ering the swelling of the surface of the membrane and the easiness of ion-exchange process on it, the soaking solution had better contain lead ion. When the surface of the membrane was polished or cut off to get rid of the thin layer of silicone rubber and the membrane was soaked in the test solution, many air bubbles adsorbed on its surface and the measurement of the potentials was impossible. The potentials of the polished membrane were unstable even after it was soaked for a long time. When a small amount of silicone oil was applied to the surface of the polished membrane, the air bubble did not adsorb in the test solution and the potentials of the membrane became stable and steady. These results mean that the surface of the membrane should be very smooth and that the intimate contact be-
tween it and the solution is indispensable. In order not to contaminate the test solution or weaken the membrane, mechanicalmethod for exposing the particles ofPbS on the surface should not be applied.
ACKNOWLEDGMENT The authors thank S. Kisaka and K . Sugihara for their encouragement in this work. Thanks are also due to Y. Hioki and H. Yamao for their X-ray and electron microscopic measurements. RECEIVED for review July 13, 1970. Accepted October 22, 1970.
Photometric Titration of Selenium(lV) with Permanganate in Sulfuric Acid Using Condensed Phosphoric Acid as Accelerator P. P. Naidu' and G . G . Rao Department of Chemistry, Andhra University, Waltair (A.P.) India VOLUMETRIC METHODS for the determination of selenium(IV) are based chiefly on oxidation-reduction reactions. In some procedures (1-5), selenium(1V) has been determined by adding a known amount of reductant and back-titrating the excess after filtering off the selenium metal. In other procedures (6-11), a known amount of oxidant is added and the excess determined. As no one so far suggested conditions under which a direct titration of selenium(1V) could be made, we have undertaken the study reported here. Since the intense violet color of the phosphate complex of manganese(II1) precludes visual detection of the end point, a potentiometric method of locating the end point was tried, in which calomel and platinum were used as reference and indicator electrodes, respectively. But, it was observed that it takes a very long time for the establishment of equilibrium potentials even at the beginning of the titration. Moreover, the break in potential at the equivalence point is small, about 1C-20 mV. Hence, we used the spectro photometric detection of the equivalence point. EXPERIMENTAL
Apparatus. A Hilger Uvispek Spectrophotometer with 1-cm cell and Klett-Summerson Photoelectric Colorimeter with 2 X 4 x 8 cm optical cell with a green filter were used. Author to whom correspondence should be sent at Central Chemical Laboratories, Airborne Mineral Surveys & Exploration, N.M.D.C. Buildings, Faridabad (Haryana State), India ~
~~
(1) G. S. Deshmukh and B. R. Sant, Analysr, 77,272 (1952). (2) W. Strecker and L. Schartow, Z. Anal. Chem., 64, 218 (1924). (3) J. H. Van dar Meulen, Chem. Weekbl., 31, 333 (1934). (4) W. C. Coleman and C. H. McCrosky, IND.ENG.CHEM.,ANAL. ED., 9, 431 (1937). ( 5 ) H. H. Willard and G. D. Manalo, ibid., 19, 167 (1947). (6) D. F. Adarns and G. S. Gilbertson, ibid., 14,926 (1942). (7) I. M. Issa and R . M. Issa, Anal. Chim. Acta, 13, 323 (1955). (8) I. M. Issa, S . A . Eidand, and R. M. Issa, ibid., 11, 275 (1954). (9) R. Starnrn and M. Goehring, Z . Anal. Chem., 120,230 (1940). (10) I. M. Issa and M. Harndy, ibid., 172, 162 (1960). (11) W. T. Schrenk and B. L. Browning, J. Amer. Chem. Soc., 48, 2550 (1926).
Reagents. CONDENSED PHOSPHORIC ACID. To 100 ml of syrupy ortho-phosphoric acid in a 250-ml borosilicate glass beaker, add 2 to 3 ml of 1 :1 nitric acid and heat until fumes of nitrogen peroxide are no longer evolved. POTASSIUM PERMANGANATE. This was prepared 0.005M and standardized against sodium oxalate. SELENIUM(IV).A 0.02M solution is prepared by dissolving sodium selenite or selenium dioxide in water and standardized by the method of McCullough et al. (12). From the spectra of the ions concerned @e4+,Se6+, Mn7+, and Mna+),it was concluded that 525 mp (green filter, maximum transmission at 530 mp) is an appropriate wavelength for the titration. Recommended Procedure. Add 1 to 10 ml of selenium(1V) solution, containing 3 to 30 mg of selenium, to an optical cell (2 x 4 x 8 cm), and add 10 to 12 ml of 20N sulfuric acid and 1.5 to 3 ml of condensed phosphoric acid. Dilute to 40 ml with distilled water. Arrange an inlet tube so that it is immersed in the solution at one corner of the cell, out of the light path. Pass carbon dioxide during the addition of permanganate solution, for mixing, and stop the passage of gas before taking a reading of the absorbance. Titrate photometrically with permanganate at 530 mp (green filter) and take readings 2 min after each addition of permanganate before equivalence point, 6 to 8 min after addition, at equivalence point, and 1 min after the end point as shown by the sudden increase in absorbance. Upon drawing a graph of absorbance US. volume of titrant, we will get two straight lines on extrapolation, and they well meet at single point which corresponds to the end point. Some typical results obtained by this procedure are given in Table I. RESULTS AND DISCUSSION
Rate of Reaction. The oxidation of selenium(1V) with permanganate is slow in either sulfuric acid or condensed phosphoric acid alone. But in a mixture of these two acids, the oxidation proceeds smoothly and completely within a short time (2 min at the start and 6 8 min at the equivalence point). Further, it has been observed that the rate of oxida(12) M. D. McCullough, T. W. Campbell, and N. J. Krilanovisch, IND.ENG.CHEM., ANAL.ED., 18,638 (1946).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
281
Taken 3.03 8.58 12.21 15.31 19.07 21.98 24.75 31.75
Table I. Determination of Selenium Amount of Selenium(1V). mp. Found Average 3.05, 3.06, 3.05 3.05 8.62 8.60, 8.66, 8.60 12.20, 12.20, 12.20 12.20 15.29 15.35, 15.36, 15.26 19.16 19.21, 19.06, 19.19 21.88 21.88, 21.88, 21.89 24.75 24.68, 24.83, 24.74 31.97 32.00, 31.98, 31.90
Table IV. Interference Study Amount Selenium Addenda added, mg found, mg ... ..* 3.03 Fe(111) 150 3.03 122 3.03 Mo(V1) 100 3.02 80 3.03 UWI) Cu(I1) 115 3.03 Ni(I1) 125 3.04 Te(V1) 200 3.03 Se(V1) 196 3.03 Ta(II1) 58 3.03 112 3.03 Sb(V) Ce(IV) 185 3.03 58 3.03 Hg(I1) Co(I1) 106 3.03 66 3.03 V(V) Al(II1) 92 3.03 250 3.03 WI) 250 3.03 &I) 68 3.03 MOW) Nitrate 165 3.03 Perchlorate 86 3.03 Sulfate 162 3.03
Table 11. Rate of Reaction at Equivalence Point A -B Effect of sulfuric acid (concn of Effect of condensed phosphoric condensed phosphoric acid acid (concn of sulfuric acid fixed, 1 M ) fixed, 3 M ) Time taken Time taken Concn for complete for complete M oxidation, min Concn oxidation, min 1.o 125 0.2 10 1.5 75 0.4 10 2.0 25 0.6 9 2.5 15 0.8 9 3.0 10 1.0 8 3.5 9 2.0 8 4.0 8 3.0 8 ~~
Table 111. Acceleration of Reaction by Condensed Phosphoric Acid at Every Percentage Oxidation of Selenium Time taken for the corresponding VOl. of of Selenium selenium permanganate oxidized, min added, ml Conditionsa oxidized, 0.4 A 20 5
x
x
0.8 1.2
B
20
1
A B A
40 40 60 60 80 80 100 100
8 2 10 3
B
1.6 2.0
A B A B
"A. 3M Sulfuric acid. B. 3M Sulfuric acid
25 5
40 8
+ 1.OM condensed phosphoric acid.
tion is strongly dependent on the sulfuric acid concentration but not affected by varying the condensed phosphoric acid concentration. Results given in Tables I1 A and I1 B give a correct knowledge of these observations. Since the reaction is complete in a short period only when condensed phosphoric acid is present, but does not depend upon its concentration, this acid may be considered an accelerator in the reaction, Mnl+ 2 Se4+ -+ Mn3+ 2 See+. As long as manganese(II1) is formed, it is taken out from the
+
282
+
+
+
+
Fe(II1) V(V) Cr(II1) A W M O W ) V(VI) Cu(I1) Se(V1) Te(V1) Os(VII1) Ta(II1) Ce(IV) NaU) K(I) Mg(II) Al(II1) NOa S01'C101-
++ + ++ + + + + ++ ++ ++
624
3.05
reaction sphere by condensed phosphoric acid as stable Mn (111)-phosphate complex, thus shifting the reaction toward the right. The values in Tables I1 A and I1 B apply only a t the equivalence point. For the clear visualization of the function of condensed phosphoric acid, experiments were carried out in the presence of condensed phosphoric acid and its absence. The data obtained are given in Table 111. From these data, one finds that the titrations in sulfuric acid plus condensed phosphoric acid are more rapid than in sulfuric acid alone; and evidently condensed phosphoric acid accelerates the recation even in the initial stage of selenium oxidation. Interferences. The usual selenium associated ions, iron(III), arsenic(V), molybdenum(VI), uranium(VI), copper(II), nickel (11), tellurium(VI), tin(IV), selenium(VI), thallium(III), antimony(V), vanadium(V), cerium(IV), cobalt(II), mercury (II), sodium(I), potassium(I), magnesium(II), calcium(II), aluminium(III), nitrates, sulfates, and perchlorates d o not interfere as can be seen from Table I V while most of these ions will interfere in earlier methods.
RECEIVED for review May 4, 1970. Accepted September 18, 1970. Financial assistance to one of us (Dr. P.P. Naidu) from Council of Scientific and Industrial Research (India) is gratefully acknowledged.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
