(4) Corbin, E. A., ANAL.CHEM.34, 1244 (1962). ( 5 ) Dhont, J. H., deRooy, C., Analyst 86, 74 (1961). (6) Dornseifer, T. P., Powers, J. J., Food Technol. 17, 118 (1963). (7) Forss, D. A., Bazinet, 11. L., Swift, J. M., J . Gas Chromatog. 2 , 134 (1964). (8) Hornstein, I., Crowe, P. F., ANAL. CHEM.34, 1037 (1962). (9) Hunter, C. L. K., Struck, R. F., Ibid.. R . 864. (10) Lyin, W. S., Steele, L. A., Staple, E., Ibid., 28, 132 (1956). (11) Meigh, D. F., Chem. Znd. (London) 1956, p. 986.
(12) Ralls, J. W., ANAL. CHEM.32, 332 (1960). (13) Rice, R. G., Keller, G. J., Kirchner, J. G., ANAL.CHEM.23, 1964 (1951). (14) Rosmus, J., Deyl, Z., J . Chromatog. 6 , 187 (1961). (15) Soukup, R. J., Scarpellino, R. J., Danielczip, E., ANAL. CHEM.36. 2255 (1964). (16) Stanley, W. L., Ikeda, R. M., T'annier. S. H.. Rolle. L. A,. J . Food Sci. 26.'43 11961). ' (17) Stephens;- R . ' L., Teszlw, A . P., ANAL.CHEM.32, 1047 (1960). (18) Teitelbaum, C. L., J . Ora. Chem. 23, 646 (1958).
(19) S'ogel, A. I., Pract. Org. Chem., 3rd ed., p. 977 (1956). DONALD F. GADBOIS JOSEPH M. MENDELSOHN LOUISJ. RONSIVALLI Bureau of Commercial Fisheries Technological Laboratory Gloucester, Mass. WORK supported by the Division of Biology and Medicine, U. S. Atomic Energy Commission, under Contract No. AT(49-7)-2443. Trade names referred to in the publication do not imply endorsement of commercial products.
Spectrophotometric Determination of Bismuth SIR: Our laboratories have been involved in a long range study of mixed heteropoly acid formation, in the course of which the molybdobismuthophosphate method for bismuth (2) was examined. As a result of this study, we have developed a new heteropoly method for bismuth. The technique described here is based on the enhancement of the blue hue of reduced sodium molybdate by bismuth. The previous heteropoly method (z), based on the bismuth enhancement of the blue hue of reduced molybdophosphoric acid, has a maximum sensitivity gram/liter, whereas the of 4 X method described here can easily measgram/liter. The molar ure 5 X absorptivity of the new bismuth system is 2 X lo6 at the 5-p.p.b. level, making this method some 10 times more sensitive than existing photometric techniques, with a recommended range of 5 to 100 p.p.b. EXPERIMENTAL
Apparatus. All spectrophotometric measurements were made on 2 Cary Model 12 recording spectrophotometer using 1.000 i. 0.002 em. quartz cells. Measurements of pH were made with a Beckman Zeromatic pH meter. Reagents. A 250-p.p.m. stock bism u t h solution was prepared by dissolving 0.2901 gram of reagent grade Bi(r\'O&.5H20 in 20 ml. of boiled HNO, and diluting t o 500 ml. T h e solution was standardized b y precipitation as BiP04. A 5% (w./v.) Xa2Mo04solution was prepared by dissolving 50.00 grams of NapMo042 H 2 0 in water and diluting to 1 liter. A 1M Ka2S04solution was prepared by dissolving 142.00 grams of S a 2 S 0 4 in water and diluting to 1 liter. A 5% (w./v.) ascorbic acid solution was prepared by dissolving 5.00 grams of ascorbic acid in water and diluting to 100 ml. This solution decomposed on standing and was prepared fresh daily. 1778
ANALYTICAL CHEMISTRY
Ibb
io
1514-
1312-
P
0807-
0 IO 20 3 0 4 0 5 0 60 70 8 0 90 CONCENTRATION OF 5% l w / v ) SODIUM MOLYBDATI IN MiLLllllERS
Figure 1. Effect of concentration of sodium molybdate a.
Bismuth solution (0.1 p.p.rn.)
b.
Blank
-4 solution containing Na2S04, Na2M004 and (reagent mixture) was prepared by adding 200 ml. of 5% Xa2hlo04, 280 ml. of 1 M KazS04, and to a %liter volu18.60 ml. of 1: 1 metric flask and diluting to the mark with water. This solution was allowed to stand 24 hours before use. Procedure. T o a solution containing 5 t o 100 p.p.b. of bismuth in a 100-ml. volumetric flask add 4 ml. of 5% ascorbic acid. Rapidly add 50 ml. of t h e reagent mixture and mix. Dilute to t h e mark and place t h e flask in a 25' C. constant temperature bath. Measure t h e absorbance at 725 mp exactly 30 minutes after mixing the solutions. The calibration curve obtained by treating various bismuth concentrations as per the recommended procedure is not linear, and decreases slowly in sensitivity with increasing bismuth concentration. RESULTS A N D DISCUSSION
Interaction between Molybdate and Sulfate. An interaction between molybdate and sulfate was suggested (4)soon after this work was undertaken and later confirmed by Dehne and Mellon (3). The influence of this inter-
action on color development was studied by measuring the absorbance of a reduced solution containing Ya2S04, HzSo4, and Sa231004 (reagent mixture) as a function of time of mixing of the reagents. Absorbance increased for the first few hours, but after 24 hours there was no further change. I n all subsequent work the reagent mixture was allowed to stand 24 hours before use. Effect of Molybdate Concentration. h series of solutions containing t h e desired amount of 5% sodium molybdate was mixed with 35 ml. of 1M sodium sulfate and enough water added t o bring the total volume to 200 ml. Acidity was adjusted to pH 1.4 with sulfuric acid and the volume taken t o 250 ml. These solutions were allowed t o stand 24 hours before being used. d 50-ml. aliquot of each solution was mixed rapidly with 4 ml. of 5y0ascorbic acid and, in the case of the bismuth-containing solutions, 1 ml. of a 10-p.p.m. bismuth solution. The solutions were diluted to 100 ml. and placed in a 25' C. water bath. The absorbance was measured at 725 mM 30 minutes after mixing. The results of this study are shown in Figure 1. The color intensity of the blank is the limiting factor in optimizing the concentration of molybdate, because there seems to be no limit to increasing the difference in absorbance. Five milliliters of 5% Na2hIo04 were chosen for further use because Na2Xo04 exhibited a substantial increase in the color intensity when bismuth was present, yet gave a workable blank. Effect of Sulfate Concentration. Preliminary studies showed Ka2S04 t o be a necessary component for a stable, reproducible color reaction. To ascertain the optimum concentration of XazSO4, a series of solutions containing the desired amount of 1X KazS04were mixed with 25 ml. of 5% Na2Mo04and enough water was added to bring the total volume to 200 ml. The acidity was adjusted to pH 1.4 and
:;I
{ 0.8
0.2
Figure 3. Effect of time on color development
0.1 1
0
0.5
Figure 2.
a. Bismuth solution (0.1 p.p.m.1 b. Blank
Effect of pH
a. Bismuth solution (0.1 p.p.m.) b. Blank
the volume taken to 250 ml. After standing 24 hours, the solutions were reduced as in the molybdate study. Seven milliliters of 1M sodium sulfate were chosen for subsequent work, because this volume gave the reproducibility and permitted a marked enhancement of the blue hue in the presence of bismuth. Effect of pH. Solutions containing 25 ml. of NazMo04, 35 ml. of 1-11 Na2S04, and varying amounts of H z S 0 4 were diluted to 250 ml. and allowed t o stand 24 hours. These solutions, in 50-ml. aliquots, were reduced as in the molybdate study (Figure 2). A p H of 1.70 was chosen for subsequent work. Effect of Concentration of Reductant. Several reductants commonly used in heteropoly acid systems were studied, including Fe+2, Sn+2, hydrazine, sulfonic acid, sulfite, and ascorbic acid. Ascorbic acid was chosen as the most satisfactory. To study the effect of ascorbic acid concentration, 50-ml. aliquots of the reagent mixture were placed in 100-ml. volumetric flasks with 1 ml. of a 10-p.p.m. bismuth solution and various amounts of 5% ascorbic acid solution. The solutions were diluted to volume, placed in a 25’ C. constant temperature bath, and absorbance was measured a t 725 mp after 30 minutes. The difference in absorbance between the bismuth-containing solution and corresponding blanks continued to increase with increasing ascorbic acid concentration until absorbance measurements could no longer be made on the solution. The magnitude of the blank became the limiting factor. To keep the blank
small and still retain sensitivity, 4 ml. of 5% ascorbic acid was chosen for future work. Effect of Time on Color Development. The absorbance of a solution prepared as per the recommended procedure was measured as a function of time (Figure 3). -4color development time of 30.0 minutes m-as selected as giving both a reasonable enhancement caused by the bismuth and a workable blank. The absorbance changes with time, and it is thus necessary to measure the absorbances within a few seconds of the specified time. Effect of Temperature. The color reaction was studied a t several temperatures above room temperature. I n each case the absolute absorbance rose, but the difference in absorbance between the blank and the bismuthcontaining solution was not appreciable. All work was subsequently carried out a t a fixed temperature of 25’ C. Effect of Diverse Ions. Solutions containing 100 p.p.b. of bismuth were treated as the recommended procedure, except that 50 p.p.m. of various ions were added. C U + ~ Fe+3, , Sb+3
A~o~-,
pod+, VO~-,
wo4-2:
Fe+2,Cr+6,Cr+3,S n f 4 , and F-interfere seriously. Other substances investigated can be tolerated in the concentrations shown in Table I. X 2y0error was considered tolerable. Accuracy of Method. The accuracy of the method was determined by analyzing S B S 53d lead-base alloy. The bismuth was separated from the other elements by the method of Bendigo, Bell, and Bright (1). An average
Table I.
Effect of Diverse Ions Amt. tolerated, Ion p.p.m. 5 15 20 15 50 20 50 50 35 10
~ 1 - 3
ISCN -
c1-
HCz04c104NOa-
BrMn +2
of six determinations yielded 0.140% bismuth compared to the certified value of 0.135’%. Standard deviation was 1.4%. LITERATURE CITED
(1) Bendigo, B. B., Bell, R. K., Bright, J., J. Res. iVatl. Bur. Std. 47. 252 ( 1951 1. (2) Campbell, R. H., Mellon, >I. G., ANAL. CHEM.32. 54 (1960). ( 3 ) Dehne, G. C.,’ Mellon, h1. G., Zbid., 35, 1382 (1963). ( 4 ) Matulis, R. &I., University of Missouri, Columbia, No., private communication, 1963.
JOHX C. GUYON LISDAJ. CLINE
Department of Chemistry University of Missouri Columbia, Mo. WORK supported by the Director of
Chemical Sciences, Air Force Office of Scientific Research under Grant AFAFOSR-205-63.
VOL. 37, NO. 13, DECEMBER 1965
1779
