GEOLOGICAL SURVEY CIRCULAR 119 

August 1951 

A FIELD METHOD FOR THE 

DETERMINATION OF TUNGSTEN IN SOILS 

By 

F. N. Ward 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary 

GEOLOGICAL SURVEY 

W. E. W\l"ather, Director 

Washington, D. C. 

Free on application to the Geological Survey, Washington 25, D. C. 
A FIELD METHOD FOR THE DETERMINATION OF TUNGSTEN IN SOILS 

CONTENTS 

Page 

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ___ 1 

Introduction. . . . . . . . . . . . . . . . . . . . . . . .. . . . • . l 

Reagents and apparatus . . . . . • . . . . . . . . . . . . 1 

Procedures. . . . . . . . . . . . . . . . . . . . . . . • . . . . . 2 

ABSTRACT 

The thiocyanate-stannous chloride method for 

the determination of small quantities of tungsten 

has been modified for use on soils in geochemical 

prospecting. A 0. 25 g sample of soil is fused 

with a modified carbonate flux in a pyrex test 

tube, extracted with water, and an aliquot of the 

aqueous extract is treated with stannous chloride 

.and: potassium thiocyanate in moderately concentrated hydrochloric acid. The tungsten thiocyanate complex ion, formed rapidly at a tempera-

·ture of 90C to lOOC, is extracted with a small 

volume of isopropyl ether, and tungsten determinations are made by comparing the yellowishgreen color of the ether layer over a sample so-

,lution with that over a standard solution treated 

'similarly. With a suitable aliquot, the field meth1od can be used on soils containing 10 to 800 ppm 

of tungsten. 

INTRODUCTION 

1 A rapid semiquantitative method for determination of trace amounts of tungsten in soils was 

needed in connection with the field work of the 

beochemical Prospecting Group of the U. S. Geological Survey. For the aid given in accomplishing 

this work, the author is grateful to Herbert E. 

J{awkes and Lyman C. Huff, geologists, for assistance in collecting samples and to Hy Almond 

~nd Laura Reichen, chemists, for many helpful 

~uggestions. 

Colorimetric procedures are more easily 

adapted than other methods to the field determination of small amounts of a given element. A 

:survey of methods currently used in the colorimetric determination of traces of tungsten shows 

lt~hat thiocyanate in the presence of a reducing 

gent like stannous chloride possesses the greatst number of advantages with respect to stability 

~nd availability of reagents and to the ease of 

transporting the simple apparatus required to 

make determinations in the field. 

The thiocyanate-stannous chloride method was 

:irst proposed for determination of tungsten by 

fi'eigl and Krumholz ( 1932) and was later modified 

Discussion..... . . • . . . . . . . . . . . . . . . . . . . . . . 3 

References cited . . • . . . . . . . . . . . . . . . . . . . . . 4 

Table 1................................. 4 

to meet various needs (Fernjan~H~, 1934; 

Fer 1yanchich, 1947; Sandell, 1946). Still other 

changes have been made in attempts to shorten the 

method and/or obtain better precision (Geld and 

Carroll, 1949; Gentry and Sherrington, 1948). 

Extraction of the tungsten thiocyanate complex ion 

with an immiscible solvent results in increased 

sensitivity (Sandell, 1950), and heating the reaction mixture decreases the time required to 

form the complex ion (Poluektov, 1941). Accordingly the proposed procedure calls for a rapid 

. formation of the tungsten thiocyanate complex ion 

at a temperature of 90C to 95C followed by an isopropyl ether extraction of the complex ion from 

the cool aqueous solution. Tungsten determinations are made by comparing the intensity of the 

yellowish-green color of the ether layer over the 

sample solution with that obtained over a standard solution treated similarly. 

REAGENTS AND APPARATUS 

Flux. A mixture by weight of 5 parts sodium carbonate, 4 parts sodium chloride, and 1 

part potassium nitrate, ground to pass an 

80 mesh silk bolting cloth sieve, thoroughly 

mixed, and passed through the sieve again. 

Potassium thiocyanate. A 25 percent solution. 

Dissolve 25 g of the salt in water and dilute 

to 100 ml. 

Hydrochloric acid. Concentrated, analytical 

reagent grade. 

Stannous chloride. Dissolve 10 g SnC12• ZHzO in 

concentrated hydrochloric acid and make 

up to 100 ml. 

Standard tungsten solution, 0. 01 percent. Solution A. Dissolve 0. 09 g Naz W04 . ZHzO in 

water and dilute to 500 ml. The solution 

contains 100 micrograms of tungsten per 

ml. 

Standard tungsten solution, 0. 0005 percent. Solution B. Because the concentration decreases on standing, it should be prepared 

daily by diluting 5 ml of Solution A with 

water to 100 ml and mixing thoroughly. 
Isopropyl ether. Practical grade is suitable. provided peroxides are absent. On the day 

during which it is to be used, shake the 

ether with one-tenth its volume of equal 

amounts of stannous chloride and potassium 

thiocyanate solutions. Without special packaging all dry ethers tend to form explosive 

peroxides on standing. To test for peroxides shake 5 ml of ether with 5 ml of an 

acidified aqueous solution of potassium 

iodide. If the latter shows more than a 

faint yellow color, due to free iodine produced by the peroxides, the ether contains 

appreciable quantities of peroxides and 

should not be used. 

Water. Purify by passing tap water through one of 

several type~ of demineralizers now commercially available. The Bantam Demineralizer manufactured by Barnstead Still and 

Sterilizer Company was used in this work. 

Agate or Mullite mortar and pestle, outside diameter of mortar, 75 mm. 

1 sieve, 80 mesh. The sieve is assembled by 

placing a piece of silk bolting cloth in an 

aluminum holder having an outside diameter of 100 mm. 

aluminum receiver to fit the sieve holder. 

camel's-hair brush, length of brush part, 18 mm. 

100 ml pyrex volumetric flask with stopper. 

ZO pyrex culture tubes ( 16 x 150 mm ) marked at 

. 5 ml. In addition each determination requires a culture tube not calibrated. 

1-1 ml serological pipette calibrated in hundredths 

of a ml. 

1-Z ml serological pipette calibrated in tenths of a ml. 

Z-5 ml serological pipettes calibrated in tenths 

of a ml. 

1 test tube rack holding at least ZO tubes. 

Torsion balance with sensitivity 0. OOZ g. 

Stevens extraction sticks. (Extractor for removing soluble constituents;in chemical 

analysis. Patent applied for by Rollin E. 

Stevens, U. S. Geological Survey. ) Prepare the sticks from pieces of pyrex glass 

tubing, 170 mm long and 9 mm outside diameter as follows: 

Constrict the tube at one end and at a point 

about ZO mm from the same end. Insert a 

plug of pyz:ex glass wool in the bulb formed 

by the two constrictions. 

1 portable gasoline stove. A Coleman pocket 

stove used by Lakin and others {Lakin, 

Stevens, and Almond, 1949) is satisfactory. 

iron rack. This rack is used to support tubes 

over the stove during the fusion. 

z 

water bath. The top of the Coleman pocket stove 

container filled with water to about twothirds of its capacity is an excellent water 

bath. 

metal support. A round rack fitting inside the 

top of the Coleman pocket stove container 

and holding 16 tubes was used. 

Cork stoppers no. 5 for the culture tubes, ( 16 x 

150 mm). 

Cork stoppers no. 00 to fit the open end of the 

Stevens extraction sticks. 

1 thermometer, llOC. 

PROCEDURE 

Preparation of sample solution 

Weigh 0. Z5 g of finely ground soil and transfer 

to a culture tube { 16 x 150 mm). Tap the tube 

gently to dislodge the sample from the sides and 

add 1. Z5 g of flux. Mix the sample and flux by 

alternately rotating and tapping the tube. Place 

the tube in a metal rack over the gasoline stove 

and allow the contents to fuse 5 to 6 min. Rotate 

the tube occasionally to insure a thorough attack 

of the sample. After the fusion is .complete, remove the tube from the rack and allow to cool. 

Pipette 5 ml purified water into the tube, and 

place the tube in a boiling water bath. After 3 

min insert a corked Stevens extraction stick into 

the culture tube and let the tube remain in the 

boiling water bath for 3 min or until air bubbles 

are escaping freely from the lower end of the extraction stick. Remove the culture tube from the 

water bath and place in a test tube rack to cool. 

As the solution cools, a clear filtrate of the aqueous extraction of the fused mass replaces the air 

displaced from the corked extraction stick at the 

higher temperature. Transfer a 1 ml aliquot of the 

filtrate {0. 05 g of soil) to a clean culture tube 

{ 16 x 150 mm) and add water to make the volume 

up to the 5 ml mark. 

Preparation of standards 

Prepare a series of standard solutions by pipetting the required volume of standard solution B 

into separate culture tubes and adding water to 

the 5 ml mark. 

Volume of Tungsten content Standard solution B 

required ml micrograms 

a o.z 1 

b 0.4 z c 0.6 3 

d 0.8 4 e 1.0 5 

f z.o 10 

g 3.0 15 

h 4.0 zo 

Determination of tungsten: 

Add 4 ml stannous chloride and 0. 5 ml potassium thiocyanate reagents to the sample solution 

and to each standard solution. Place the tubes in 
a water bath maintained at a temperature of 90C 

to 95C for 5 min or until one observes a moderate 

evolution of hydrogen sulfide. If the altitude 

causes the water to boil below the specified temperature, allow the tubes to remain in the water 

bath until hydrogen sulfide evolves freely. Remove the tubes from the water bath, cool, add 

0. 5 ml potassium thiocyanate reagent and 0. 3 

ml isopropyl ether. Insert a clean cork into the 

tube and shake 5 sec to extract the tungsten thiocyanate complex ion into the ether. Compare the 

intensity of the yellowish-green color of the. ether 

layer over the sample with that obtained over the 

standard solution. If the color of the ether layer 

over the sample is amber instead of yellowishgreen, add about 5 ml of· concentrated hydrochloric acid and shake to..ti.each the amber color. 

Since a small amount of the immiscible solvent is 

lost during this process, add an additional 0. 1 ml 

isopropyl ether in order to make valid comparisons. 

Multiply the number of micrograms of tungsten 

found in the aliquot by ZO to convert the results to 

parts per million. The procedure is suitable for 

soils containing from ZO to 400 ppm of tungsten; 

·but, if one adjusts the aliquot, the lower limit can 

be decreased to 10 ppm and the upper limit increased to a point where the quantity of tungsten 

present can be determined more conveniently by 

other field methods (North and Grimaldi, 1946). 

After the analysis is complete, wash and retain the calibrated culture tube and extraction. 

stick for future use. Destroy the tube used for the 

fusion as the action of the flux makes further use 

impracticable. 

DISCUSSION 

The Stevens. extraction stick is used to effect a 

rapid filtration of the aqueous extract of the fused 

mass under field conditions. In practice the open 

end of the tube is corked and the extraction stick 

is inserted in the culture tube containing the hot 

aqueous extract. Air in the stick is forced 

through the plugged end until equilibrium is attained or the culture tube is removed from the 

boiling water bath. After a few minutes the culture tube containing the aqueous extract and stick 

is removed from the boiling water bath and set 

aside to cool. Upon cooling, a partial vacuum is 

formed within the tube and causes the aqueous 

extract to filter through the glass plug. Thus,. 

from a practical standpoint, the fused mass is 

extracted and filtered in one oper·ation. 

Since the pyrex culture tube allows a limited 

amount of heat transfer, and, under field conditions, the Coleman stove provides less heat 

than an ordinary laboratory burner, sodium carbonate alone cannot be used as a flux in the proposed procedure. The melting point is too high. 

However, finely ground sodium carbonate mixed 

with an equal quantity of finely ground sodium 

chloride fuses at a temperature attainable with 

the gasoline stove under field ·conditions. Subsequently, a small amount of potassium nitrate was 

added to provide an oxidizing flux. The resulting 

flux, a mixture by weight of 5 parts sodium carbonate, 4 parts sodium chloride, and 1 part potassium nitrate possesses the advantages of a 

3 

carbonate flux and at the same time fuses at a 

temperature low enough for field use. 

The carbonate fusion followed by an aqueous 

extraction separates tungsten from iron, titanium, 

and other elements which form insoluble hydrdxides. Little is known about the retention of tungsten by· th.e leached residue; but since losses due 

to coprecipitation are usually smaller in a fusion 

procedure than in a precipitation procedure 

(Sandell, 1946), one can expect negligible losses 

of tungsten during the preparation of the sample 

solution. 

Since rhenium occurs only to the extent of 

0. OOlppm in the earth's crust (Goldschmidt, 1937) 

and the isopropyl et.her does not extract small 

amounts of tantalum, uranium, phosphorus, and 

boron, interferences due to these elements are 

unlikely. Moreover, under the stipulated conditions, vanadium in amounts as great as 1000 

micrograms imparts no color to the organic 

layer. Both fluoride and nitrate tend to bleach 

the yellow color of the tungsten thiocyanate ion. 

However, the amount of fluorine' in soils is not 

appreciable and the amount of nitrate present remains constant because the quantity added in the 

flux is much greater than that reported in soils. 

Large amounts of copper react with thiocyanate 

to form the white insoluble cuprous thiocyanate, 

but the reaction apparently causes no interference 

in the method. 

On the basis of the results obtained in our laboratory, molybdenum interference is the exception rather than the rule. Only Z of 15 soil samples analyzed in our laboratory contained enough 

molybdenum in the final aliquot to impart an 

amber color to the isopropyl ether layer. Since 

the intensity of the color of the molybdenum thiocyanate complex ion is considerably less than that 

of the tungsten thiocyanate ion (Grimaldi and 

North, 1943; Sandell, 1950), and since tungsten 

is 4 to 5 times as abundant as molybdenum in the 

earth's crust (Goldschmidt, 1937), these results 

are not surprising. The adaptation of a method 

to field use requires the omission of unnecessary 

steps, particularly those using additional or 

larger quantities of reagents; therefore, except 

for the rare but easily discernible case of molyb-

·

denum interference, f:Oncentrated hydrochloric 

add is not added to tp.e sample solution (See 

p. ). With standard solutions containing 10 

micrograms each of tungsten and molybdenum, 

the ye.Llowish-green color of the tungsten thiocyanate complex ion was readily observed after 

the addition of concentrated hydrochloric acid 

and a thorough mixing of the solution. 

At the higher temperature used in the proposed 

field method, the stannous chloride also reduces 

the potassium thiocyanate causing evolution of 

hydrogen sulfide. If this reduction is allowed to 

continue more than Z or 3 minutes, black stannous 

sulfide is precipitated. In order to avoid this reaction and other possible side reactions, the 

analyst can allow the complex tungsten thiocyanate io~ to form at room temperature with different standing periods. Within the limits of experimental error, the same results were obtained on 

5 soil samples (a) by allowing the reaction to take 
place rapidly at a high temperature, (b) by allowing the sample solution plus reagents to stand at 

room temperature for Z hours, shaking occasionally, or (c) by allowing the sample ·solution plus 

reagents to stand at room temperature overnight, 

shaking immediately before making color intensity comparisons. In all cases the color intensities obtained over sample solutions are compared 

with those obtained over standard solutions 

treated similarly. 

In order to test the usefulness of the method, 

several soils were analyzed by an accepted 

colorimetric laboratory procedure using the same 

reaction (Grimaldi and North, 1943), and by the 

proposed field method. The results are shown in 

table I. The values obtained by the field method 

are in good agreement with those obtained by the 

accepted procedure. 

The proposed method is simple and capable of 

intensive application. The reagents and equipment are commercially available and both are 

easily fitted in a small carrying case. With little 

experience the analyst can determine tungsten on 

ZS or more soil sa~ples per day with sufficient 

accuracy for programs of geochemical prospecting as well as other research programs requiring 

a rapid accumulation of data. 

REFERENCES CITED 

Feigl, F., and Krumholz, P., 193Z, Colorimetric determination of smal~ amounts of tungsten: Zeitschr. angew. Chemie, vol. 45, pp. 

674-5. 

Fernjan~i~. S., 1934, Rapid method for determining small quantities of tungsten in ores: 

Zeitschr. anal. Chemie, vol. 97, pp. 33Z-4. 

Fer'yanchich, F. A., 1947, Colorimetric determination of tungsten in minerals in the presence of arsenic, antimony, molybdenum, titanium, and phosphorous: Zavodskaya Lab., vol. 

13, pp. 668-76. 

Geld, Isidore and Carroll, Jacob, 1949, Colorimetric determination of columbium and tungsten in high-temperature alloys: Anal. Chemistry, vol. Zl, pp. 1098-1101. 

Gentry, C. H. R., and Sherrington, L. G., 1948, 

Photometric determination of tungsten: Analyst, 

vol. 73, pp. 57-67. 

Goldschmidt, V. M., 1937, Geochemische 

Verteilungsgesetze der Elemente 9, Die 

Mengenverhaltnisse der Elemente und der 

Atom-Arten: Norske vidensk-akad. Oslo Skr., 

Mat. -naturw. Klasse 4, p. 148. 

Grimaldi, F. S., and North, Victor, 1943, Determination of tungsten in low-grade tungsten 

ores: Ind. Eng. Chemistry, Anal. Ed., vol. 

15, pp. 65Z-4. 

Lakin, H. W., Stevens, R. E., and Almond, Hy, 

1949, Field Method for the determination of 

zinc in soils: Econ. Geology, vol. 44, pp. 

Z96-306. 

North, Victor and Grimaldi, F. S. , 1946, A field 

test for the detectio'n and estimation of tungsten: 

U. S. Geological Survey Bull. 950, pp. 1 Z9-31. 

Poluektov, N. S., 1941, Colorimetric determination of tungsten: Zavodskaya Lab., vol. 10, pp. 

9Z-3. 

Sandell, E. B., 1946, Determination of tungsten 

in silicate rocks: Ind. Eng. Chemistry, Anal. 

Ed., vol. 18, pp. 163-7. 

Sandell, E. B., 1950, Colorimetric determination of traces of metals, Zd ed., 673 pp., 

New York, Interscience Publishers. 

Table I. --Determination of tungsten in soils 

Tungsten found, ppm 

Sample No. Location Laboratory Field 

method method 

1 North Carolina 0 10 

z II II 8 10 

3 II II 10 10 

4 II II zo 10 

5 Washington Z8 60 

6 North Carolina 40 40 

7 II " 90 50 

8 Washington 108 60 

9' " 114 90 

10 II Z60 zoo 

11 II 3Z5 300 

lZ II 500 600 

13 II lZOO 1000 

14 II 1~00 1000 

15 II 3300 Z500 

4 INT.-DUP. SEC., WASH., D.C ... .,2;: