Colorimetric Determination of Microgram Amounts of Tungsten in Uranium-Tantalum-Tungsten Alloys CLARK E. BRICKER' and GLENN R. WATERBURY The University o f California, 10s Alamos Scientific laboratory, 10s Alamos, N. bMicrogram amounts of tungsten in uranium-tantalum-tungsten alloys may b e estimated colorimetrically a i 478 mp using hydroquinone in concentrated sulfuric acid solutions. For 14 determinations of 50 to 1 5 0 y of tungsten in solutions containing various amounts of uranium and tantalum, the average for the tungsten found was 99.8%, with a standard deviation of 1.1 %. Molybdenum, iron, chromium, titanium, vanadium, and ruthenium interfere seriously with the determination; a reliable correction is made for the small absorbance of uranium a t the wave length used. Because of the simplicity of the procedure, 10 to 12 determinations may b e performed in 3 hours.
C
for the determination of tungsten were investigated because of the need for a rapid and accurate procedure for the determination of about 0.5y0 of tungsten in uranium-tantalum-tungsten alloys. Analyses of these alloys had been made using a modification of a gravimetric method described by Bagshawe and Elwell (2), but the low tungsten concentrations and the difficulties encountered in separating the tantalum and tungsten made the gravimetric procedure long and tedious. Therefore, a n independent method was sought which would provide reliable analytical results and be better suited for the low tungsten concentrations. Several reagents have been described for the colorimetric determination of tungsten. These include thiocyanatestannous chloride (5, 12, 1 5 ) , dithiol (1, 11), Rhodamine B (8), reagents that reduce tungsten to colored lower oxidation states ( 1 4 , reagents that form colored heteropoly acids with tungsten ( 7 , 16), titanium(II1) plus malachite green (6), and hydroquinone (4, 8, I S ) . Of these reagents, hydroquinone seemed the most promising because the tungsten-hydroquinone color was formed directly and immediately upon mixing the reagents. Furthermore, tantalum was reported not to interfere with this reaction (3, I O ) . OLORIMETRIC METHODS
Preeent address, Chemistry Department, Princeton University, Princeton,
N. J.
M.
The present work describes the determination of tungsten in uraniumtantalum-tungsten alloys by a modification of the hydroquinone procedure of Heyne (8) and gives the effect of several other metals on the color reaction. Although methods to eliminate the interference of some other metals are reported (3, I O ) , no investigation of this aspect of the problem was made because the uranium alloys to be analyzed were of high purity. Only the effect of uranium and tantalum was of immediate importance. APPARATUS AND REAGENTS
Beckman hIodel D U spectrophotometer with matched 1-cm. Corex cells. Hydrofluoric acid, 49%, reagent grade, Sulfuric acid, specific gravity 1.84, reagent grade, protected from atmospheric moisture. Nitric acid, specific gravity 1.42, reagent grade. Hydroquinone solution, 55 mg. per ml. Dissolve 5.50 grams of Eastman White Label hydroquinone in concentrated sulfuric acid and dilute to 100 ml. with this acid. Dispense the reagent from a 5-ml. Koch microburet equipped with silica gel drying tubes a t the top of the buret and a t the reservoir vent. Tungsten standard solution, 1.0 mg. per ml. Dissolve 1.80 grams of sodium tungstate dihydrate in water and dilute to 1 liter. Determine the tungsten concentration gravimetrically (9). Prepare other standard solutions by diluting accurately measured volumes of the stock solution with water. Uranium standard solution, 20 mg. per ml. Dissolve 1.0 gram of uranium metal or 1.18 grams of uranium oxide (UsOs) in nitric and sulfuric acids and dilute to 50 ml. RECOMMENDED PROCEDURE
Treat a weighed sample of the uranium-tantalum-tungsten alloy in a platinum dish R-ith 2 ml. of water, 1 ml. of concentrated sulfuric acid, and 5 ml. of nitric acid for each gram of sample. After the initial reaction subsides, cautiously warm the mixture under a heat lamp until effervescence ceases. Add 0.5 ml. of concentrated hydrofluoric acid for each gram of sample and warm the reaction mixture again. Repeat the hydrofluoric acid treatment until solution is complete and then dilute the solution to a known volume.
Transfer to a platinum dish a n aliquot of the sample solution which contains 10 to 250 y of tungsten and which also contains approximately the same amount of uranium as used in preparing a calibration curve. Add 2 to 3 ml. of sulfuric acid and heat to strong fumes of sulfur trioxide. Quantitatively transfer the solution to :t IO-ml. volumetric flask using concentrated sulfuric acid t o wash the dish. The volume of the solution and washings should be less than 7.0 ml. Allow the solution to cool to room temperature and add 3.00 ml. of the hydroquinone solution, Dilute to 10 ml. with concentrated sulfuric acid and mix thoroughly. Measure the absorbance of this solution a t 478 mp against a reagent blank containing 3.00 ml. of hydroquinone solution and sufficient sulfuric acid to make 10 ml. Calibration Curve. Evaporate to strong fumes of sulfur trioxide known amounts of the standard tungsten solution plus a fixed amount of the standard uranium solution and 2 t o 3 ml. of sulfuric acid. T h e amount of uranium solution taken depends upon the tungsten concentration of the sample. For alloys containing 0.10 to 1.0% tungsten, 20 mg. of uranium should be taken with 20 to 200 y of tungsten. After fuming, quantitatively transfer the sulfuric acid solution of the uranium and tungsten to a 10-nil. volumetric flask. Cool the solution, add 3.00 ml. of hydroquinone reagent, dilute to 10 ml. with sulfuric acid, and mix thoroughly. Measure the absorhance at 478 mp against a reagent blank containing 3.00 nil. of hydroquinone solution and sufficient sulfuric acid to make 10 ml. Plot the absorbances as a function of the tungsten concentration. To calculate the amount of tungsten in the sample aliquot, determine from the calibration curve the micrograms of tungsten corresponding to the absorbance of the sample. RESULTS AND DISCUSSION
Interferences. The use of the color reaction between tungsten and hydroquinone has been limited largely in the past t o the analysis of steel samples. For these determinations the effect on the color reaction of iron and the alloying elements in the steel was investigated and various methods were employed t o eliminate interferences. Bogatski ( 3 ) and Johnson (IO) used zinc or stannous chloride to reduce VOL. 29, NO. 7, JULY 1957
1093
iron(II1) and molybdenum(V1) prior to the color development; phosphoric acid was added to hinder the reduction of tungsten and the precipitation of tin sulfate. I n addition, sufficient water !vas added to prevent the precipitation of iron sulfate and to eliminate inter-
Table 1. Effect of Foreign Metals on Tungsten-Hydroquinone Color Reaction
Foreign Netal Mg. nIo 3.0 0.1 3.0 Ti 0.1 3.0 Ta 1.0 3.0 Sr 11g 3.0 Cd 3.0 3.0 Ce( IV) 3.0 3.0 Bi 3.0 Zr 3.0 Nd 3.0 A1 3.0 K 50.0 TI 3.0 3.0 Fe( 111) 0.5 0.1 Fe(I1) 3.0 1.0
2
0.5
Sn
hIn( 11)
Hg Cr(II1) Cr(V1) Ce(II1) Xi Mn(VI1) As
V Zn
cu Ru
0.1 3.0 1.0 0.5 0.1 3.0 1.0 3.0 1.0 3.0 0.5 0.1 3.0 0.5 0.1 3.0 1.0 0.5 0.1 3.0 0.5 0.1
3.0 1.0 0.5
3.0 1.0 0.5 0.1 3.0 0.1 3.0 1.0 3.0 1.0 0.5
3.0
ReProbable covery," Cause of yo Interferenceb Infinity 1 1 111.5 1 Infinity 1 418 1 113.8 None 99.8 None 100.2 None 100.2 None 100.0 None 100.5 101.0 None 99.8 None None 101.5 99.5 None 101.7 None 97.6 None 99,5 None None 99.3 2 174.4 2 122.0 2 102.4 2 118.8 2 116.4 2 114.0 2 104.0 2 130.5 2 116.7 None 100.7 None 99.8 2 or 3 107.9 None 99.3 2 106.1 None 100.2 2 389 2 103.6 None 101.9 2 and 3 611 2 and 3 104.3 2 and 3 102.2 2 111.0 2 104.4 2 110.9 None 99.3 2 or 3 103.7 2 or 3 104,s None 99.5 3 110.5 3 102.7 None 99.5 2 114.9 2 104.4 2 105.8 None 101.0 3 734 3 116.6 105.1 2 (?I None 101.2 1 or 3 122.0 1 or 3 108,2 None 100.7 278
3
3 125.6 3 112.3 0.05 3 103.9 3.0 co None 101.9 1.0 a 100 y of tungsten taken in each case. b 1 = reaction with hydroquinone form colored product; 2 = precipitate formed; 3 = natural color of element interferes; none = no interference. 1094
0.5
ANALYTICAL CHEMISTRY
0.01
400
1
450
I
500
I
550
I
600
1
650
WAVE LENGTH, MILLIMICRONS Figure 1. Absorption curves for tungsten-hydroquinone complex with 100 y of tungsten A. No uranium 6. 12.5 mg. of uranium
0. 40 mg. of uranium E. 50 mg. of uranium
C. 20 mg. of uranium
ference by tantalum. Measurements were taken a t 570 to 580 mp, about 100 nip above the wave length of maximum absorbance for the tungsten-hydroquinone complex, to avoid interference caused by organic material, nitrate, and other substances that produce a yellow color with hydroquinone ( I S ) . Under these conditions iron, chromium, molybdenum, nickel, cobalt, vanadium, tantalum, aluminum, copper, manganese, silicon, phosphorus, and sulfur caused little or no interference. However, the effect of uranium on the reaction was not reported. I n the present work the effect of uranium, tantalum, and several other metals was investigated on the color reaction in concentrated sulfuric acid. For amounts of tantalum equal to the tungsten concentration, no interference was found. Large amounts of uranium caused a small positive interference and changed the absorption characteristics of the tungsten-hydroquinone color. Absorption curves are given in Figure 1 for solutions containing 100 y of tungsten, 165 mg. of hydroquinone, and various amounts of uranium in a final 10-ml. volume of concentrated sulfuric acid, Below 450 mp uranium causes the absorption curve t o vary markedly from that of the tungsten-hydroquinone complex alone. However, for readings made a t the wave length of maximum absorbance for the tungsten-hydroquinone complex, 478 mp, the effect of the uranium is small. Calibration curves for tungsten in the presence of 12.5 and 50 mg. of uranium were prepared as described under the recommended procedure. Parallel curves were obtained that followed Beer's law, showing that the uranium
interference is independent of the tungsten concentration a t 478 mp. Because the uranium interference is small and constant with tungsten concentrations, a reliable correction may be applied to the absorbance reading obtained for uranium-tantalum-tungsten alloy samples or the tungsten concentration may be obtained directly from a calibration curve. Heyne (8) reported that moisture dulled the tungsten-hydroquinone color, and Johnson (IO) used water to eliminate the interference caused by tungsten and niobium in the determination of titanium using hydroquinone Because moisture was not used in the present method to prevent interferences, water was excluded from the solutions toobtain maximum sensitivity for the tungsten determination. I n addition, the use of a concentrated sulfuric acid system has the further advantage that the correct acid concentration could be attained readily by evaporating the solutions to fumes of sulfur trioxide; most anions were eliminated by this process also. To protect the hydroquinone solution and the sulfuric acid from moisture, these reagents were stored in containers equipped with drying tubes. The effect of some 30 other metals on the tungsten-hydroquinone color reaction was also determined. Known amounts of the foreign element from 0.05 to 50 mg. were added to 100 y of tungsten and 2 ml. of concentrated sulfuric acid in a platinum dish. The solutions were evaporated to strong fumes of sulfur trioxide and taken through the color development described above. The results of this investigation
(Table I) show that strontium, niagnesium, cadmium, cerium(IV), silver, lead, bismuth, zirconium, neodymium, duminum, and thallium cause no interference even when the ratio of the foreign metal to tungsten is 30 to 1; ~iotassiumcauses no interference at a ratio of 500 t.o 1 . Molybdenum, titanium, iron, vanadium, and ruthenium iriterfere for weight ratios as low as 1 to I ; arsenic, cerium(III), nickel, and chromium(II1) and (VI) interfere a t ratios of 5 to 1 or greater; copper, t.in, :ind manganese(VI1) inkrfere for ratios of 10 to 1 or greater; and tantalum, nianganese(II), mercury, zinc, and c4obalt interfere when the ratio is 30 t o 1 or greater. The interference of wine of the metals was caused by the formation of precipitates and was erratic as shown in Table I. The most serious interference was caused by molybdenum. chromium, titanium, vanadium, and ruthenium. However, because the alloy samples analyzed by this method were of high purity. no attempt was made to eliminate the interference causcd by any foreign metal. Reliability. Because no standard samples of tungsten-tantalum-uranium alloys are awilable, the reliability of the method is based upon analyses of known solutions and upon a comparison of the analytical results for samples obtained by this color methodand by t h e gravimetric method ( 2 ) . An average for the tungsten found in 14 solutions of known concentration cmntaining 50 to 150 y of tungsten and wrious amounts of uranium and tantalum was 99.8%, with a standard c!vviation of 1.1% (Table 11). The results for eight unknown alloy simples analyzed by the two methods
are shown in Table 111. The average of the differences between the results obtained by the two methods was 0.02% absolute, or 4y0 relative. No bias between the methods is shown. Although several metals interfere with the direct colorimetric method for tungsten, the simplicity of the proTable 11.
Tungsten, Mg. 0.100 0,100 0.100 0.100 0 100 0.100 0 . 100 0.150 0.050 0.100 0.150 0.050 0.100 0.150
Effect of Foreign Metals on Color Reaction
Tantalum, Mg.
TungUranium, sten Mg. Found, Q/o 99.8 100.0 99.8 100.5 100.0
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
12.5
12 . .i
~~
25.0 25.0 25.0 50.0 50.0 50.0 Av . Av. dev. Std. dev.
99.8 59.8 100.0 56.0 100.4 100.4 100.0
100.4 100.1 99.8 0.5 1 1
Table ’”’ Analytical
for Uranium-Tantalum-Tungsten Alloys
Tungsten Found, % ColoriGraviSample metric metric 1 0.44 0.42 9 0 50 0.50 0.44 3 0 48 0.48 4 0.49 0.52 5 0.51 0.45 6 0.51 0.50 7 0.52 8 0.57 0 51
Diff., yo
0.02
-
0.00
0.04 0.01 0.01 0.02
0.02
0 06
Av. 0.02
cedure permits rapid analyses with highly reproducible results. A single sample may be dissolved and analyzed in about 2 hours, but 10 or 12 determinations may be performed simultaneously in 3 or 4 hours. The reproducibilit,y of this method is within 2% for multiplicate determinations of 20 y or more of tungsten. LITERATURE CITED
(1) Allen, S. H., Hamilton, M. B., Anal. Chim. Acta 7, 483 (1952). (2) Bagshawe, B., Elwell, W. T., J . Soc. Chem. Ind. (London) 66, 398 (1947). (3) Bogatski, G., Z . anal. Chem. 114, 170 (1938). (4) Defacgz, E., Compt. rend. 123, 308 (1896). (5) Fernjacic, S., 2. anal. Chem. 97, 332 (1934). (6) Goto, H., Ikeda, S., J . Chem. SOC. Japan, Pure Chem. Sect. 73, 654 (1952). (7) Gullstrom, D. K., hlellon, M. G., ANAL. CHEM. 25, 1809 (1953). (8) Heyne, G., 2. angew. Chem. 44, 237 (1931). (9) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A,, Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 689, Wiley, New York, 1953. (10) Johnson, C. M., Iron Age 157, No. 14, 66 (1946). (11) Miller, C. C., Analysf 69, 109 (1944). (12) Nishida, H., Japan Analyst 3, 25 (1954). (13) Sandell, E. B., “Colorimetric Determination of Traces of h/Ietals,” 2nd ed., p. 587, Interscience, New York, 1950. (14) Travers, A,, Compt. rend. 165, 408 (1917). (15) Westwood, W., Mayer, A., Analyst 72, 464 (1947). (16) Wright, E. R., Mellon, M. G., IND. ENO. CHEM., ANAL. ED. 9, 251 (1937). RECEIVEDfor review December 26, 1956. Accepted March 7, 1957.
DifferentiaI Thermal Analysis of Some Polyglucosans HIROKAZU MORITA Chemis?ry Division, Canada Department of Agriculture, Ottawa, Canada
b Alpha- and P-linked polyglucosans manifest distinct differential thermographic features. The results are offered as a contribution to organic differential thermal analysis.
T
of differential thermal analysis to natural polymers is under active investigation in this laboratory and some results extend and correlate earlier findings. Details pertaining t o the analytical procedure have been reported (11). HE APPLICATION
In the present work, 75-mg. vacuumdried samples were used to prepare the compressed ‘(sandwich” packing. A heating rate of 10” C. per minute was used throughout. Studies (9, IO) with the starch fractions and the dextrans have shown t h a t polysaccharides having predominantly the anhydroglucose linkages give thermograms featured by characteristic endotherms in the 100” t o 310” C. region which are due to dehydration and molecular rearrangements. It is
now shown that other polysaccharides possessing similar linkages give analogous thermograms. Reliable chemical evidence suggests that glycogens are closely related in their general macromolecular outline to starches. Similar conclusions can be inferred from differential thermal analysis. Rabbit liver glycogen (Figure 1, A ) shows endotherms a t 130” and 255’ C. Similar thermographic patterns have been observed with ox liver and northern pike liver glycogens. VOL. 29, NO. 7, JULY 1957
1095
