Spectrophotometric Determination of Tungsten in Tantalum, Titanium, and Zirconium Using Dithiol PAUL GREENBERG Electro Metallurgical
Co.,A
Division o f Union Carbide and Carbon Corp., Niagara Falls, N. Y.
b The increased use of high-purity tantalum, titanium, and zirconium necessitates a method for determining < O s l o % tungsten in these materials. The tungsten-dithiol complex can b e extracted with amyl acetate without interference and the blue-green color measured spectrophotometrically. A method is described by which it is possible to analyze six samples in 2.5 to 3 hours.
T
HE PROBLEM of determining small amounts of tungsten in the presence of other readily hydrolyzable metals such as titanium, tantalum, and zirconium has become more acute with the increased use of these metals. Tungsten can be introduced either from the ore or during melting. The method described, originally developed for determination of tungsten in tantalum, has been extended to include determination in titanium and zirconium. The hydroquinone method for tungsten (2) could not be used for determination in tantalum metal because it lacked sufficient sensitivity on a sample size that could be used without having tantalum hydrolyze, and the presence of even small amounts of columbium and titanium interfered with the tungsten complex. The thiocyanate method ( S , J ) was not satisfactory because the tantalum hydrolyzed under the conditions necessary for reduction of tungsten(V1) to tungsten(V), and the color of the tungsten thiocyanate complex was not stable. Several authors (I,&-7) reported use of the organic reagent, 4-methyl-1-2-dimercaptobenzene, commonly called dithiol, for the determination of traces of tungsten in a variety of materials. This reagent seemed to be of sufficient sensitivity and specificity to warrant investigation. In 1944, Miller (5)made a rather complete semiquantitative study of the determination of tungsten with dithiol. Tungsten reacts with dithiol under reducing conditions in 10 to 11N hydrochloric acid solution to form a bluegreen complex, which is extractable with isoamyl acetate. In her study of 58 cations, six acids, and 17 anions. she
896
ANALYTICAL CHEMISTRY
light fumes of sulfuric acid. Cool and add 5 drops of hydrofluoric acid 10.2 ml.). TANTALUM. Transfer the sample containing not more than 0.20 mg. of tungsten and weighing not more than 1 gram to a 50-ml. platinum dish. Add 5 ml. of hydrofluoric acid and nitric acid dropmise until a clear solution is obtained. Add 3 ml. of sulfuric acid, This next step is critical. Evaporate using moderate heat until a light, white precipitate is visible. If the evaporation is carried too far, excessive precipiAPPARATUS A N D REAGENTS tation of tantalum occurs, and the sample should be discarded. Cool and All absorbance measurements were add 5 drops of hydrofluoric acid. made in 1.00-cm. cells with a Beckman ZIRCOXIUX Transfer the sample Model DU spectrophotometer. Dithiol containing not more than 0.20 mg. of was obtained from the Eastern Chemical tungsten and weighing not more than 1 Corp., Newark, N. J., and the British gram to a 50-ml. platinum dish, Add 5 Drug Co., Ltd., Toronto, Canada. ml. of water and hydrofluoric acid dropAmyl acetate, from the Fisher Scienmise until a clear solution is obtained. tific Co., was solvent-purified. Evaporate, using moderate heat, until Dithiol. Dissolve 0.5 gram of dithiol the first crystals of salts separate. Cool in 100 ml. of amyl acetate. and add 5 drops of hydrofluoric acid. Stannous chloride. Dissolve 20 Development of Color. Transfer grams of stannous chloride dihydrate the solution obtained in any of the in 50 ml. of hydrochloric acid and dilute above procedures to a 125-ml. Erlento 100 ml. with water. meyer flask, using 15 ml. of hydroStandard tungsten solution. Dischloric acid (1 to 2). Cool to 2OOC. solve 1.260 grams of pure tungstic oxide Add 3 ml. of stannous chloride solution in 100 ml. of 10% sodium hydroxide and 10 ml. of dithiol solution. Shake solution, dilute to 1 liter with water, for 10 minutes. Transfer the solution and store in a caustic-resistant bottle. to a separatory funnel, using two 3Determine concentration of tungsten to 5-ml. washings of amyl acetate. by the conventional cinchonine method. Keep the organic layer to less than 20 Tungsten addition solution. Dilute ml. When the layers have separated, an aliquot of the standard tungsten draw off the acid layer into the original solution with water to give a tungsten flask and reserve Wash the amyl concentration of 0.02 mg. per ml. acetate layer with two 2- to 3-ml. porUse this solution within a few hours tions of hydrochloric acid (1 to 1) and after preparing. return these washings to the flask. Standard molybdenum solution. DisThe acetate layer now contains any solve 1.500 grams of pure molybdic oxide molybdenum present. The molybin 100 ml. of 10% sodium hydroxide denum can be determined a t this point solution and dilute to 1 liter with water. by diluting to 25 ml. with amyl acetate Determine concentration of the molyband measuring the absorbance at 685 denum by the conventional cu-benzoinmp. Use a reagent blank carried oxime method. through all steps of the procedure as Molybdenum addition solution. Prethe reference solution in the spectropared in the same way as the tungsten photometer. Obtain the percentage of solution. molybdenum present by referring to a standard curve prepared by measuring PROCEDURE the absorbance of solutions containing Dissolution of Sample. TIT.~NIUJI. known amounts of standard sodium molybdate solution ranging from 0.01 Transfer the sample containing not to 0.20 mg. of molybdenum, which have more than 0.20 mg. of tungsten and been treated as described. weighing not more than 1 gram to a Add 25 ml. of hydrochloric acid and 50-ml. platinum dish. Add 5 ml. of 0.2 gram of titanium sponge to the rewater, then hydrofluoric acid and nitric served solution and heat gently until acid dropwise until a clear solution is the solution is a medium purple due to obtained. Add 3 ml. of sulfuric acid the presence of titanium(II1). [If the and evaporate, using moderate heat, to found few interferences. Under the conditions necessary for the determination of tungsten, none of the interfering elements was expected to be present in sufficient quantity of proper valence state to cause difficulty, with the exception of molybdenum. As this study progressed, it was found that conditions could be varied sufficiently to give separate quantitative recoveries of tungsten and molybdenum.
sample is titanium metal, additional titanium is not required. However; heating until titanium(II1) is formed is necessary.] Add 10 ml. of dithiol solution and heat 20 minutes in a water bath at 80' to 90" C., swirling the flask frequently. Transfer the solution to a separatory funnel, using small washings of amyl acetate, and keeping the volume of the organic layer to less than 20 ml. Draw off and discard the acid layer. Wash the acetate layer twice vith 3 to 5 ml. of hydrochloric acid (4 to 1). Transfer the amyl acetate layer to a dry 50-ml. graduated cylinder. If the amyl acetate layer is cloudy, filter through a dry filter paper into the cylinder. Dilute the solution to 25 ml. with amyl acetate. Measure the absorbance a t 640 nip in a 1-em. cell, using amyl acetate a s the reference solution. Determine the tungsten concentration of the sample by referring to a standard curve prepared by measuring the absorbance of solutions containing known amounts of sodium tungstate qolution ranging from 0.01 to 0.20 mg. of tungsten which have been treated as described SELECTION OF REDUCTANT
In an attempt t o determine the valence of tungsten in the tungstendithiol complex, a known amount of tungsten(V1) was reduced to tungsten(V) by the method of Crouthamel and Johnson ( 3 ) . The reaction of tungsten(V) with dithiol was instantaneous. and with additional heating and shaking, recoveries of 0.095 nig. of tungsten were 0.094 and 0.097 mg. From these results, it would appear that the tungsten in the dithiol complex has a valence of (V). K i t h this in mind, it was felt that the method of Bagshawe and Truman ( I ) for the determination of tungsten in steel would be adequate. However, several modifications were necessary to prevent tantalum hydrolysis and formation of other insoluble tantalum products. The addition of phosphoric. acid was omitted, and the sul-
furic acid volume was increased sufficiently to prevent tantalum hydrolysis. Under these conditions, stannous chloride did not completely reduce tungsten, and recoveries were low and erratic. Short ( 7 ) found that tungsten(V) could be readily formed by using the titanium(II1) formed in solution as the reductant and that stannous chloride was not a strong enough reductant for tungsten but was adequate for molybdenum. As tungsten-free titanium sponge is readily available, it was felt that it would serve as a good reductant for tungsten. However, titanous chloride added as a solution is equally effective. On two samples containing 0.0347, tungsten, results of 0.036 and 0.038% were obtained when 0.2 gram of titanium(II1) was added in hydrochloric acid (1 to 1) solution. Additional hydrochloric acid was added to bring the solution acid concentration to 9 to 10N hydrochloric acid. I n the procedure described, addition of 0.2 gram of titanium as sponge was selected because of volume considerations rather than for any chemical advantages.
drofluoric acid solution was evaporated to a low volume prior to extraction. This procedure gave complete tungsten recoveries. EFFECT OF TUNGSTEN CONCENTRATION
The colored compound of tungstendithiol exhibits two absorption bands: one centered a t 360 mp and the other a t 640 mp (Figure 1). However, the absorbance a t 360 mp was much too great to be usable in the range of tungsten to be analyzed. The 640 mp band was selected because of the desirable sensitivity for the amounts under investigation.
Table 1. Presence
Recovery of Tungsten in Varying Amounts of Molybdenum (Matrix, 0.3 gram of tantalum)
of
Molyb- Tungsten Tungsten denum ReRePresent, .4dded, covered, covery, A4g. Mg. Mg. % 0.099 0.0'3'3 0,099 0,099
0.0108 0.0216 0.0324 0.054
0.102 0.101 0,102 0.100
103.2 102.1 103.2 101.1
DISSOLUTION OF SAMPLE
Titanium and zirconium can be dissolved in hydrochloric acid, but tantalum cannot. The dissolution is not rapid in any case; hydrofluoric acid and nitric acid, in combination, will dissolve all these metals rapidly. Unfortunately, nitric acid interferes with the dithiol reaction. Even with careful addition and controlled evaporation to remove the nitric acid, the results were erratic and frequently low. Light fuming prior to extraction with sulfuric acid gave complete recovery of tungsten in tantalum and titanium, but tungsten recoveries were low when zirconium was treated in the same manner. As zirconium will dissolve in dilute hydrofluoric acid, the addition of nitric acid and sulfuric acid was eliminated, and the dilute hy-
The green-blue colored system measured a t 640 mp follows Beer's law from 0.01 to 0.20 mg. of tungsten in 25 ml. of amyl acetate when 1-cm. cells are used (absorbance values from 0.050 to 1.0). If the concentration of dithiol solution is increased from 0.5 to 1%, the colored system follows Beer's law from 0.01 to 0.40 mg. of tungsten in 50 ml. of organic solvent, when 1-cm. cells are used. Where more than 0.2 mg. of tungsten is expected, a 1% instead of a 0.5% solution of dithiol can be used. If the measured absorbance is less than 1.0 in a 50-ml. volume, all the tungsten present has reacted. CARE
OF REAGENT
One of the most important causes of erratic results with dit'hiol is air oxidation of the reagent. It should be stored under refrigeration and if possible in an inert atmosphere. Dithiol, on decomposition, forms a yellow liquid. If the crystalline dithiol contains this yellow liquid, even though stored under refrigeration, it should be discarded. For best results, the reagent solution should be made up just before use. INTERFERENCES
0 5 0
1
1 600 W A V E LENGTH
1
- mu
Figure 1. Absorption spectra molybdenum dithiol complexes
of
I
roo tungsten
I and
The possibility of simultaneously reducing, extracting, and determining molybdenum and tungsten with dithiol was studied by Bagshawe and Truman (1) and Short ( 7 ) . They found that under conditions necessary to reduce tungsten(V1) to tungsten(V)-9 to VOL. 29, NO. 6, JUNE 1957
897
Table
Matrix Titanium
Sample, G.
II.
Recovery of Varying Amounts of Tungsten
Present in sample
Tungsten, hIg. Added
Recovered
Recovery, %
0.200
0
0.038 0.057 0.095 0.133
0.035 0.062 0.100 0.128
92 108 105 96
Zirconium
0.200
0
0,038 0.057 0.095 0.133
0.012 0.059 0.100 0.128
110 5 103 5 105 3 96 2
Tantalum
0.200
0
0,038 0.057 0.095 0.133
100 100 103 2 103 0
0.038 0.057 0,095
0.038 0.057 0,098 0.137 0.068 0.104 0.130 0.161
0.000 0.019 0.038 0.057
0.013 0.033 0,053 0.071
0.068
Tantalum
Table 111.
1.000
0.013
Kone
Determination of Tungsten in Watertown Arsenal Titanium Samples
Participating Laboratory 1" 20 3= 4"
70Tungsten WA22 0.05, 0.05 0.05, 0.06 0.06, 0.06 0.06; 0.06 0.06. 0 . 0 6 . 0 . 0 6 0.06; 0.06; 0.06 0.05, 0.05 0.05, 0.06 0.054, 0.055
\VAS 0.41, 0.41 0.42, 0.39 0.40,O.41 0.41; 0.41 0.42.0.42.0.43 0.42; 0.42; 0.41 0.40,0.40,0.40,0.36 0.40,0.40,0.40,0.39,0.38 0.38, 0.38
Metals Research Laboratoriesb aiBenzoinoxime precipitation-hydroquinone colorimetric method. b Dithiol method.
1LY hydrochloric acid and a strong reductant such as titanium(II.1)-molybdenum recoveries were low. For quantitative recoveries of molybdenum, more moderate reducing agents and weaker acid solutions, 6N hydrochloric acid or less, were necessary. The supposition by both authors was that under conditions necessary for tungsten reduction, some of the molybdenum was reduced to molybdenum(III), which did not form a colored dithiol complex. Since sequential extraction of molybdenum and tungsten had been utilized by Bagshawe and Truman (1) and Short (?), conditions for this type of extraction were investigated. It was found that molybdenum could be reduced and extracted in 4N hydrochloric acid by using 20% stannous chloride in 1 to 1 hydrochloric acid at a temperature of 20" C. (Table I). A small amount of sulfuric acid (not more than 4 ml.) could be present. No tungsten was extracted even after shaking 20 minutes. The acidity was then increased to 9 to 11N hydrochloric acid, titanium(II1) used as the reductant, and tungsten extracted. 898
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ANALYTICAL CHEMISTRY
Hydrofluoric acid in low concentrations did not interfere and served two useful purposes; it prevented tantalum hydrolysis and facilitated dissolution of the added titanium sponge. RESULTS A N D DISCUSSION
As no acceptable metal standards were available, precision and accuracy were checked using synthetic solution standards. Tungsten - bearing metals were used to check precision. All the metals used were first analyzed with dithiol for tungsten. I n order to determine reproducibility, five portions of a sample of titanium containing 0.095 mg. of tungsten were analyzed. An average of 0.096 mg. of tungsten was recovered with a maximum difference of 0.003 mg. Varying amounts of tungsten (0.038 to 0.133 mg.) were added to titanium, zirconium, and tantalum. As is shown in Table 11, recoveries were satisfactory. Two titanium samples prepared by the Watertown Arsenal and issued to the panel on methods were submitted to these laboratories for determination by
1 8 3
2
94 7 108 8 97 9
105 2 105 2 101 7
the dithiol method (Table 111). Values obtained by other laboratories using an a - benzoinoxime precipitation-hydroquinone colorimetric method are also given in Table 111. The molybdenum extraction, in most cases, may be omitted, as molybdenum is usually present in negligible quantities or not a t all. However, the addition of stannous chloride was continued because conversion of titanium (metal) to titanium(II1) was more rapid. The advantages of this method are its sensitivity, specificity, rapidity, and simplicity. ACKNOWLEDGMENT
The author is grateful to Galen Porter, Arnold R. Gahler, and other members of the Metals Research Laboratories for helpful suggestions. LITERATURE CITED
(1) Bagshawe, B., Truman, R. J., -4nalyst 72, 189 (1947). (21 ~, Boaatski, G.. 2. anal. Chem. 114, 170 fi938): ' (3) Crouthamel, C. E., Johnson, C. E., ANAL.CHEM.26, 1284 (1954). . . (4) Gentry, C. H. R., Sherrington, L. G., Analyst 73,57 (1948). (5) Miller, C. C., Ibid., 69, 109 (1944). (6) Sandell, E. B., "Colorimetric Determination of Traces of Metals,', pp. 586, 595, Interscience, New York, 1950. (7) Short, 11. G., dnalyst 76, 710 (1951).
RECEIVEDfor review May 21, 1956. Accepted February 28, 1957. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 27, 1956.
