ACKNOWLEDGMENT The authors are grateful for helpful discussions with H. D. Beckey, C. R. McKinney, and C. Brunke. E. M. C H A I T ~ T. W. SHANNON~ J. W. AMY F. W. MCLAFFERTY Department of Chemistry Purdue University, Lafayette, Indiana 47907
RECEIVED for review September 25,1967. Accepted January 31, 1968. The generous support of the National Institutes of Health, GM12755 and FR00354, is gratefully acknowledged. 1 Present address, Central Research Dept., E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. Present address, Eastern Research Laboratories, Dow Chemical Co., Wayland, Mass. 3 To whom inquiries should be addressed.
Spectrophotometric Determination of Tungsten as the Tungstate by Heteropoly Acid Formation SIR: In many commonly used techniques for the spectrophotometric determination of tungsten, molybdenum is a serious interference ( I , 2 , 3). In our method an excess of molybdenum as molybdate is used as the complexing reagent. In short, the method involves the formation of an assumed molybdotungstic heteropoly acid in a weakly acidic medium and subsequent reduction to the heteropoly blue after a short period for complex formation. The method is simple and quite accurate for tungsten as tungstate in the concentration range of 0 to 15 ppm.
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EXPERIMENTAL Apparatus. All absorbance measurements were made with a Cary Model 12 recording spectrophotometer using matched 1-cm quartz cells. All pH measurements were made with a Beckman Zeromatic I1 pH meter. Reagents. A 0.02M sodium molybdate solution was prepared by dissolving 4.8396 grams of NalMoOs 2 H z 0 in 100 ml of water and diluting to 1000 nil. A 2 z w/v hydrazine dihydrochloride solution was prepared by dissolving 20 grams of NH2NH2.2HCl in 100 ml of water and diluting to 1000 ml. Two sulfuric acid solutions were prepared, one 3 % v/v and the other 10% v/v. A standard tungsten solution (0.5 mg Wjml) was prepared by dissolving Na2WOa,2H20 in water and diluting to the appropriate volume. Procedure. To a solution containing 0 to 1.6 mg of tungsten as tungstate add 10 ml of 0.02M NanMoOa. Dilute samples to 50 ml with HzO. Adjust the pH to 3.3 with 3 % v/v sulfuric acid. After allowing 15 minutes for complex development, add 1.5 ml of 10% v/v sulfuric acid by syringe, followed in 20 seconds by the addition of 4 ml of 2 z w/v hydrazine dihydrochloride. Transfer immediately to a 100-ml volumetric flask and dilute to the mark with water. A blank is prepared in the same manner but omitting the tungsten. Allow 1 hour for color development and read the absorbance at 765 mp.
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RESULTS AND DISCUSSION In a study of the amount of time necessary to form this unusual unreduced complex it was found that maximum absorbance values occurred between 10 and 15 minutes after (1) B. Bagshawe and R. J. Truman, Analyst, 72,189 (1947). (2) C. E. Bricker and G. R. Waterbury, ANAL. CHEM.,29, 1093 (1957). (3) G. Gottschalk, Z. A m / . Clrern., 187,164 (1962).
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Figure 1. Study of the amount of acid needed to minimize blank A . Contains 5 ppm W B. Contains 0 ppm W C. Difference
mixing of reagents. After 15 minutes, absorbance values dropped at the rate of 0.04 absorbance unit per hour. The colored species formed by the reduction of this assumed molybdotungstic heteropoly acid shows a broad absorption band between 840 and 740 mp with the maximum at 765 mk, The absorption at 765 mp rises rather rapidly for 50 minutes after reduction and then rises much more slowly. A study of the optimum p H for formation of the molybdotungstic acid complex was carried out. It was found that there was a linear decrease in absorption with decreasing pH beVOL. 40, NO. 4, APRIL 1968
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Table I. Diverse Ion Amount permitted (ppm) 90 50 10
c1 e1 c1 10 e1 25
40
CI c1 C1-complete interference
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mi 2% Hydrazine Hydrochloride
tween p H 6.23 and 1.93. An optimum p H 3.3 was chosen to minimize blank absorption. Figure 1 shows the results of a study to determine the optimum amount of 10% sulfuric acid needed t o minimize blank absorbance. A volume of 1.5 ml was chosen t o give sufficient sensitivity and a small blank correction. Several reductants are commonly used in heteropoly systems. Of these, ascorbic acid, Fe(Il), and 1-amino-2-naphthol4-sulfonic acid failed t o reduce the complex under the described conditions. Chlorostannous acid is a rapid reductant but forms turbid solutions a t the p H of the system. Hydrazine dihydrochloride gave the most satisfactory results. Four milliliters of 2 z hydrazine dihydrochloride was chosen as the optimum reductant concentration (Figure 2). It was found that the time interval between the addition of the sulfuric acid to minimize the blank and the addition of reductant was not critical. A plot of this time interval vs. final absorbance is a straight line up to 60 seconds. Longer time intervals seem to favor the reduction of the molybdo-
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ANALYTICAL CHEMISTRY
Figure 2. Reductant concentration study A . Contains 5 ppm W B. Contains 0 ppm W C. Difference
tungstic acid complex, while minimizing the effect of interfering elements. The calibration curve does not obey Beer’s law. The molar absorptivity a t 5 ppm tungsten is 20,850. Table I gives the levels of interference of a few of the diverse ions studied. Most other common ions d o not interfere. JOHNC. GUYON JERRYY. MARKS Department of Chemistry University of Missouri Columbia, Missouri
