(HP04), C O ( ~ ~ ) ~ P O ~ .and X HDowex ~ O 1-X8anion exchange resin in the (HP04)2- form are given in Figure 3. The electrode which gave the best response was the one containing Co(en),P04.H20. The response to (HP0d2-) was linear over the range 10-I to and had a slope of 45 mV per decade change in concentration. As the concentration of K2HPOd in the reference solution is increased, lower potentials are observed. However, the slope of the response curve remains the same. The drift of emf with time was determined by taking the initial potentials and the potential readings after 5 minutes. The drift becomes appreciable only in the range of and lOPM, and all the electrodes were found to be stable and essentially free of drift. The rate of the precipitate exchange reactions were not measured. However, since the steady state response was achieved within 1 or 2 minutes, the rate of exchange must be fairly rapid. Selectivity. The response of the ferric phosphate impregnated silicone rubber electrode to a number of common anions is shown in Figure 4. Such a response was observed with all the other electrodes prepared. The electrode is not selective to any one type of anion, but responds to all anions. At 0.1M concentrations, the order of response is [HP042-] > [H2P04-] > [NO3-] > [Cl-1 > [C104-], but this order is reversed at lower concentrations. Studies of the reproducibility of potential measurements of a ferric phosphate impregnated silicone rubber electrode indicate that measurements in the range 10-' to lOW3Mare fairly reproducible from day to day, but a deviation of about =t7% can be expected for concentrations of loe4 to 10-5M.
CONCLUSIONS
It can be concluded from this study that although precipitate impregnated or phosphate ion-exchange electrodes can be prepared that are reproducible and are linearly responsive to phosphate ion, such electrodes (either in heterogeneous or homogeneous form) lack selectivity and will be responsive to all anions. These electrodes will indicate, rather, the total concentration of anions in solution. The main reason for lack of selectivity is because the supporting material of the electrodes-if the precipitate particles are in contact with each other across the membrane-do not influence the electrochemical behavior of the membrane electrodes. Because the resistance of all impregnated electrodes is low (1300 to 10,000 a), the electrochemical responses cannot be attributed to a diffusion process alone. The potentials observed are due to a combination of the Donnan potential and the so-called diffusion potential (16). As similar response characteristics were obtained with acrylamide, paraffin, and other membranes, it can be concluded that if an ion selective phosphate electrode is to be obtained, a technique other than the precipitate impregnatedmembrane or ion-exchange must be used. RECEIVED for review February 19, 1969. Accepted April 29, 1969. One of us (P. J. B.) gratefully acknowledges the financial support of Fisher Scientific (Pittsburgh) .in the form of a graduate fellowship. (16) E. Pungor, K. Toth, and J. Havas, Acta Chim. Acad. Sci. Hung. Tomus, 48, 19 (1966).
Determination of Selenium(1V) in the Presence of Selenium(V1) Using Sulfur Dioxide Loring R. Williams and Phillip R. Haskettl Chemistry Department, University of Nevada, Reno, N e v . 89507
THEFACT that sulfur dioxide readily reduces selenium(1V) to elemental selenium in acid solution has been recognized as the basis for the detection and determination of selenium (IV); however, investigators have not been in agreement as to whether selenium(V1) is reduced by sulfur dioxide in acid solution. Gilbertson and King ( I ) used the reduction of selenium(1V) by sulfur dioxide as a qualitative test to determine if the oxidation of selenous acid to selenic acid was complete but Caley and Henderson (2) reported that selenium(V1) was reduced by this reagent in a sulfuric acid solution and stated that the sulfur dioxide test was unsuitable for the detection of selenous acid in selenic acid. It seemed possible that the lack of agreement between the results of previous investigations may have been caused by
the presence of small amounts of selenium(1V) in the samples of selenic acid used. Polarography offered an excellent means of detecting small amounts of selenium(1V). An extensive investigation of the various oxidation states of selenium over a wide range of pH in a variety of supporting electrolytes was conducted by Lingane and Niedrach (3) using polarographic techniques. They reported that the diffusion current was directly proportional to the concentration of selenium(1V) present and also that no polarographic wave was obtained for selenium(V1). In more recent studies, Christian, Knoblock, and Purdy (4-6) reported that selenium(1V) can be determined polarographically at concentrations as low as 10-6 M which is about 79 ppm. The purpose of the present investigation was to examine
1 Present address, United States Bureau of Mines, Reno, Nev. 89505
(3) J. J. Lingane and L. W. Niedrach. J . Amer. Chem. SOC.,70,
(1) L. I. Gilbertson and G. B. King, J. Amer. Chem. SOC.,58, 180 (1959). (2) E. R. Caley and C. L. Henderson, ANAL.CHEM.,32,975 (1960). 1138
ANALYTICAL CHEMISTRY
4115, (1948). (4) G. D. Christian, E. C. Knoblock, and W. C. Purdy, ANAL. CHEM.,35, 1128 (1963). ( 5 ) Zbid., 37, 425 (1965). (6) G. D. Christian, E. C. Knoblock, and W. C. Purdy, J. Assoc. Ofic. Agr. Chemists, 48, 877 (1965).
starting material and the oxidation was carried out in a basic solution, the polarogram showed no wave for selenium(1V) in the calcium selenate. Reduction of Selenium(1V) by Sulfur Dioxide. It was found that the reduction of selenium(1V) can be accomplished quantitatively in 6N hydrochloric acid, 3N sulfuric acid, and 3N perchloric acid using sulfur dioxide which was generated internally from sodium sulfite or added as a gas. The results as summarized in Table I indicate that the most consistent results are obtained when perchloric acid is used. The average deviation of the mean of the averages is about 1 ppt which is well within the generally accepted limits for good precision. The slightly higher values obtained when hydrochloric and sulfuric acids were used maj’ have been the result of occlusion because the precipitates in perchloric acid solutions were more finely divided.
The polarograms from the filtrates from each of the determinations showed no selenium(1V) wave, which indicated that no selenium was present in amounts larger than 79 ppm. The results from the atomic absorption determination are recorded in Table I1 and also indicate a quantitative reduction of selenium(1V) by sulfur dioxide. Determination of Selenium(1V) in the Presence of Selenium (VI). When perchloric acid was used as the acid medium, an average of 41.05 0.01% of selenium was obtained as compared to an average of 41.04 f 0.03% when no calcium selenate was present. The polarograms showed no selenium(1V) wave, indicating a quantitative separation of selenium(1V) in the presence of selenium(V1).
*
RECEIVED for review December 23, 1968. Accepted March 20, 1969.
Spectrophotometric Determination of Hafnium as Reduced Molybdosulfatohafnic Acid C. C. Clowers, Jr., and J. C. Guyon Department of Chemistry, University of Missouri, Columbia, Mo. 65201 SEVERAL chromogenic reagents have been utilized in spectrophotometric determinations of hafnium (1-10). The zirconium work of Grosscup ( I ] ) , Liberti (12), and Shakova (13), and studies by Dehne (14) indicated the possibility of formation of a sulfato heteropoly and species involving hafnium. The purpose of this work was two-fold: to furnish qualitative existence of a simple heteropoly molybdohafnic acid or a related species, and to develop a quantitative spectrophotometric method for the determination of hafnium based on some mechanism of formation of its heteropoly molybdate. EXPERIMENTAL
Apparatus. All spectrophotometric measurements were made using a Cary Model 12 automatic recording spectrophotometer with 1.000 + 0.002-cm fused quartz cells. All pH measurements were made using either a Beckman Zeromatic or Beckman Expanded-Scale direct-reading pH meter. (1) Y . Hoshino, Nippoii Kagakir Zasshi, 80, 738 (1959). (2) K. N. Bagdasarov, Ref. Zh, Khim., 1963, Abstract No. 7G82. (3) L. I. Kononenko and N. S. Poluektov, Zacodsk. Lab., 28, 794 ( 1962). (4) A. D. Horton, ANAL.CHEM., 25,1331 (1953). ( 5 ) K. L. Cheng, Talanta, 2, 81 (1959). (6) G. Banerjee, Anal. Chim. Acta, 16, 62 (1957). (7) F. R. Sheyanova and V. L. Ganina, Trudy p o Khimii i Khim. Tekhnol., 3, 101 (1960). (8) K. L. Cheng, Anal. Chim. Acta, 28,41 (1963). (9) S. D. Biswas, Talarita, 12, 119 (1965). (10) K. Pan, A. Lin, S. Lin, Y . Wu, and E. Chen, J. Chinese Chem. SOC.,10, 24 (1963). (11) C. G. Grosscup, J. Amer. Chem. SOC.,52, 5154 (1930). (12) A. Liberti, La Ricerca Sei., 25, 880 (1955). (13) Z. F. Shakova, Zh. Neorg. Khim., 6, 330 (1961). (14) G. C. Dehne with M. G. Mellon, ANAL.CHEM.,35, 1382 (1963).
1140
ANALYTICAL CHEMISTRY
Temperature was controlled with a water bath in conjunction with a Sargent Model 3554 thermoregulating unit. Reagents. Stock solutions of ammonium heptamolybdate, approximately 5 % (w/v), Na2S04, approximately 10% (w/v), and chlorostannous acid, 0.025M, were used. A stock hafnium(1V) solution was prepared from HfO(NO& and standardized by precipitation of the hydrous oxide with ignition to HfOn. Recommended Procedure. PREPARATION OF CALIBRATION CURVE. Transfer 0.0, 0.5, 1.0, 2.0, 3.0, and 5.0 ml of a standard solution containing approximately 1.O mg/ml of hafnium to 150-ml beakers. Add 4.0 ml of a 10% (w/v) solution of NazS04,adjust the volume to approximately 35 ml with water and add 10 ml of a 5 (w/v) solution of ammonium heptamolybdate. Adjust the pH in each beaker to 1.3, transfer the solutions to 100-ml volumetric flasks and place the flasks in a water bath (45 “C f 0.5) for 90 minutes. Add, by means of hypodermic syringes, 5.0 ml of 1:1 sulfuric acid and, exactly 10 seconds later, 3.0 ml of 0.025Mchlorostannous acid to the solutions while still warm. Dilute to the mark, mix, and read the absorbance at 725 mp exactly 20 minutes after addition of reductant, using distilled water as the reference. Construct a calibration curve of absorbance cs. concentration. GENERAL PROCEDURE. Dissolve the sample to be analyzed for hafnium and treat the resulting solution to remove interfering ions. Concentrate the resulting solution to about 30 ml. Adjust the amount of sulfate present to between 225 and 475 mg by adding sodium sulfate solution. Add 10 ml of 5 (w/v) ammonium heptamolybdate solution and proceed as outlined under “preparation of calibration curve,” beginning with the pH adjustment step. Effect of Experimental Variables. Preliminary studies indicated that reduction of the heteropoly species would be necessary if the method were to have sufficient sensitivity. The absorption spectrum of the reduced species showed a maximum at 725 mp, and all measurements were made at this wavelength.
