again evaporate the filtrate to apparent dryness; then continue heating the dry residue 

for 1 h on the steam bath. 

26. Cool and drench the residue with 10 ml of 

concentrated HCI. Wet the salts with the acid 

as before. 

27. Add approximately 50 ml of water to the dish 

and continue with the digestion, decantation 

filtering and washing, catching the filtrate in 

the 400-ml beaker as before, except use 9-cm 

#40 filter paper for the filtration. 

28. Lift the paper and wash the funnel stem into 

the beaker. Reserve the solution. 

29. Carefully fold the paper in the funnel with 

the aid of a platinum stirring rod and add to 

the crucible containing the main silica precipitate. 

30. Place the cover on the crucible in such a manner that the crucible is only partially covered, 

10 

and place in a cold electric muffle furnace 

with the door partially open. 

31. Allow the temperature of the furnace to rise 

slowly to 800°C and maintain the temperature 

of the furnace at 800°C until all carbon is completely consumed. 

32. Finally, heat at 1,200°C until the weight remains constant. The weight of the crucible 

and the silica minus the weight of the crucible 

is the weight of the impure silica. 

33. Cautiously wash down the crucible lid into 

the crucible with a few drops of water. 

34. Wash down the sides of the crucible onto the 

silica. 

35. Add six drops of dilute H2S04 (1 + 1) and 10 

ml of concentrated HF to the crucible and set 

on a steam bath and evaporate to about 1 mi. 

36. Place the crucible and its contents on a hot 

plate and carefully evaporate until fumes of 

SOa cease. 

37. Cautiously heat the crucible over a small 

flame until the residue is dry; then place the 

covered crucible in an electric furnace at 

1,100°C for 15 min. 

38. Cool in a desiccator and weigh. 

39. Repeat steps 33-38 until the weight remains 

constant. The difference between this weight 

and the weight of the crucible plus the impure 

silica represents the amount of Si02 determined by double dehydration. To this amount 

of Si02 add the residual Si02 and calculate the 

percentage of Si02 by multiplying this weight 

by 100 and dividing by the weight of the sample. 

40. Record the weight of the residue from the 

volatilization of the silica. If this weight is 

more than 5 mg, the sample probably contains 

large amounts of titanium or zirconium. In 

any case, the residue should be examined visually to determine if it is more than a reddish-brown stain on the bottom of the crucible. 

41. If the weight of the HF residue is less than 

5 mg, reserve the crucible and its contents for 

the ignition of the R20 3 precipitate. 

42. If the weight of the residue is more than 5 

mg, add a few milliliters of warm 1 + 1 HCl to 

the crucible and digest for a few minutes at 

room temperature. 

43. Transfer the solution to the filtrate reserved 

for the R20 3 precipitation. 

44. If the residue is not dissolved in the warm 
HCl (1 + 1), filter it into the main solution, 

and burn off the residue. 

45. Add about 1 g of N~C03 to the crucible and 

fuse the residue in an electric furnace at 

950°C for 20 min. 

46. Add a little water to the melt and transfer 

the solution to the filtrate reserved from the 

double dehydration of silica. 

4 7. Reserve this solution for the precipitation of 

the ammonium hydroxide group. 

DETERMINATION OF THE R,()3 GROUP 

A total of 10 to 15 ml of HCl in a volume of 200 

ml is necessary to ensure that enough NH4Cl will 

be formed when the HCl is neutralized with 

NH40H to prevent the coprecipitation of magnesium with the R20 3 group. Iron, aluminum, and 

the like can be quantitatively separated from manganese by precipitation with NH40H in boiling 

solution, provided the solution is boiled for no 

more than 2 min after careful adjustment of the 

alkalinity and is then immediately filtered. If contamination of the R20 3 by the introduction of 

platinum is indicated, the ignited R20 3 precipitate 

may be fused with N ~C03 at 950°C and the R20a 

reprecipitated. 

REAGENTS 

Bromocresol purple indicator (0.1 percent}-Wet 

0. 5 g of bromocresol purple with a few milliliters of NH40H and dilute to 500 ml with 

water. Add just enough 1 + 1 HCl to make 

acid. 

NH4N03 wash solution (2 percent}-Add one drop 

of bromocresol purple and 20 g of NH4N03 to 

1 L of hot water; then add NH40H until the 

solution is just alkaline. 

NH40H-Add 500 ml of water to a 1-L 

polyethylene bottle, set in an ice bath, and 

add NH3 gas until the volume increases to 1 

L. 

H20+NH3 =NH40H 

HCl--(1 + 1 and concentrated) 

PROCEDURES 

PRECIPITATION OF THE R203 GROUP 

1. Heat the solution reserved for precipitation 

of the R20 3 group, which should be about 200 

11 

ml in volume and contain about 15 ml of HCl, 

to incipient boiling. 

2. Remove from the heat and add NH40H 

slowly to the hot solution by pouring NH40H 

from a graduate cylinder until a precipitate 

that just fails to redissolve begins to form. 

3. Add about two drops of bromocresol indicator 

to the solution and continue the addition of 

NH40H dropwise while stirring until the solution is purple. The presence of much iron 

will make recognition of the endpoint difficult. 

Test for completeness of neutralization during 

the dropwise addition of NH40H by tilting 

the beaker slightly and noting the color of the 

solution as a drop of ammonia or a drop of indicator strikes the surface. 

4. Add two drops of 1 + 1 HCl to the solution, 

which should be just acid, and bring it to a 

vigorous boil. 

5. Boil1 min while stirring. 

6. Remove the beaker from the heat and add 

one or two drops ofNH40H. 

7. Confirm that the solution is purple; then 

place the beaker on the steam bath and let 

the precipitate settle for no more than 5 min. 

8. Immediately filter the hot solution through 

an 11-cm #40 paper into a 600-ml beaker. 

9. Transfer the precipitate to the paper and 

wash once with the hot 2-percent NH4NOa 

wash solution. 

10. Wash the beaker and the precipitate four 

more times with the 2-percent NH4N03 wash 

solution. 

11. Lift the filter paper just enough to allow the 

liquid in the stem of the funnel to drain into 

the 600-ml beaker. 

12. Add a few drops of concentrated HCl to the 

solution and reserve for the filtrate from the 

second precipitation of the R20a group. 

13. Remove the bulk of the precipitate from the 

filter paper with a stirring rod, and place the 

precipitate and the rod in the beaker used 

from the first R20 3 precipitation. 

14. Pipet 5 ml of hot 1 + 1 HCl around the sides 

of this beaker. 

15. Rub the lip of the beaker with a small piece 

of filter paper and a drop of acid, and add the 

paper to the funnel containing the main precipitate. 

16. Place the beaker under the funnel and pipet 

5 ml of hot 1 + 1 HCl around the top edge of 

the filter paper and slowly over the precipi-
tate, allowing it to run through the funnel 

into the beaker. 

17. Pipet 5 mi of hot water over the precipitate 

in the same manner. 

18. With a small platinum stirring rod, lift the 

fold of the filter paper and dissolve the precipitate from the paper into the beaker with 

four more 5-mi portions of hot 1 + 1 HCl and 

hot water. 

19. Finally, wash the paper thoroughly with 

water, allowing the washings to drain into the 

beaker. 

20. Remove the filter paper and reserve. 

21. Rinse the funnel into the beaker and dilute 

the solution to about 150 mi. 

22. Heat the solution just short of boiling. 

23. Add about 1 mi of indicator and slowly add 

NH40H from a graduate cylinder to the hot 

solution until a precipitate begins to form. 

24. Add NH40H dropwise and precipitate the 

R20 3 group as described above. 

25. Add the reserved filter paper and macerate 

it. 

26. Add NH40H dropwise, if necessary, to ensure that the solution is just basic. 

27. Add 1 + 1 HCl dropwise until the solution is 

just acid. 

28. Bring the solution to a rapid boil, boil 1 min, 

then set on the steam bath. 

29. Add NH40H dropwise until the solution is 

just purple; then let it stand on the steam 

bath for no more than 5 min. 

30. Confirm that the endpoint has been reached 

and filter the hot solution through an 11-cm 

#40 filter paper into the 600-mi beaker containing the first filtrate. 

31. Quantitatively transfer the precipitate to the 

paper with the aid of a small piece of filter 

paper, the wash solution, and a policeman. 

32. Wash the beaker and the precipitate four 

times with hot 2-percent NH4N03 wash solution. 

33. Drain the funnel. 

34. Cover the precipitate with a watch glass and 

let it air dry; then transfer the paper and the 

precipitate to the previously weighed 

platinum crucible containing the HF residue 

from the silica determinations and reserve for 

the precipitate obtained from the recovery of 

R20 3 from the beaker. 

35. Make the filtrate just acid to HCl and evaporate to about 100 mi. 

12 

36. Rinse the face of the watch glass used to 

cover the beaker with a few milliliters of 1 + 1 

HCl, catching the washings in the beaker. 

37. Rinse down the sides of the beaker with a 

few milliliters of HCl and reserve for the recovery of R20 3 from the beaker. 

RECOVERY OF R203 FROM THE BEAKER 

1. Dilute the solution reserved for the recovery 

of R20 3 to about 100 mi. 

2. Heat just short of boiling and add one or two 

drops of bromocresol purple indicator. 

3. Add NH40H dropwise until the solution just 

turns purple; then add HCl dropwise until the 

solution is just acid. 

4. Bring the solution to a rapid boil and boil 1 

min. 

5. Remove from the heat and add NH40H dropwise until the solution is just purple. 

6. Digest on the steam bath no longer than 5 

min. 

7. Add a little paper pulp, and filter through a 

9-cm #40 filter paper into the 600-mi beaker 

containing the main portion of the filtrate 

from the R20 3 precipitation. 

8. Wash the beaker and the precipitate with 2-

percent · NH4N03 wash solution as described 

in the procedure for the precipitation of the 

R20 3 group, quantitatively transferring the 

precipitate to the papers. 

9. Add the papers and precipitate to the main 

R20 3 precipitate and reserve. 

10. Rinse the funnels into the 600-mi beaker containing the filtrate. 

11. Make the solution just acid with 1 + 1 HCl 

and heat just short of boiling. 

12. Evaporate the solution to about 150 mi; then 

make just ammoniacal. 

13. Digest on the steam bath, and if any precipitate appears, filter as before and add to the 

main R20 3 precipitate. 

14. If no precipitate appears, proceed with the 

ignition of R20 3 precipitate and the removal 

of ammonium salts from the filtrate. 

IGNITION OF R203 PRECIPITATE 

1. Place the cover on the crucible in such a manner that the crucible is only partially covered 

and place in a cold electric muffle furnace and 

leave the door partially open. 
2. Allow the temperature of the furnace to rise 

slowly to soooc and ensure that air has free 

access to the furnace during the initial stages 

of the ignition. At no time should the paper 

be allowed to burst into flames. 

3. Maintain the temperature of the furnace at 

800°C until all carbon is completely consumed. 

Keep the lid of the crucible displaced during 

the heating to prevent reduction of iron. 

4. Finally, heat at 1,150°C to constant weight. 

5. If the HF residue from the silica determination was less than 5 mg and the R20a precipitate has been added to the crucible containing 

this residue, subtract the weight of the crucible before determining silica from the weight 

of the crucible plus the R20 3 to determine the 

weight of the R20 3• If the HF residue has 

been fused and added to the solution before 

precipitating the R20 3 group, subtract the 

weight of the crucible after the fusion from 

the weight of the crucible plus the R20a in 

order to obtain the weight of the R20a. 

Percent R203 =(weight R20a in 

grams/sample weight in grams) x 100 

6. Reserve the R203 precipitate for the determination of residual Si02, Ti02, and total iron as 

Fe20a. 

REMOVAL OF AMMONIUM SALTS 

The method of J. Lawrence Smith is used in 

this laboratory to remove ammonium salts. By 

this operation, which requires no attention, 100 g 

of ammonium chloride can be separated as easily 

and safely as 1 g from a few milligrams of alkalies, 

magnesium, or the alkaline earths. Maxwell (1968) 

suggested that the NH4 salts be removed by this 

method. Peck (1964) did not recommend the removal of NH4 salts. Hillebrand and others (1953, 

p. 622-623) stated that the correct amount of ammonium chloride to have present is a problem; a 

large excess reduces the precipitation of magnesium and barium but at the same time retards the 

precipitation of calcium and more especially of 

strontium. They stated that no removal of ammonium salts is necessary unless special operations have introduced excessive amounts. 

I recommend that the ammonium salts be removed before calcium is precipitated because excess salts interfere with the precipitation of calcium and because the removal of ammonium salts 

is an operation that requires very little of the 

chemist's time. 

REAGENTS 

HCl 

HN03 (concentrated) 

Bromocresol purple indicator (0.1 percent) 

PROCEDURE 

1. Add 3-4 drops of bromocresol purple indicator to the filtrate from the R20a precipitation, make just. acid with 1 + 1 HCl, and 

evaporate to about 100 ml. 

2. Cover with a speedy-vap glass, cautiously 

add 50 ml of concentrated HN03, and evaporate to dryness in the steam bath. 

3. Rinse the cover and sides of the beaker with 

water and dilute to 100 ml. 

4. Heat the solution on the steam bath. 

5. If insoluble salts appear, repeat the addition 

of concentrated HN03 and the evaporation 

until a clear solution of about 100 ml volume 

is obtained. 

6. Make the solution just ammoniacal, then just 

acid to 1 + 1 HCl and continue with the procedure for the removal of manganese by ammonium persulfate. 

REMOVAL OF MANGANESE BY 

AMMONIUM PERSULFATE 

After the double precipitation of the ammonia 

precipitate as described above, most, if not all, of 

the manganese will be in the filtrate and will be 

precipitated with calcium and magnesium if it is 

not removed before the determination of these 

elements. This manganese must then be determined and the magnesium and calcium values corrected for its presence. 

The separation of manganese with ammonium 

persulfate in the presence of zirconyl hydroxide 

(Peck and Smith, 1961) is a simple and rapid 

method for the quantitative removal of manganese 

and should be used routinely before proceeding 

with the determination of calcium. 

REAGENTS 

ZrCl2 solution (5 percent)-Dissolve 25 g of 

ZrCl2 • 8H20 in 200 ml of water containing 5 

ml of 1 + 1 HCI. Let the solution stand over13 
night. Filter it through a #42 paper or its 

equivalent into a 500-ml volumetric flask and 

dilute to volume. 

HCl (1+1) 

NH40H (concentrated) 

Ammonium persulfate 

NH4N03 (2 percent}-Dissolve 20 g of NH4N03 in 

1 L of water and heat just short of boiling. 

Add one or two drops of bromocresol purple 

and NH40H dropwise until solution just turns 

purple. 

PROCEDURE 

1. To the solution reserved after the removal of 

ammonium salts, which should have a volume 

of 100 ml, add 1 ml of the 5-percent ZrC12 solution and a little paper pulp. 

2. Make the solution just ammoniacal to 

bromocresol purple, then just acid with 1 + 1 

HCI. 

3. Add 1 g of ammonium persulfate. 

4. Stir the solution and heat it for 20 min on the 

steam bath. 

5. Add 1 ml concentrated NH40H and continue 

heating the solution for another 5 min. 

6. Stir the solution and filter it at once through 

a #40 paper into a 400-ml beaker. 

7. Wash the inside of the beaker three times 

with the hot 2-percent NH4N03 solution, and 

transfer the washings to the filter. 

8. Wash the precipitate on the paper 10 times 

with the wash solution. 

9. Discard the precipitate. 

10. Make the filtrate just acid to HCl and reserve 

for the determination of calcium. 

11. Clean the glassware used for the removal of 

manganese with a dilute nitric acid solution 

containing hydrogen peroxide. 

GRAVIMETRIC DETERMINATION OF CALCIUM OXIDE 

Calcium is precipitated as calcium oxalate by 

adding ammonium oxalate to the hot hydrochloric 

acid solution from which all the other elements, 

save the alkali metals, magnesium, and the other 

alkaline earth metals, have been removed and 

neutralizing with ammonium hydroxide. 

The alkali metals are removed by double precipitation of the oxalate. Small amounts of barium 

as are usually found in rocks are usually removed 

when double precipitations are made. Strontium is 

14 

nearly completely precipitated with calcium. If the 

calcium value needs to be corrected for the presence of strontium and barium, a separation can be 

made by the nitric acid method described by 

Kolthoff and Sandell (1943, p. 746). 

Maxwell (1968, p. 366) stated that "Any 

platinum which separates with the calcium oxalate 

will be reduced to platinum during the ignition 

and can be removed by dissolving the ignited 

oxide and reprecipitating the calcium; the direct 

solution and reprecipitation of the calcium oxalate 

will not suffice." 

Magnesium is the most common contaminant of 

calcium oxalate. When calcium is in excess, it is 

usually separated from magnesium by double precipitation with ammonium oxalate. Satisfactory 

separations can be made if the amount of magnesium equals or is in moderate excess over that of 

calcium. Where an extremely accurate determination of a very small amount of calcium is desired, 

especially in the presence of much magnesium, the 

magnesium pyrophosphate precipitate must be 

analyzed for its calcium pyrophosphate content. 

The weight of the CaO recovered from the magnesium pyrophosphate precipitate is added to the 

weight of CaO found by double precipitation with 

(NH4)2C204. 

REAGENTS 

Ammonium oxalate (6 percent)-Dissolve 60 g of 

(NH ~C 4 in 1 L of hot water. Filter the 

solution into a 1-L flask. 

Ammonium oxalate (0.1 percent)-Take 16.67 ml 

of 6-percent ammonium oxalate solution and 

dilute to 1 L 

Bromocresol purple indicator (0.1 percent) 

HCl (concentrated, 5 percent, and 1 + 1) 

NH40H (1+1) 

HN03 (concentrated) 

PROCEDURE 

1. Add a few drops of bromocresol purple indicator to the filtrate from the manganese removal and make it just acid to HCI. 

2. Add 5 ml of concentrated HCl in excess, and 

while it is boiling vigorously, slowly add 50 ml 

of the 6-percent ammonium oxalate solution. 

3. Set the beaker on the steam bath and add 

(1 + 1) NH40H dropwise while stirring until 
the solution just turns purple; then add about 

1 ml in excess. 

4. Digest at room temperature overnight. 

5. Filter the solution through a 9-cm #42 filter 

paper into a 600-ml beaker, retaining as much 

of the precipitate in the original beaker as 

possible. 

6. Wash the precipitate in the beaker two or 

three times with cold 0.1-percent ammonium 

oxalate solution and pour the washings onto 

the filter. 

7. Wash the precipitate on the paper two or 

three times with the 0.1-percent ammonium 

oxalate wash solution. 

8. Remove the 600-ml beakers from the funnel 

and acidify the filtrate with a few drops of 

1+ 1 HCI. 

9. Reserve for the filtrate from the second precipitation of calcium. 

10. Place a 250-mi beaker under the funnel. 

11. Pipet about 10 mi of hot HCl (1 + 1) down the 

sides of the beaker. 

12. Pour the solution through the filter into the 

250-ml beaker. 

13. Wash the beaker thoroughly with hot 5-percent HCl and finally with hot water, pouring 

all washings through the filter into the 250-mi 

beaker. 

14. Lift the fold of the paper with a small 

platinum rod and wash the paper with the hot 

5-percent HCl solution and finally with water. 

The final volume should be aproximately 100 

mi. 

15. Transfer the paper to a weighed platinum 

crucible and reserve for the second calcium 

precipitate. 

16. Rinse the funnel into the solution with water. 

Add two or three drops of bromocresol purple 

to the beaker containing the dissolved calcium 

oxalate, place it on a steam bath, and heat 

just short of boiling. 

17. Add 1 + 1 NH40H dropwise until the solution 

just turns purple; then make just acid with 

HCl (1+ 1). 

18. Set the beaker on a wire gauze over a Meker 

burner and bring the solution to a vigorous 

boil while stirring. 

19. Add 10 mi of 6-percent ammonium oxalate to 

the boiling solution while continuing to stir. 

20. Set the beaker on a steam bath and add 1 + 1 

NH40H dropwise until the solution just turns 

purple. 

21. Add 1 mi of 1 + 1 NH40H in excess and keep 

the solution at room temperature overnight. 

22. Filter through a #42 9-cm paper into the 

beaker containing the filtrate from the first 

precipitation of calcium. 

23. Transfer the precipitate quantitatively to the 

paper with the aid of a policeman and the cold 

0.1-percent ammonium oxalate wash solution. 

24. Wash the precipitate and paper moderately, 

then fold the paper over the precipitate and 

add it to the crucible containing the paper reserved from the first calcium precipitation. 

25. Reserve the 250-mi beakers for the precipitation of calcium oxalate after dissolving the ignited precipitate. 

26. Place the crucible in a cold electric furnace. 

27. Allow the temperature of the furnace to increase gradually to 800°C in order to char the 

paper slowly. Do not increase the temperature of the furnace above 800°C until the carbon has been consumed. 

28. Finally, ignite the precipitate at 1, 150°C for 

30 min; cool and weigh. 

29. Repeat the heating and cooling until constant 

weight is obtained. 

30. Acidify the filtrate with a few drops of 1 + 1 

HCl and evaporate it to about 100 mi. If ammonia salts fall out of solution, remove them 

as before except add only 10 ml of concentrated HN03 before evaporating the solution 

to dryness. 

31. Reserve the filtrate for the determination of 

ma211esium. 

32. Add a few milliliters of water cautiously to 

the ignited precipitate and then cautiously 

add 10 ml of HCl (1 + 1). 

33. Transfer the solution to a 250-mi beaker and 

precipitate the calcium oxalate as before. 

34. Allow the solution to stand overnight, filter, 

wash, and ignite as before. 

If the weight of the precipitate obtained in step 

34 is the same as the weight of the precipitate before redissolving, calculate the percentage of CaO 

by multiplying the weight of CaO, in grams, by 

100 and dividing by the weight of the sample. If 

the weight of CaO is less than that after the first 

ignition, the precipitate should be redissolved and 

reprecipitated until the weight of the precipitate 

remains constant. 

Check the magnesium ammonium phosphate 

precipitate for the presence of calcium as directed 

in the procedure for the determination of magne15 
sium and correct the calcium value to include any 

calcium found in that precipitate. 

GRAVIMETRIC DETERMINATION OF MAGNESIUM 

Magnesium is precipitated as magnesium ammonium phosphate, ignited to magnesium 

pyrophosphate, and weighed as such. 

(2MgNH4P04}-+ Mg2P201 + 2NH3 + H20 

Two precipitations are always made. An excess 

of ammonium phosphate is added to the first precipitation to ensure quantitative precipitation of 

magnesium. In the reprecipitation, the excess of 

reagent is limited to the small amount required to 

reduce the solubility of the precipitate, thereby 

limiting the coprecipitation of other substances. 

An additional precipitation is made on the filtrate 

in order to verify that the precipitations have 

been complete. 

The precipitate is finally washed with ammonium nitrate wash solution to aid in the ignition 

of the precipitate. Special care must be taken in 

burning off the filter paper containing the magnesium ammonium phosphate precipitate. Rapid ignition at a high temperature usually leaves unburned carbon which remains even after prolonged heating. The phosphate can be reduced by 

strong heating in the presence of carbon, causing 

damage to the crucible. The paper is slowly 

burned off in an electric furnace at as low a temperature as possible before gradually raising the 

temperature to 1,100°C in an electric furnace. At 

this temperature, the precipitate can usually be 

heated to constant weight without decomposition. 

REAGENTS 

(NH4)2HP04-Dissolve 50 g of (NH4)2HP04 in 200 

ml of water and filter. 

NH40H (1 + 1, 10 percent, and concentrated) 

NH4NOa wash solution-Dissolve 200 g of 

NH4N03 in water, add 200 ml of concentrated 

NH40H, and dilute to 1 L 

Bromocresol purple indicator (0.1 percent) 

HCl (1:20) 

PROCEDURE 

1. Place the filtrate from the calcium determination, which should have a volume of about 225 

ml and should be just acid to HCl, in an ice 

bath and cool to about 10°C. 

16 

2. Add 15 ml of (NH ~HP04 solution (25 percent) and one or two drops of bromocresol 

purple indicator to the cold solution. 

4. Continue the stirring for several minutes and 

add a 10-percent excess of C<'ncentrated 

NH40H. 

5. Set the solution aside for at least 8 h in a cool 

atmosphere. 

6. Filter the solution through an 11-cm #42 

paper into a 400-ml beaker. The bulk of the 

precipitate should be kept in the beaker. 

7. Wash the beaker and precipitate moderately 

with 10-percent NH40H wash solution and 

pour the washings through the filter, adding 

all washings through the funnel into the filtrate. 

8. Evaporate the filtrate to about 100 ml andreserve for the second filtration. 

9. Set a 250-ml beaker under the funnel and dissolve the magnesium ammonium phosphate 

into the beaker with alternate 5-ml portions 

of 1:20 HCl and hot water pipeted around the 

sides of the beaker and poured through 

the funnel into the 250-ml beaker. 

10. Continue to dissolve the magnesium ammonium phosphate in the beaker and on the 

paper in this manner until it has been quantitatively transferred to the 250-ml beaker. 

The volume of the solution should be about 

100 mi. 

11. Transfer the filter paper to a weighed 

platinum crucible containing about 0.5 g of 

ammonium nitrate, char the paper, and 

slowly burn off the carbon at as low a temperature as possible with free access of air. 

12. Reserve for the main portion of the precipitate. 

13. Cool the solution to about 10°C; pipet 1 mi of 

the (NH4)2HPO 4 into the solution. 

14. Add one or two drops of bromocresol purple 

indicator and then add 1 + 1 NH40H dropwise 

while stirring constantly until the solution 

just turns purple. 

15. Continue the stirring as in the first precipitation while adding enough 1 + 1 NH40H to 

make a 5-percent solution by volume. 

16. Allow the solution to stand overnight and filter on an 11-cm #42 paper, quantitatively 

transferring the precipitate to the paper and 

washing moderately with the cold 10-percent 

NH40H solution. 
17. Catch the filtrate and washings in the beaker 

containing the filtrate from the first magnesium precipitation. 

18. Finally, wash the precipitate on the filter 

with two 10-ml portions of the NH4N03 wash 

solution; then transfer it to the platinum crucible containing the residue from the first magnesium precipitation. 

19. Evaporate the filtrate to about 200 ml andreserve for the final recovery of magnesium. 

20. Set the crucible in a cold electric furnace and 

slowly dry and char the paper without inflaming. Do not allow the temperature of the furnace to exceed 450°C until the paper is consumed. 

21. Allow the temperature of the furnace to 

reach 800°C gradually and maintain this temperature until the precipitate is a light gray. 

22. Finally ignite the precipitate at 1,100°C to 

constant weight. 

23. Cool the filtrate reserved from the previous 

precipitation of magnesium to about 10°C, 

make just acid to HCl, and then add 15 ml of 

(NH4)2HP04 solution (25 percent) and one or 

two drops of bromocresol purple indicator. 

24. Add 1 + 1 NH40H dropwise while stirring 

constantly until the solution just turns purple. 

25. Continue the stirring for several minutes and 

add a 10-percent excess of concentrated 

NH40H. 

26. Set the solution aside for at least 8 h in a cool 

atmosphere. 

If any precipitate is observed, it should be filtered, redissolved, reprecipitated, and ignited to 

constant weight. This weight is added to the 

weight of the main precipitate. 

Calculate the percentage of MgO as follows: 

Percent MgO =(weight Mg2P20 7 

x 0.3622/sample weight in grams) x 100 

Reserve the ignited precipitate for the recovery 

of calcium in the magnesium pyrophosphate precipitate if this step is deemed necessary, and correct the magnesium value obtained as directed if 

any calcium is found to be present. 

RECOVERY OF CALCIUM FROM 

THE MAGNESIUM PYROPHOSPHATE PRECIPITATE 

Calcium was found in the magnesium pyrophosphate precipitate of standard samples PCC-1 and 

DTS-1. The accepted MgO value of PCC-1 is 43.18 

percent, and that of DTS-1 is 49.80 percent. The 

accepted value for CaO content of PCC-1 is 0.51 

percent and that for DTS-1 is 0.15 percent. In 

both samples, all the CaO was found in the magnesium pyrophosphate precipitate. The method 

used here is based on one that was devised by 

Hillebrand for the recovery from the magnesium 

pyrophosphate precipitate of the small amount of 

calcium that escapes precipitation as oxalate. The 

method as described by Hillebrand and others 

(1953, p. 613) and Maxwell (1968, p. 367) omits the 

usual precipitation with oxalate, and calcium is 

precipitated as phosphate along with the magnesium and is recovered from the magnesium ammonium phosphate precipitate as described below. 

When several samples of varying MgO and CaO 

content are run routinely, it is convenient to precipitate the calcium with oxalate in all the samples 

and to recover the calcium in the magnesium 

pyrophosphate precipitate only for those samples 

containing very small amounts of calcium oxide 

and large amounts of magnesium. 

17 

H2S04 (1+ 1) 

Ethyl alcohol 

REAGENTS 

Ammonium oxalate (6 percent) 

Water-alcohol wash solution (15:70) 

HN03 (concentrated) 

HCl (5 percent) 

Bromocresol purple indicator (0.1 percent) 

PROCEDURE 

1. Transfer the ignited and weighed magnesium 

ammonium phosphate precipitate to a 250-ml 

beaker. 

2. Add H2S04 (1 + 1) slowly by pipet to dissolve 

the precipitate, avoiding more than approximately 0.5 ml in excess. If the precipitate dissolves with difficulty, add 1-2 ml concentrated HN03 , boil, and then evaporate to 

fumes of 803• Cool. 

3. Dissolve the residue in the minimum amount 

of water and add 70 ml of ethyl alcohol for 

each 15 ml of solution. 

4. Mix thoroughly and let stand overnight. 

5. Filter through a 9-cm #42 filter paper, quantitatively transferring the precipitate to the 

paper. 
6. Wash moderately with the water-alcohol 

wash solution. 

7. Dissolve the precipitate into a 250-ml beaker 

with hot 5-percent HCI. 

8. Lift the fold of the paper with a small 

platinum rod and wash the paper and funnel 

with 5-percent HCl and finally with water. 

9. Dilute the solution to about 100 mi. 

10. Add two or three drops of bromocresol purple to the solution. 

11. Heat just short of boiling and precipitate calcium with ammonium oxalate as described in 

the procedure for the gravimetric determination of calcium. 

12. Ignite the precipitate to constant weight. 

13. Add this weight to the weight of calcium 

oxide previously found and correct the weight 

of Mg2P 20 7 for co precipitated calcium by subtracting the weight of Ca3(P04)2 equivalent to 

the CaO recovered from the Mg2P 20 7 precipitate. 

1.8438 

CaO~Caa(P04)2 

0.5424 

SOLUTION OF THE R203 PRECIPITATE 

AND THE DETERMINATION OF RESIDUAL SILICA 

Many tests have been made and reported in the 

literature showing that silica is not completely recovered in two dehydrations. Various methods 

have been suggested for the recovery of this 

silica. Usually the ignited R20 3 is dissolved in 

potassium pyrosulfate, sulfuric acid is added, and 

the silica is recovered by dehydration. Total iron 

and titanium are determined on the filtrate. 

Dissolved platinum interferes in the determination of total iron; therefore, the fusion of the main 

portion of the precipitate is made in a vycor crucible, and the platinum crucible is cleaned by a 

short fusion at as low a temperature as possible. 

Fusion of the R20 3 can be effected efficiently in 

vycor crucibles at a relatively low temperature. 

However, I found that when vycor crucibles containing 7 g of potassium pyrosulfate were heated 

over a Meker burner, cleaned, and. reweighed four 

times, they consistently weighed 2 mg less after 

each fusion. Fusions made in vycor crucibles in an 

electric furnace at 600°C showed no loss of weight. 

The pyrosulfate fusion of the ignited R20 3 precipitate is next transferred to a beaker or to a 

platinum dish, sulfuric acid is added, and after the 

18 

cake is dissolved, the solution is brought to fumes 

of S03 to dehydrate the silica. When the silica was 

fumed in Pyrex beakers, visual observation indicated attack of the beakers. Therefore, the use of 

platinum dishes is recommended for the dehydration of the residual silica. 

The ignited precipitate must be heated finally at 

a temperature of 1,150°C--1,200°C until constant 

weight is obtained, and the treatment with sulfuric acid and hydrogen flouride to volatilize the 

silica must be repeated until constant weight is 

obtained. 

REAGENTS 

~0.,-In our laboratory, we have obtained 

good results by using Bakers Instra-analyzed 

acid-flux grade. 

H2S04 (1+1) 

HF (concentrated 

PROCEDURE 

1. Weigh 6 g of ~07 and carefully transfer 3 

g of it to the crucible containing the ignited 

R20a residue. 

2. Stir with a platinum rod to mix. 

3. Working over a sheet of powder paper, place 

a 30-ml vycor crucible upside down over the 

platinum crucible, rinse the stirring rod in the 

flux, and transfer the contents of the 

platinum crucible to the vycor crucible. 

4. Add 1 g of ~07 to the platinum crucible. 

Rinse the platinum rod in the K~0 • 

5. Place the cover on the vycor crucible and 

place in an electric furnace at 400°C. 

6. Gradually increase the temperature of the 

furnace to 700°C for another 30 min. 

7. Set the vycor crucible and its contents on a 

triangle supported by a tripod and placed 

over a small flame. 

8. Gradually increase the temperature until S03 

fumes just begin. 

9. Grasp the crucible with the Blair tongs and 

swirl over the flame for 1 or 2 min. 

10. Verify that the fusion is complete and set the 

crucible aside to cool. 

11. Place the platinum crucible in a furnace at a 

temperature of 600°C for 5 min; then swirl 

over a small flame for about 30 s. Do not heat 

the crucible above a dull red. 

12. Cool. 
13. Wash the cover into the crucible and digest 

the crucible and its contents on the steam 

bath until the melt disintegrates. 

14. Quantitatively transfer the contents of the 

crucible to a 75-ml platinum dish and evaporate on the steam bath. 

15. Heat the platinum crucible at 1, 150°C for 

about 15 min. Cool and weigh. 

16. Compare this weight with the weight of the 

empty crucible before igniting the R20 3 precipitate. 

17. The loss in weight should not exceed more 

than a few tenths of a milligram as the solution for determining iron should be platinum 

free. 

18. Wash the top of the vycor crucible into the 

vycor beaker with a little water. 

19. Set the crucible on the steam bath until the 

melt has dislodged from the crucible; then 

quantitatively transfer the melt and the solution to the 75-ml platinum dish. 

20. Add enough water to make the volume 50-60 

ml and heat on the steam bath until the melt 

has dissolved. 

21. Transfer the platinum dish and its contents to 

a hot plate and evaporate to about 40 ml. 

22. Carefully pipet 20 ml of 1 + 1 H2S04 into the 

dish and continue the evaporation. 

23. Finally increase the surface temperature of 

the hot plate to about 350°C. 

24. Bring the solution to fumes of S03 with occasional stirring. 

25. Fume for about 10 min; then remove from the 

heat and cool. 

26. Dilute the solution to about 70 ml, set on the 

steam bath, and digest until only a fleecy residue of silica remains. Add water as necessary to keep the volume of the solution about 

70ml. 

27. Cool the solution and filter through a 9-cm 

#42 paper into a 100-ml volumetric flask. 

28. Quantitatively transfer the precipitate· to the 

filter with the aid of a small piece of paper 

and a policeman. 

29. Wash the precipitate and paper with a few 

milliliters of water; then transfer to a 

weighed platinum crucible. 

30. Reserve the filtrate. 

31. Carefully ash the paper in the crucible. 

32. Finally, ignite the precipitate at 1,200°C until 

constant weight is obtained. 

33. Add one or two drops of 1 + 1 H2S04 and 

19 

about 5 ml of concentrated HF to the crucible 

and volatilize the silica. 

34. Repeat the volatilization and ignition until 

constant weight is obtained. 

35. The weight of residual silica equals the 

weight of the crucible plus the silica minus 

the weight of the crucible plus the residue. 

36. Add 1 g of ~07 to the residue from the 

volatilization of the residual silica and fuse in 

an electric furnace at 600°C. 

37. Cool. 

38. Add a little water to the melt and digest on 

the steam bath until dissolved. 

39. Filter into the 1()0-ml volumetric flask containing the main solution from the residual 

silica. 

40. Make the filtrate to volume and reserve for 

the determination of total iron and titanium. 

DETERMINATION OF TITANIUM OXIDE 

ON THE SOLUTION OF THE R203 GROUP 

Peck (1964) recommended that titanium be determined on a portion of the solution of the R203 

precipitate. The transmittance of a portion of the 

sample is measured, hydrogen peroxide is added 

to the cell, and the transmittance is measured 

again. 

The amount of titanium oxide equivalent to the 

transmittance of the unperoxidized solution is subtracted from the equivalent of the transmittance 

of the peroxidized solution in order to obtain the 

correct value for titanium oxide. The solution in 

the cell and the rinsings are reserved and combined with the remainder of the R203 solution 

and, after evaporation to reduce the volume and 

to expel hydrogen peroxide, are used for the determination of total iron. 

Maxwell (1968, p. 423) recommended that total 

iron be determined by use of the solution from the 

ferrous iron determination and stated that if the 

sulfuric acid solution of the pyrosulfate fusion of 

the R203 group residue is used for the colorimetric determination of Ti02, the solution should be 

evaporated to a small volume to expel all hydrogen peroxide, that H2S should be passed through 

the solution to remove platinum, and that the solution should be evaporated to expel all traces of 

H2S before the titration of the iron. 

Kolthoff and Sandell (1943) determined titanium 

on the solution remaining after the titration of 

iron with permanganate. 
Scheffer (in Kolthoff and Elving, 1961) determined titanium on a separate sample. Hillebrand 

and others (1953, p. 581) stated, "In rock 

analyses, the test is usually made after the volumetric determination of iron in the sulfuric acid 

solution of the pyrosulfate fusion of the weighed 

ammonia precipitate." Titanium and iron are determined in our laboratory on separate aliquots of 

the sulfuric acid solution of the R20 3 precipitate. 

This procedure has proven to be convenient and 

accurate. Platinum need not be removed prior to 

the iron titration as only a negligible amount of 

platinum will be introduced into the analysis if the 

precautions mentioned above are observed. 

Because 3-percent hydrogen peroxide deteriorates rapidly, it is prepared from the very stable 

30-percent peroxide as needed. Beer's law was observed for solutions containing as much as 0.2 mg 

Ti02 per milliliter. 

Chromium, vanadium, and iron interfere in the 

colorimetric determination of titanium because of 

the color of their solutions and may be present in 

the R20 3 residue to such an extent that a correction is necessary. This correction is made by subtracting the reading of an aliquot of the sample 

without hydrogen peroxide from the reading obtained for another aliquot to which hydrogen 

peroxide is added. 

All readings are made at the same room temperature to eliminate the effect of temperature on 

the color intensity. 

REAGENTS 

Standard Ti02 solution (1 ml = 1.00 mg TiO~ 

Weigh 1.013 g of U.S. National Bureau of 

Standards sample No. 154 (Ti02 98.7 percent) 

that has been dried overnight in an oven at a 

temperature of 105°-110°C, and transfer it to 

a 400-ml beaker. Add a magnetic stirring bar 

and 100 ml of concentrated H2S04 to the 

beaker. Set the beaker and its contents on a 

magnetic stirring hot plate (such as Fisher 

Thermix) and bring the solution to a constant 

boil while stirring. Continue the heating and 

stirring until the solution is complete. Cool, 

then rapidly pour it into about 500 ml of 

water and allow it to stand at room temperature overnight. 

Filter the solution through a 11-cm #42 filter paper into a 1-L flask and wash the 

beaker and the paper with water. Place the 

20 

paper in a vycor crucible. Slowly char the 

paper and carefully burn off the resulting carbon. Add 1 g of ~07 to the crucible and 

fuse the residue in an electric furnace at 

700°C for 30 min. Cool the melt, add a few 

milliliters of the Ti02 solution to dissolve the 

melt, transfer it to the flask, and make to volume. 

~07 solution (16 percent}-Dissolve 80 g of 

~07 in about 400 ml of hot water. Cool to 

room temperature and filter into a 500-ml volumetric flask. Wash the beaker and paper 

moderately with water and dilute the solution 

to 500 ml. 

H20 2 (3 percent}-Pipet 2.5 ml of 30-percent H20 2 

and transfer it to a 25-ml volumetric flask and 

dilute to volume. Prepare just before using. 

H~04 (1+1) 

H~04 (1+1) 

PROCEDURE 

This procedure is to be followed as outlined if 

the solution of the R20 3 precipitate contains 8 g of 

~07 and is 10 percent in H~0 • Adjust the 

~07 and H~04 content of the standards if 

other than these amounts are used. 

1. Pipet two 20-ml aliquots of the solution of the 

R20 3 precipitate and transfer each to a separate 25-ml volumetric flask lettered A or B. 

2. Dilute the sample in flask A to 25 ml with 

water. 

3. Reserve flask B. 

4. Prepare a series of standard solutions containing 0, 0.10, 0.50, 1.00, 2.00, 3.00, and 5.00 

ml of standard Ti02 solution (1 ml = 1 mg 

Ti02) in 25-ml volumetric flasks; each flask 

should contain 4 ml of 1 + 1 H2S04 and 10 ml 

of 16-percent solution of ~0 • 

5. Dilute these solutions to about 20 ml; then 

add 1.25 ml of 3-percent H20 2 solution to each 

flask and to flask B, which contains an aliquot 

of the sample. 

6. Dilute the solutions to 25 ml and mix thoroughly. 

7. Arrange the standards and the sample in 

flask B in order of the intensity of the color 

of the solutions. 

8. Measure the absorbance of each solution in a 

1-cm cell in this order, using distilled water as 

a reference solution and measuring the absorbance of the solutions in flask A. 
9. Correct the absorbance reading of solution B 

for any color given to the solution by interfering elements by subtracting the absorbance 

reading of solution A from the absorbance 

reading of solution B. 

10. Prepare a curve from the absorbance reading 

of the standards and obtain the weight of 

Ti02 measured from this curve. 

11. Calculate the percentage of Ti02 in the sample as follows: 

Percent Ti02 = [weight of Ti02 in aliquot/ 

weight of sample in aliquot ( = 200 mg)] x 100 

Percent Ti02 =weight of Ti02 in aliquot/2 

DETERMINATION Of TOTAL IRON ON 

THE SOLUTION Of THE R203 GROUP 

An aliquot of the solution of the R203 group is 

used for the determination of total iron. The 

method is essentially that recommended by Peck 

(1964) except that the solution from the titanium 

determination is not reserved and evaporated, and 

the necessity of removing hydrogen peroxide is 

thereby eliminated. 

APPARATUS AND REAGENTS 

Silver reductor (fig. 4)-A Jones-Blair reductor by 

Kimax (Kimble No. 39000) or Sargent-Welch 

(No. S-72227) filled with a 1:1 ratio by volume 

of electrodeposited and copper-reduced silver 

(Dinnin, 1960, p. 295-,.296; Kolthoff and 

Belcher, 1957, p. 12-16). 

A 10-ml micro-buret 

Standard K2Cr207 solution (0.1 N)-Weigh 9.810 

g of primary standard grade K2Cr20 7 into a 

weighing bottle. Record the weight of the 

bottle plus the reagent. Place the bottle and 

its contents in an oven at 105°-ll0°C and dry 

for 2 h. Cool in a desiccator and weigh. Repeat the heating (at 105°-ll0°C) and cooling 

until constant weight is obtained. With the 

aid of a small funnel, wash the contents of the 

weighing bottle into a 2-L volumetric flask. 

Dry the weighing bottle in the oven, cool, and 

reweigh to obtain the true weight of the 

K2Cr207 used. Add about 1 L of water to the 

flask and mix to dissolve the reagent; then dilute to volume with water and mix thoroughly. Standardize against standard iron solution. 

Stannous chloride solution-Dissolve 5 g of FIGURE 4.-Silver reductor. 

21 
SnC12·H20 in 10 mi of HCl and dilute to 100 

mi. Add scraps of iron-free granulated tin and 

boil until the solution is clear. Keep the solution in a closed dropping bottle containing 

metallic tin. 

NH4Cl solution (15 percent)-Dissolve 300 g of 

NH4Cl in water, filter, and dilute to 2 L. 

Standard Fe20 3 solution-Transfer 1.2815 g of 

standard sample Sibley iron ore (U.S. N ationa! Bureau of Standards standard sample 

#27) that has been previously dried in an 

oven for 2 h to a previously weighed weighing 

bottle. Record the weight of the bottle and its 

contents. 

Place the bottle and its contents in an oven 

at 105°-110°C and repeat the heating and 

weighing to constant weight. 

Quantitatively transfer the iron ore to a 

250-mi beaker with the aid of 10 mi of concentrated HCI. Add 40 ml more concentrated 

HCl to the beaker, cover the beaker, and digest the solution just short of boiling for 

about 30 min. If the iron ore has not completely dissolved, add a little of the stannous 

chloride dropwise, and continue the digestion 

until all the dark particles have dissolved. 

Dilute the solution to about 100 mi with 

water and add 3 ml of bromine water to destroy any excess SnC12; then boil vigorously 

to destroy the bromine water; Cool and filter 

into a 250-mi volumetric flask, wash the paper 

moderately with water, ash the paper, treat 

with a little concentrated HCl, then evaporate to dryness and fuse the residue with 

about 0.5 g of N~C0 • Dissolve the melt in a 

little water, transfer to the volumetric flask, 

and make to volume. 

Sodium diphenylamine sulfonate (0.2 percent)-

Dissolve 0.2 g of sodium diphenylamine sulfonate in 100 mi of hot water. 

HCl (1 + 1 and 5 percent) 

NH40H (1+1) 

H3P04 (1+ 1) 

PROCEDURE 

1. To a series of 250-mi beakers, add aliquots of 

standard Fe20 3 solution to make solutions 

containing 0-100 mg of Fe20 3 • Dilute to 50 

mi. 

2. Add aqueous 1 + 1 NH40H while stirring 

until a precipitate that does not dissolve just 

22 

begins to form; then add 1 + 1 HCl dropwise 

to just dissolve the precipitate. 

3. Add 25 ml of 15-percent NH4Cl solution to 

adjust the volume of the solution to 100 mi. 

4. Pour 500 mi of 5-percent HCl through thereductor and allow the solution to drain until 

the level is about 1

/s in above the silver. 

5. Add 20 mi of 1 + 1 H3P04 to a clean 600-mi 

beaker and lower the reductor into the beaker 

until the delivery tube dips into the acid. 

6. Transfer about half the iron solution to the 

reservoir. 

7. Fully open the stopcock on the reductor, and 

as the solution drains, transfer the remainder 

of the iron solution to the reservoir and let 

the solution drain until its level is about lh in 

above the silver. 

8. Lift the reductor as the volume of the 

effluent increases, but keep the tip of the delivery tube below the level of the liquid. 

9. Wash the inside of the reservoir with a small 

amount of 5-percent of HCl and continue 

draining until the volume of the solution inthe 

beaker is almost 300 mi. 

10. Close the stopcock and lift the delivery tube 

clear of the liquid. 

11. Wash the tip of the delivery tube with water. 

12. Add two or three drops of 0.2-percent solution of sodium diphenylamine sulfonate to the 

solution and immediately titrate with standard K2Cr20 7 solution, while stirring, until 

the purple color caused by the addition of one 

drop of titrant disappears slowly. 

13. Wash down the inside of the beaker with a 

fine stream of water; then continue adding 

the K2Cr20 7 solution in fractional drops while 

stirring the solution until a distinct purple 

color forms that persists for at least 30 s. 

14. Calculate the normality of the solution as follows: 

Normality of K2Cr20 7 solution 

mg of Fe20 3 titrated by 1 ml 

of 0.1N K2Cr207 (7.985 mg) 

= mg Fe20 3 (mg Fe20 3 m standard sample) 

titrated by the K2Cr20 7 used 

15. Transfer 10 ml of the solution of the R20 3 

precipitate that was reserved for the iron determination to a 250-ml beaker and proceed as 

directed in steps 1-3 above. 

16. Calculate the percentage of total iron in the 

sample as follows: 
Percent total iron as Fe20 3 

= (M x N x 79.34/sample weight 

in milligrams) x 100 

where 

M = milliliters of K2Cr207 

N = normality of the K2Cr20 7 

79.34 = iron equivalent 159.68/2 

DETERMINATION Of FERROUS IRON 

The Cooke method (1867) of decomposing the 

sample in a hydrofluoric-sulfuric acid mixture in 

the absence of air and titrating the ferrous iron in 

the solution with permanganate has been studied 

and modified by many investigators. Because of 

the rapid oxidation of ferrous sulfate in the presence of hydrofluoric acid, air must be removed before hydrofluoric acid is added to the sample and 

must be excluded thereafter 

Pratt (1894) simplified the method by eliminating the Cooke apparatus and using a large 

platinum crucible fitted with a perforated cover 

for the introduction of carbon dioxide into the 

crucible containing the mineral powder and boiling 

sulfuric and hydrofluoric acids. 

Hillebrand and others (1953, p. 915) used carbon 

dioxide only at the start and depended on the 

steam of the boiling solution to exclude air. They 

recommended the use of a 100-ml platinum crucible for the decomposition of the sample. 

Barnebey (1915) suggested the use of potassium 

dichromate for the titrant and the addition of boric 

acid to stabilize the ferrous iron against the action 

of the air during the titration with potassium ferricyanide and also to avoid a large displacement of 

the observed endpoint in the presence of hydrofluoric acid. 

Knop (1924) worked out the details for the use 

of diphenylamine as an internal indicator for this 

reaction. Sarver (1927) recommended the use of 

dichromate together with diphenylamine as an internal indicator. 

Kolthoff and Sandell (1943) described a simple 

procedure for the determination of ferrous iron in 

which carbon dioxide is not used during the decomposition. The sulfuric acid is heated nearly to 

boiling before the hydrofluoric acid is added. The 

steam from the boiling sulfuric acid excludes the 

air from the crucible. This method has been found 

satisfactory for most silicates. A 100-ml platinum 

crucible is used for the decomposition, and the 

crucible and its contents are then immersed beneath the surface of a sulfuric-boric acid solution; 

the ferrous iron is titrated with a standard solution of K2Cr20 7 and diphenlyamine sulfonic acid is 

used as an indicator. 

Research on the problems associated with the 

determination of ferrous iron has been extensive. 

Many of the problems that may arise in analyzing 

unusual samples have been discussed by Hillebrand and others (1953), Maxwell (1968), and Peck 

(1964). 

APPARATUS AND REAGENTS 

Platinum crucible of 100-ml capacity 

Blair-type tongs made of nichrome, the handle 

bent at an angle of 90° 

Timer 

Surface thermometer 

H3B03 solution, 5 percent (saturated) 

Barium diphenylamine sulfonate indicator (0.2 

percent) 

H2S04 (1+ 1) 

HF (concentrated) 

H3P04 (concentrated) 

Standard K2Cr20 7 solution (see p. 21) 

23 

PROCEDURE 

1. Weigh 0.500 g of sample into a 100-ml 

platinum crucible having a tightly fitting 

cover. 

2. Disperse the sample in about 5 ml of water 

and add two or three drops of 1 + 1 H2S04 to 

decompose any carbonate materials present. 

3. Cover the crucible and allow it to stand until 

all reaction has ceased. 

4. Add 10 ml of 1 + 1 H~04 to a plastic 

graduate cylinder, and add 5 ml of concentrated HF to another plastic graduate cylinder. 

5. Set the crucible on a hot plate that has a surface temperature of about 400°C. 

6. Pour the 5 ml of HF into the graduate cylinder containing the 10 ml of 1 + 1 H2S04 

slightly displace the crucible lid, and quickly 

and carefully add the H2S04-HF solution to 

the crucible. 

7. Replace the cover on the crucible and boil the 

sample gently for 10 min. 

8. During this 10 min, add 10 ml of concentrated 

H3P04 to a 600-ml beaker containing 200 ml 

of water, 50 ml of saturated H3B03 solution, 

and 35 ml of 1 + 1 H2S04 and place the beaker 

near the crucible. 
9. Grasp the crucible with the Blair-type tongs, 

and while pressing the top of the crucible 

with a glass rod, immerse the crucible in the 

H3B03 solution. 

10. Remove the tongs and turn the crucible on its 

side. 

11. Swirl it to allow its contents to go into the 

solution. 

12. Remove the crucible with the aid of a glass 

rod and wash particles clinging to it into the 

solution with the aid of a little water. 

13. Add four or five drops of 0.2-percent barium 

diphenylamine sulfonate indicator and titrate 

with standard K2Cr20 7 solution until the purple color caused by the addition of one drop of 

K2Cr20 7 fades slowly. 

14. Wash down the inside of the beaker and continue the titration by adding fractional drops 

of K2Cr20 7 until a purple color forms that 

lasts for 30 s. 

15. Calculate the percentage of FeO as follows: 

Percent FeO = (M x N x 71.85 x 100/ 

sample weight in milligrams) x 100 

where 

M = milliliters of K2Cr20 7 

solution used 

N = normality of K2Cr20 7 

71.85 = iron equivalent 

DETERMINATION OF AI,03 

Aluminum is one of the elements precipitated 

by ammonium hydroxide from the filtrate reserved from the double dehydrations of silica. This 

precipitate is commonly called the R20 3 group, 

and the weight of aluminum oxide is obtained by 

deducting the weight of the other elements from 

the total weight of the group. 

The total iron and the Ti02 (determined on the 

solution of the dissolved precipitate), the P20 5 and 

the residual silica are elements other than 

aluminum usually found in the R20 3 precipitate of 

a silicate when the suggested procedure is followed. 

If present in the sample, zirconium, chromium, 

vanadium, the rare earths, beryllium, and members of the H2S group will be precipitated also and 

must be determined and subtracted from the total 

weight of the oxides or they will be counted as 

Al203 • However, most silicate rocks contain only 

negligible amounts of these elements, and they 

are not determined in routine analyses. 

24 

Calculations: 

Percent Al20 3 =percent R20 3 - percent total Fe 

as F~0 -percent P205-percent Ti02 

PREPARATION OF THE SOLUTION FOR 

THE DETERMINAnON OF 

MANGANESE AND PHOSPHORUS 

A nitric acid solution free of carbon compounds, 

chlorides, silica, and fluorine is prepared for the 

determination of manganese and of phosphorous. 

Soluble organic matter, if present, would interfere 

with the determination of manganese by reducing 

permanganate. Mineral acids other than nitric will 

interfere in the precipitation of ammonium phosphomolybdate. Therefore, the decomposition of 

the sample is effected by nitric and hydrofluoric 

acid, and silicic acid is removed by dehydration. 

Boric acid is added to complex any remaining 

fluoride. The small residue is fused with sodium 

carbonate, the melt is leached with water to extract the phosphate, and the solution is filtered 

into the main portion of the sample. The solution 

is made to volume, and separate aliquots are 

taken for the determination of manganese and 

phosphorous. 

REAGENTS 

HN03 (concentrated) 

HN03 (1 + 1) that has been boiled and cooled toremove oxides of nitrogen 

HF (concentrated) 

HaB03 (5 percent) 

Sodium sulfite 

N~COa 

PROCEDURE 

1. Weigh 1 g of the sample and transfer it to a 

platinum evaporating dish (75 to 100-ml capacity). 

2. Cover the dish with a platinum cover and set 

it on a triangle over a low flame. 

3. Gradually increase the flame until the bottom 

of the dish is red. 

4. Partially open the dish and continue the heating for 5 min. 

5. Allow the dish to cool; then swirl the powder 

in a few milliliters of water, breaking up any 

lumps with a platinum stirring rod. 

6. Set the dish on a steam bath and add 20 ml 
of concentrated HN03 and 20 ml of concentrated HF to it. 

7. Stir the solution with a platinum rod and let 

it evaporate to dryness. 

8. Wet the dried salts by washing down the 

sides of the dish with. a little water; then add 

10 ml of concentrated HN03 followed by 10 

ml ofHF. 

9. Repeat the evaporation to dryness. 

10. Add 10 ml of concentrated HN03 to the dish. 

11. Wash the salts down into the acid with a little water; then evaporate the solution to dryness. 

12. Repeat the addition of concentrated HN03 

and the evaporation to dryness. 

13. Heat the solution for an additional 30 min on 

the steam bath after the salts appear to be 

dry. 

14. Cool. 

15. Add 40 ml of 1 + 1 HN03 that has been previously boiled and cooled to remove oxides of 

nitrogen. 

16. Add 20 ml of 5-percent H3B03 solution to the 

dish. 

17. Cover and digest for 2 h on the steam bath. 

18. Stir the solution occasionally, and if a brown 

insoluble precipitate persists, add a few crystals of sodium sulfite to reduce the manganese 

and render a clear solution. 

19. Spread a thin film of stopcock grease on the 

underlip of the platinum dish, add a little 

paper pulp, and filter through a 9-cm #40 

paper into a 100-ml volumetric flask. 

20. Wash the dish and the paper with a little 

water. 

21. Police the platinum dish with the aid of a 

small piece of filter paper. 

22. Add the paper to the funnel; then carefully 

char the papers in a small platinum crucible. 

23. Burn off the carbon and fuse the residue with 

about 0.5 g of Na.zC03 • 

24. Leach the melt with a few milliliters of 

water, add a few milliliters of the main solution to the crucible, and filter into the volumetric flask. 

25. Wash the crucible and the filter four or five 

times with water. 

26. Discard the filter paper and make the solution to volume and mix. 

27. Transfer a 25-ml aliquot of the solution to a 

50-ml volumetric flask and reserve for the colorimetric determination of manganese. 

25 

28. Transfer a 50-ml aliquot of the solution to a 

50-ml beaker and reserve for the determination of phosphorous. 

29. Reserve the remaining solution for determining whether there is any interference in the 

manganese determination caused by the presence of any elements that form colored solutions. 

DETERMINATION OF MANGANESE 

The colorimetric method for determining manganese as described by Willard and Greathouse 

(1917) is essentially the accepted method for determining small amounts of manganese (Maxwell, 

1968; Hillebrand and others, 1953; Kolthoff and 

Sandell, 1943; Peck, 1964). It is based upon the 

oxidation of manganese to permanganate and the 

comparison of the color of the solution with that of 

standard solutions containing known amounts of 

manganese present as permanganate. The oxidation is effected in a hot acid solution using potassium periodate as the oxidizing agent. Phosphoric 

acid is added to complex ferric iron and to prevent 

the precipitation of manganese compounds and 

ferric periodate. The solutions can be heated conveniently by immersing the volumetric flask 

(which is supported by a test-tube holder) in a 

boiling water bath (Peck, 1964, p. 77). 

REAGENTS 

Standard manganese solution A (1 ml= 1 mg 

MnO)-Transfer 0.6754 g of manganese ore 

(U.S. National Bureau of Standards sample 

25c) that has been dried for 2 h at 120°C to a 

250-ml beaker. Add 20 ml of 1 + 1 HN03 and 

digest on the steam bath until no dark residue 

remains. Bring to a vigorous boil to expel the 

oxides of nitrogen; cool and filter into a 500-

ml volumetric flask. Wash the beaker and 

paper moderately with water. Ash the paper 

and fuse the residue with about 0.5 g of 

anhydrous N a.zC03 • Leach the cake with 

water and add to the flask containing the 

main portion of the solution. Dilute to volume 

and mix thoroughly. 

Standard manganese solution B (1 ml = 0.1 mg 

MnO)-Transfer a 25-ml aliquot of solution A 

to a 250-ml volumetric flask, add 10 ml of 

1 + 1 HN03 and dilute to volume. 

KI04 solution (1 percent)-Weigh 5 g of potassium 

periodate (KI04) into a 400-ml beaker. Add 
about 100 ml of 1 + 1 HN03 and digest on the 

steam bath until the salts dissolve; then 

transfer the solution to a 500-ml volumetric 

flask, cool, dilute to volume, and mix. 

HaP04 (1+ 1) 

HN03 (concentrated) 

PROCEDURE 

1. Add 0-, 0.1-, 0.5-, 1.0-, i.5-, and 2-mg portions of standard manganese solution B to a 

series of 50-ml volumetric flasks and add 10 

ml of concentrated HN03 and 10 ml of KI04 

solution. 

2. Wash down the inside of each flask with a 

few milliliters of water and mix the solution. 

3. To the 25-ml aliquot of the sample that was 

reserved for the determination of manganese, 

add 5 ml of concentrated HN03 and 10 ml of 

KI 0 4 solution. 

4. Wash down the inside of the flask with a few 

milliliters of water and mix the solutions. 

Continue the procedure for samples and standards as follows: 

5. Slip the second ring of the steam bath cover 

over the neck of the volumetric flask; fasten 

a test-tube clamp to the upper part of the 

neck and insert the flask into the boiling bath 

in such a manner that the upper part of the 

flask extends above the surface of the steam 

bath as the clamp rests on the top of the 

steam bath. 

6. Heat the solution for 1 h or until heating 

ceases to cause color changes. 

7. Cool the solution and add 5 ml of 1 + 1 H3P04 

8. Make to volume with water and mix well. 

9. Arrange the solutions according to the depth 

of the color. Starting with the lightest in 

color, measure the absorbance at 525 Mm in 

a 1-cm cell, using distilled water as a reference blank. 

10. Measure the absorbance of the original sample solution obtained in step 29 of "Preparation of the Solution for the Determination of 

Manganese and Phosphorous," using distilled 

water as a reference blank, and correct the 

absorbance readings of the samples by subtracting this reading divided by two, from the 

absorbance reading of the sample. Make a 

calibration curve from the absorbance readings of the standards and determine the 

26 

number of milligrams of MnO in the sample 

aliquot from this curve. 

Percent MnO =(milligrams of MnO/milligrams 

of sample) x 100 

=(milligrams of Mn0/250) 

DETERMINATION OF PHOSPHOROUS 

Phosphorus is first separated from interfering 

elements by precipitation as ammonium phosphomolybdate (Peck, 1964). The precipitate is dissolved in ammonium hydroxide, and phosphorus is 

determined gravimetrically after its precipitation 

as magnesium ammonium phosphate or colorimetrically by the molybdovanadophosphoric acid 

method (Kitson and Mellon, 1944, p. 379). The 

molybdovanadophosphoric acid method is considered to be the most specific spectrophotometric 

method for the determination of phosphorus. 

When an excess of a molybdate solution is added 

to an acidified solution of a vanadate and an orthophosphate, a yellow hue forms, which presumably indicates the presence of molybdovanadophosphoric acid. 

Mission (1908) first proposed using this reaction 

as a basis for a colorimetric method of determining 

phosphorus in steel. Kitson and Mellon (1944) 

made a spectrophotometric study of the method 

and recommended its use for measuring phosphorus colorimetrically. Some of the extensive literature on the nature of the complex and the best 

conditions for its formation is discussed below. 

Phosphorus must be present as the orthophosphate, and as it is usually present in this form in 

silicate rocks, the analyst need only put the sample into solution before adding the acid, vanadate, 

and molybdate for the color to appear. However, 

as other elements also form a colored complex, 

their interference must be eliminated. 

The conditions for the precipitation of ammonium phosphomolybdate as reported in the literature are varied and indefinite. The temperature at which the precipitation is made and the 

amount of ammonium molybdate and the amount 

of HN03 needed greatly influence the completeness of the precipitation and the composition of 

the precipitate. 

Randolph and Grimaldi (1953) described a 

routine procedure for the determination of P 20 6 in 

amounts from 0.1 mg to 2.0 g that requires no 

changes in conditions or quantities of reagents. 

Results reported for standard samples and phos-
phate rocks indicate that the molybdate precipitate is of constant composition, and the P 20 5 is determined gravimetrically from the weight of the 

molybdate precipitate. 

Workers in this laboratory have found that the 

values obtained for P 20 5 when the ammonium 

phosphomolybdate was dried at 100°C were 

slightly higher than the values obtained when the 

precipitate was ignited to P20 5·24 Mo03 at 480°C. 

The values obtained for P 20 5 in each of these 

methods were higher than the accepted values 

that were obtained when the ammonium phosphomolybdate was redissolved and P20 5 was determined either colorimetrically or gravimetrically. 

The conditions described here for the precipitation of ammonium phosphomolybdate are bascially 

those recommended by Randolph and Grimaldi 

(1953). Formic acid is not added to routine determinations as only small amounts of vanadium are 

expected to be present in silicate rocks. 

The P 20 5 content of the sample is determined 

colorimetrically when it is less than 0.8 percent 

and gravimetrically, after precipitation as magnesium ammonium phosphate, when it is greater 

than 0.8 percent. The colorimetric method may be 

used for samples containing more than 0.8 percent 

P 20 5 if the ammoniacal solution of the ammonium 

phosphomolybdate is made to volume and phosphorus is determined on an aliquot containing less 

than 4 mg of P20 5 in 100 mi. If more than 4 mg 

of P 20 5 is present in the ammoniacal solution used 

for the colorimetric determination of phosphorus, 

a precipitate that will not redissolve will form 

when the solution is made acid. 

Procedures involving a wide variety of conditions for the formation of the yellow complex have 

been recommended. Nitric acid must be added 

first; then vanadate and molybdate are added 

separately or as one reagent containing both 

chemicals (Barton, 1948). 

The concentrations of nitric acid recommended 

range from 0.04 to 1.0 N (Rieman and Beukenkamp, in Kolthoff and Elving, 1961). Kitson and 

Mellon (1944, p. 379) stated that the optimum 

amount of acid is 10 ml of nitric acid (1:2) in 100 

ml volume (0.5 N). Peck (1964) recommended a 

0.48 N solution, and Maxwell used a 0.6 N nitric 

acid solution. Kitson and Mellon (1944) and Maxwell (1968) recommended using 10 ml of 0.25-percent ammonium vanadate solution and 20 ml of 

ammonium molybdate solution added separately. 

Peck (1964) recommended using 10 ml of 0.25-percent ammonium vanadate solution and 10 ml of 

ammonium molybdate solution added as one reagent containing both chemicals. The conditions for 

the color formation described in the "Procedure" 

below are those recommended by Peck (1964). 

REAGENTS 

H3P04 (85 percent)-Analytical grade. 

Ammonium molybdate solution-Dissolve 160 g of 

ammonium molybdate ([NH41M~024·4H20) 

in 900 ml of hot water. Cool and filter. 

NH4N03 wash solution-Dissolve 20 g of NH4NOa 

in about 1 L of 1 + 99 HN03• 

NH40H solution (5 percent and 1 + 1) 

Magnesium mixture-Dissolve 50 g of 

MgC12-6H20 and 100 g of NH4Cl in 500 ml of 

water. Add one drop of bromocresol purple 

indicator (0.1 percent); then make the solution 

slightly alkaline with NH40H and allow it to 

stand at least 12 h. Filter, acidify with H Cl, 

and dilute to 1 L. 

Citric acid solution (35 percent)-Dissolve 35 g of 

citric acid (C6H80 7) in 100 ml of water. 

Ammonium Vanadate and ammonium molybdate 

solution-Dissolve 5.000 g of ammonium vanadate in 400 ml of hot water. Add 100 g of 

ammonium molybdate, and stir the solution 

until the molybdate dissolves. Let the solution stand overnight; then filter it into a 2-L 

volumetric flask. Wash the beaker and paper 

with water. Add 10 ml of concentrated 

NH40H to the filtrate; dilute the solution to 

volume and mix it. Store in a polyethylene 

bottle. 

Bromocresol purple indicator (0.1 percent) 

HN03 (concentrated and 1 + 1) 

HCl (5 percent and 1 + 1) 

N2H40a (2 percent) 

PROCEDURE 

STANDARDIZATION OF THE PHOSPHORUS SOLUTION 

27 

A. Precipitation of phosphorus as ammonium 

phosphomolybdate 

1. Dilute 2.2 ml of 85-percent H3P04 to 200 

ml. This is phosphorus standard solution 

A. 

2. To two series of four 150-ml beakers, 

pipet 0, 2.0, 5.0, and 10.0 ml of phosphorus standard solution A and dilute to 

about 10 mi. 
3. Add two or three drops of bromocresol 

purple, then 1 + 1 NH40H until the solution just turns purple. 

4. Dilute to 40 ml. 

5. Add 8 g of NH4N03 (solid) and warm on 

the steam bath until it dissolves. 

6. Set the beakers in a water bath and keep at 

a temperature of about 60°C. 

7. Add 15 ml of concentrated HN 0 3 andimmediately add 45 ml of the ammonium 

molybdate solution slowly while stirring. 

8. Add water, if necessary, to make the 

total volume 100 ml and let stand at room 

temperature until the precipitate settles. 

B. Gravimetric determination of phosphorus as 

magnesium ammonium phosphate 

1. Decant the solutions from which phosphorus was precipitated as ammonium 

phosphomolybdate through a 9-cm #42 

filter paper that has been washed with 5-

percent NH40H wash solution. 

2. Wash the precipitate left in the beaker 

several times with the NH4N 0 3 wash solution, pouring the washings through the 

filter. 

3. Wipe the underlip of the beaker with a 

small piece of filter paper and expel the 

wash solution from the stem of the funnel. 

4. Discard the filtrate. 

5. Wash down the inside of the beaker with 

the 5-percent NH40H solution and stir to 

dissolve the precipitate. 

6. Set the beaker under the funnel and dissolve the precipitate from the paper into 

the beaker by washing over the precipitate and around the upper edges of the 

filter paper with several small portions of 

the 5-percent NH4 0H solution until no 

yellow color remains. 

7. Wash the paper three times with small 

portions of water, twice with 5-percent 

HCl, and finally three times with water. 

8. Discard the filter paper and -wash the 

funnel into the beaker. 

9. Add 1 + 1 HCl dropwise to the solution 

until it is just acid to bromocresol purple; 

then add to it 1 ml of 35-percent citric 

acid to hold traces of iron, if any, in solution. 

10. Cool the solution to about 10°C in an ice 

bath. 

28 

11. Add 10 ml of the magnesium mixture and 

then, while stirring constantly, add 1 + 1 

NH40H drop by drop until the solution is 

just alkaline to bromocresol purple. 

12. Continue the stirring for several minutes, 

then add a 10 percent by volume excess 

of concentrated NH40H. 

13. Set the solution aside for at least 8 h; 

then filter through an 11-cm #42 paper 

and wash it a few times with the 5-percent NH4 0H wash solution. 

14. Dissolve the precipitate into the original 

beaker by pipeting 5-ml portions of 1 + 1 

H Cl over the precipitate and around the 

upper edges of the filter paper using 

about 25 ml of 1 + 1 HCl in all. 

15. Wash the filter paper thoroughly with 5-

percent HCl solution and finally with 

water. 

16. Dilute the solution to 50-100 ml. 

17. Cool to about 10°C, add 2 ml of magnesium mixture; then while stirring constantly, add 1 + 1 NH40H slowly to the 

solution as in step 11 above. 

18. Allow the mixture to stand overnight and 

filter on an 11-cm#42 paper, quantitatively transferring the precipitate to the 

paper and washing moderately with cold 

5-percent NH40H wash solution. 

19. Finally, wash the precipitate on the filter 

with two 10-ml portions of 5-percent 

NH40H wash solution. 

20. Transfer the precipitate to a weighed 

platinum crucible, slowly char the paper, 

and carefully burn off the resulting carbon in an electric furnace. 

21. Allow the temperature of the furnace to 

reach 800°C gradually, and maintain this 

temperature until the Mg2P20 7 precipitate is white. 

22. Finally, ignite the precipitate at 1,050°C 

to constant weight. 

23. Obtain the average weight of Mg2P20 7 in 

1 ml of phosphorus standard solution A. 

Multiply this value by 0.6379 (molecular 

ratio of P20 5 to Mg2P20 7) to obtain the 

weight of P20 5 in 1 ml of standard solution A. 

24. Transfer a suitable aliquot of solution A 

to a 1-L volumetric flask to make phosphorus standard solution B (1 ml = 0.20 

mg P20 8 ). Dilute to volume and mix. 
DETERMINATION OF PHOSPHORUS IN THE SAMPLE 

A. The gravimetric method 

1. Transfer to a 150-ml beaker an aliquot of 

standard solution B that contains 4 mg of 

P 20 5 and dilute to about 40 ml. 

2. Add about 40 ml of water to another 

beaker. 

3. To these solutions and to each of the sample 

solutions reserved for the determination 

of phosphorus, add two or three drops of 

bromocresol purple, then 1 + 1 NH40H 

until the solution just turns purple. 

4. Add 8 g of NH4N03 (solid) and continue 

with the precipitation of phosphorus as 

ammonium phosphomolybdate as directed 

above in steps 5-8 of section "A. Precipitation of Phosphorus as Ammonium Phosphomolybdate." 

5. Visually determine whether the volume of 

the ammonium phosphomolybdate precipitate in any of the samples exceeds the 

volume of the precipitate in the 4-mg standard. 

6. Colorimetrically determine the P 20 5 content of one blank and of all the samples 

having a precipitate smaller than the precipitate in the 4-mg standard. 

7. Determine the P 20 5 content of one blank 

and of any samples having a precipitate 

larger than the precipitate for.med in the 4-

mg standard by the gravimetric method 

described above in section "B. Gravimetric Determination of Phosphorus as Magnesium Ammonium Phosphate." 

8. Correct the weight of the Mg2P 20 7 found in 

the sample for the weight of Mg2P 207 

found in the blank and calculate the percent of P 20 5 as follows: 

where 

Percent P20 5 = W x 63. 79/S 

W = milligrams ofMg2P 207 

S = milligrams of sample 

used, and 

63.79 = molecular ratio ofP 20 5 

toMg2P207 

(0.6379) multiplied 

by 100. 

B. The colorimetric method 

1. Decant the solutions from which ammonium phosphomolybdate was precipitated through a 9-cm #42 paper. 

29 

2. Wash the precipitate left in the beaker several times with the NH4N 0 3 wash solution 

and transfer the washings to the filter. 

3. Wipe the underlip of the beaker with a 

small piece of filter paper and add the paper 

to the funnel. 

4. Expel the wash solution from the stem of 

the funnel. 

5. Discard the filtrate. 

6. Place a 100-ml volumetric flask under the 

funnel. 

7. Wash down the inside of the beaker with a 

few milliliters of 5-percent NH40H solution. 

8. Transfer this solution to the filter. 

9. Continue to dissolve all the yellow precipitate from the beaker and the filter with 

small portions of 5-percent NH40H solution. The total volume of the NH40H solution should not exceed 25 ml. 

10. Finally, wash the paper with water and 

wash the funnel into the flask, keeping the 

volume below 60 ml. 

11. Remove the flasks and add 1 + 1 HN 03 to 

just neutralize the solutions. 

12. Dilute those samples containing more 

another 100-ml volumetric flask and dian 

appropriate aliquot and transfer it to lute to 

about 60 ml. Reserve. 

13. To a series of 100-ml volumetric flasks, 

pipet 0, 1, 2, 5, 10, 15, and 20 ml of phosphorus standard solution B and dilute to 

about50ml. 

14. Add 8 ml of 1 + 1 HN03 to each of the 

samples, the reagent blank, and the standard solutions; then add 10 ml of ammonium vanadate-ammonium molybdate 

solution to each and mix. 

15. Dilute each solution to volume and mix 

thoroughly. 

16. After 30 min, measure the absorbance of 

the molybdovanadophosphoric acid complex at 460 Mm in a 1-cm cell; use the 

reagent blank as the reference. 

17. Prepare a calibration curve 1 from the absorbance readings of the standards and 

determine the weight of the P 20 5 in the 

sample solution from this curve. This 

weight divided by the weight of the sample in the solution x 100 is the percentage of P 20 5 in the sample. 
DETERMINATION OF Na20 and K20 

The IL443 Flame Photometer is used for the 

quantitative determination of sodium and potassium after the silicate rock is decomposed and dissolved in hydrochloric acid. The accuracy of the 

data obtained and the simplicity of operation of 

this instrument recommend its use for determination of sodium and potassium in silicate rocks, 

even though this method is not classical. Such design features as ratioed internal standard operation, high ionization-suppressant concentration, 

and low flame temperature contribute to the accuracy of the instrument. 

A constant 15 M solution of lithium is added to 

all samples and standards as an internal standard. 

The samples are closely bracketed with the standards when the readings are made, and the sample solutions are read at least twice; an average 

reading is used for calculating the number of milligrams of sodium and potassium in the solution. If 

the second reading is made from a second aliquot 

containing a different amount of the sample solution, possible interferences from elements in the 

sample can be detected. 

A 1-g sample is decomposed in a Teflon beaker 

with nitric acid, sulfuric acid, and hydrofluoric 

acid and is dissolved in hydrochloric acid. 

Tests were made to determine the effect of various levels of sodium on the potassium determination and the effect of various levels of potassium 

on the sodium determinations. No effects were observed. 

Determinations made on different size samples 

of a group of silicate rocks gave no indication of interferences from other elements present in the 

sample. Solutions containing from 2 to 10 percent 

HCl gave identical readings; however, there was 

some indication of attack on the atomizer bowl 

when solutions of higher acidity were aspirated. 

In our laboratory, a 4-percent HCl solution is presented for aspiration. 

APPARATUS 

Laboratory flame photometer IL443 having goldplated connections for the aspirator bowl. 

Teflon beakers--Design of Teflon vessel for decomposition of rock samples by Shapiro (1959). 

400-ml vycor or other pure silicon beakers 

30-ml plastic beakers--use for aspirating samples 

Plastic or Teflon stirring rods 

30 

REAGENTS 

Potassium stock solution (1 ml = 1 mg K20)--Dissolve 1.583 of KCl in about 50 ml of water and 

dilute to 1 L. Transfer the solution to a 1-L 

plastic bottle for storage. 

Sodium stock solution (1 ml = 1 mg Na20)--Dissolve 1.886 g of reagent-grade N aCl in a little 

water and dilute to 1 L. Transfer the solution 

to a 1-L plastic bottle for storage. 

H2S04 (1 + 1) 

HF (concentrated) 

HN03 (1 + 1) 

HCl (1:2) 

Lithium stock solution (10 ml = 10.41 mg Li)--Dissolve 6.360 g of LiCl in a little water and dilute to 1 L. Transfer the solution to a 1-L 

plastic bottle for storage. 

PROCEDURE 

DECOMPOSITION OF THE SAMPLE 

1. Transfer a 1-g sample to a Teflon beaker and 

disperse with a little water. 

2. Add 10 ml of 1 + 1 H2S04 to the beaker and 

immerse in the steam bath. 

3. Add 10 ml of concentrated HF and then 10 ml 

of1+1 HN03 . 

4. Cover the beaker and digest the contents for 

6-8 h. 

5. Remove the cover and wash material adhering to it into the beaker. Continue the evaporation on the steam bath until the volume of 

the solution is about 5 ml. 

6. Set the uncovered beaker on a hot plate that 

has a surface temperature of 470°- 500°C and 

heat until fumes of S03 cease. 

7. Cool. 

8. Add 120 ml of 1:2 HCl to a 400-ml vycor 

beaker. 

9. Transfer about 20 ml of this HCl to the Teflon beaker and transfer the contents of the 

Teflon beaker to the vycor beaker with the 

aid of a little water and a Teflon policeman. 

10. Bring the solution to a vigorous boil and boil 

about 3 min. A clear solution should be obtained; if it is not, filter, ignite the residue, 

and treat with HF and H2SO4 as in steps 2 

and 3 above. Dissolve the residue in a portion 

of the main solution and add to that of the 

main portion of the sample. 
11. Transfer this solution to a 200-ml volumetric 

flask. Make to volume with water and then 

transfer it to a dry polyethylene bottle. 

12. Pipet 20 ml of this solution and transfer it to 

a 100-ml volumetric flask. Add 10 ml of 

lithium solution (10 ml = 10.41 mg Li). Make 

to volume, then transfer to a plastic bottle. 

13. Reserve. 

PREPARATION OF STANDARD SOLUTIONS 

1. Add volumes of standard solutions equal to 0, 

0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 mg each of 

Na20 and K20 to a series of 100-ml volumetric flasks. 

2. Add 8 ml of 1 + 1 HCl to each flask. 

3. Add 10 ml of lithium solution (10 ml = 10.41 

mg Li) 

4. Make to volume and transfer to plastic bottles. Reserve. 

5. Following the procedures outlined under 

"Section Six--Operation" of the instruction 

manual for the flame photometer (Instrumentation Laboratory, Inc., 1977), aspirate the 

standards and record the readings. 

6. Aspirate the sample solution. 

7. Select the standard solution having the nearest lower concentration of potassium and 

sodium and the standard solution having the 

nearest higher concentration of potassium and 

sodium and bracket the reading of the sample 

solution. 

8. Calculate the milligrams of K20 and the milligrams of N a20 in the solution from the following equation. 

[U mg = (Ru- Rls)(mgHs- mgLs/ 

where 

Rhs- Rls) +mgLs] 

U mg =milligrams 

ofK20 or 

Na20 found 

in the 

sample solution, 

R u =reading for 

sample solution 

Rls = reading for 

lower standard 

mgHs =milligrams 

of K20 or 

Na20 in 

the higher standard 

31 

mgLs =milligrams 

ofK20 or 

Na20 in 

the lower standard 

Rhs =reading for 

higher standard 

Rls = reading for 

Percent 

Na20 

or K20 

in the 

lower standard 

sample= [U mg (for the 

sample)-

U mg (for 

the blank)/mg 

sample in the 

solution] x 100 

SUMMATION OF ANALYTICAL RESULTS 

The 13 constituents for which the procedures 

are given and C02 are the constituents usually determined in a routine rock analysis. The results 

are customarily reported in terms of percent of 

oxides in the air-dried sample. 

Kalthoff and Sandell (1943, p. 283) stated that 

careful analysts usually aim to obtain an accuracy 

corresponding to an error of one part per 

thousand in the determination of a constituent occurring in relatively large amounts in a sample. 

They submitted the following table to show the 

degree of accuracy expected for smaller amounts 

of constituents. The digit expected to vary is represented by X. 

Percentage constituent 

in sample 

1.00 

0.1 

0.01 

0.001 

Permissible variation 

in percent 

o.ox 

O.OX-O.OOX 

o.oox 

O.OOX-O.OOOX 

Maxwell (1968, p. 18 - 35) discussed precision and 

accuracy and gave an extensive list of references. 

A good analysis will usually give a summation 

that is very close to 100 percent. Hillebrand and 

others (1953, p. 802) said "Given the purest obtainable reagents, an ample supply of platinum, 

facilities for working, and a reasonably clean labo-
TABLE 1. Compa:ri.3on of results (in percent) obtained in this laboratory by th.e methods recommended in this work 

and recommended averages from Flanagan (1976, p. 171) 

[For each sample, the ftrst row of results is from Flanagan (1976), and the second row of results contains values obtained in this laboratory] 

Total 

Fe as 

Sample Si02 Al~s FezOa FeO MgO CaO NB!O K~ H2o+ H2o- Ti~ P205 MnO CD! Total Fe20s 

G-2 ______ 69.11 15.40 1.08 1.45 0.76 1.94 4.07 4.51 0.55 0.11 0.50 0.14 0.034 0.08 99.73 2.65 

69.24 15.44 1.05 1.40 .71 1.94 4.13 4.55 .58 .06 .46 .14 .036 .10 99.84 2.61 

G-1 ______ 72.64 14.04 .87 .96 .38 1.39 3.32 5.48 .34 .06 .26 .09 .03 .07 99.93 1.94 

72.28 14.32 .80 .95 .50 1.41 3.36 5.51 ~ 

.34 .06 .29 .07 .02 .06 99.97 1.86 

t-.:l 

GSP-L __ 67.38 15.25 1.77 2.31 .96 2.02 2.80 5.53 .57 .12 .66 .28 .042 .15 99.84 4.33 

67.30 15.18 1.74 2.30 1.12 2.06 2.76 5.48 .59 .10 .65 .29 .04 .14 99.75 4.30 

DTS-L__ 40.50 .24 1.21 7.23 49.80 0.15 0.007 .0012 .46 .06 .013 .002 .11 .08 99.86 8.64 

40.56 .68 .75 7.25 49.60 0.29 <0.01 .006 .45 .07 0 .005 .11 .08 99.86 8.81 

AGV-1 __ 59.00 17.25 4.51 2.05 1.53 4.90 4.26 2.89 .81 1.03 1.04 .49 .097 .05 99.91 6.76 

59.17 16.98 4.58 2.00 1.52 4.89 4.20 2.92 .78 .97 1.08 .49 .11 .06 99.75 6.80 

W-1 ______ 52.64 15.00 1.40 8.72 6.62 10.96 2.15 .64 .53 .16 1.07 .14 .17 .06 100.26 11.09 

52.50 14.47 1.79 8.68 6.59 10.90 2.18 .63 .57 .14 1.08 .10 .16 .08 99.87 11.44 

BCR-1 __ 54.50 13.61 3.69 8.80 3.46 6.92 3.27 1.70 .77 .80 2.20 .36 .18 .03 100.28 13.40 

54.43 13.59 3.48 8.81 3.51 6.90 3.24 1.68 .78 .79 2.24 .32 .18 .04 99.99 13.27 
ratory, there is usually little excuse for failure on 

the part of a competent chemist to reach a summation within the limits 99.75 and 100.50. 

"If the analysis has to be made in other 

vessels than those of platinum and careful 

blank determinations are not carried along, a 

higher limit than 100.50 is certainly called for, 

but it is extremely doubtful if a lower one 

than 99.75 should be accepted." 

Peck (1964, p. 10) considered 99.50 to 

100.25 acceptable. The summations for the 

summary values for eight USGS rocks listed 

by Flanagan (1976, p. 171) range from 99.73 

to 100.28. Kolthoff and Sandell (1943, p. 262 

- 298), Groves (1951, p. 224 - 236), Hillebrand 

and others (1953, p. 802 - 806), and Maxwell 

(1968, p. 14 - 15 and p. 18 - 35) discussed summation of analytical results and possible errors in the analysis and listed many references on these subjects. 

Values listed by Flanagan (1976, p. 171) as 

recommended averages obtained from a compilation of values obtained for USGS standards by analysts worldwide and the values 

obtained in this laboratory by the methods recommended here are listed in table 1. 

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33 

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34 

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