Lead Sulfide-ImpregnatedSilicone Rubber Membranes as Selective Electrodes for Lead Ion Hiroshi Hirata and Kenji Date Wireless Research Laboratory, Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan
MANYEFFORTS have been made to develop the precipitateimpregnated membranes as selective electrodes (I). Pungor (2) successfully developed silicone rubber membranes as selective electrodes for chloride, bromide, iodide, and sulfide ions. Rechnitz ( 3 ) summarized the responses of Pungor’s electrodes toward anions. As for cation selective electrodes, various kinds of precipitate-impregnated membranes were prepared ( I ) . However, they did not exhibit the desired response, stability of potentials, analytical range, or selectivity for the cations concerned. Copper(1) sulfide-impregnated silicone rubber membranes have recently been developed as selective electrodes for cupric ions in our laboratory (4). The internal electrode and solution which are commonly used in selective membrane electrodes have been eliminated by attaching the membrane to the surface of a metal directly. The membrane had to be soaked in the solution of CuSOl for a week to develop the stable potentials. The present paper describes the development of selective electrodes for lead ions using lead sulfide-impregnated silicone rubber membranes, which have the same construction as described above. It also presents the investigation of the soaking effect of the membranes before the measurement. EXPERIMENTAL Apparatus and Reagents. Hitachi-Horiba Model F-5 pH meter was used to make the potential measurements. A double-junction calomel electrode, Horiba 2530-05T, was used as a reference electrode. Silicone rubber was obtained from Shinetsu Kagaku Co., and all the reagents were Nakarai Chemical reagent grade. Preparation of PbS-Impregnated Silicone Rubber Membrane Electrode. A fine powder of PbS was obtained by heating a mixture of lead powder and sulfur in molar ratio of 1 :1 at 500 “C for 2 hours in an atmosphere of H2S and grinding the heated mixture to a particle size of less than 10 pm. The powdered PbS was mixed with 25% w/w of silicone rubber. A platinum plate (10-mm diameter) or wire (1-mm diameter) which directly connected with a leading wire was coated with the mixture to a thickness of about 0.5 mm. After being polymerized, the membrane was held in a plastic holder filled with epoxy resin. Figure 1 is an example of the cross section of the membrane electrode. The membrane electrode was then soaked in 10--2Msolution of Pb(N0& at 25 “C for a week. Measurement of Potentials. The potentials developed by the membrane electrode in the test solution were measured ES. SCE as the reference electrode at 25.0 f 0.1 “C as shown in Figure 1. RESULTS AND DISCUSSION Lead sulfide is a semiconducting polar crystal with highelectron conductivity at room temperature (5), and Ag2S is (1) R. A. Durst, “Ion-Selective Electrodes,” N.B.S. Monograph 314, U.S. Govt. Printing Office, Washington, 1969. (2) E. Pungor, ANAL.CHEM., 39 (13), 28A (1967). (3) G. A. Rechnitz, Chem. Eng. News, 45 (25), 146 (1967). (4) H. Hirata and K. Date, Tulunfu, 17, 883 (1970). ( 5 ) R. L. Petritz and W. W. Scanlon, Phys. Rev., 97, 1620 (1955).
. Figure 1. Cross section of PbS-impregnated silicone rubber membrane electrode
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A. PbS-impregnated silicone rubber membrane, B. Platinum plate, C. Shielded wire, D . Epoxy resin, E. Plastic holder, F. SCE, G. Test solution, H. pH meter
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Figure 2. Potential response of PbSimpregnated silicone rubber membrane electrode in the solution of Pb(NO& added to provide ionic conductivity by silver ions in the usual selective electrode for lead ions (9. Nevertheless, PbS-impregnated silicone rubber membrane responds to lead ions and shows good selectivity as will be described below. The powdered PbS was analyzed and impurities were found as traces. Copper(I1) sulfide-impregnated silicone rubber membrane ( 4 ) and HgS-impregnated polyvinyl acetate membrane (7) selectively respond toward cupric and mercuric ions, respectively, though CuS and HgS are the crystals with highe!ectron conductivity (8, 9). However, it may be premature to decide from the above results whether PbS-impregnated silicone rubber membrane electrode operates by electronic (6) G. A. Rechnitz, ANAL.CHEM., 41 (12), 109A (1969). (7) S. Araki, S. Suzuki, and Y. Tomita, Symposium on Analytical Chemistry, Fukuoka, Aug. 1970, No. 2B09. (8) H. Devaux and J. Cayrel, Compt. Rend., 198, 1339 (1934). (9) A. G. Mikolaichuk, L. I. Ivankiv, and R. S. Velichko, Visa L’viv. Derzh. Univ., Ser. Fiz., 1965, 74; Chem. Abstr., 66, 23205x (1967).
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Figure 3. Electron micrhscopic photographs of the surfaces of PbS(A) 10-lM solution of Pb(N03)%,and (B) pure water, and (0usoak-- ,,,____,
conductivity, or by ionic conductivity provided by traceable impurities or the lattice defects in the crystal. Potential Response of PbS-Impregnated Silicone Rubber Membrane Electrode. .Figure 2 shows the potential response of PhS-impregnated silicone rubber membrane electrode in the solution of Ph(NO&. The activity was derived from the concentration by means of the ion activity coefficients tabulated by Kielland (IO). The slope was 29.0 mV in the concentration range from lo-' to 10-5 M of lead ion and the electrode can he used to determine lead ion from 10-1 to 10-6 M. The response time of the measuring cell was less than 2 minutes. Inffuence of Diverse Ions. K+, Na+, Mg", Ca", AI8+, Cr3+, Znz+, Few, Co", Ni*, NH,+, NO3-, C104-, and CHCOO- could he tolerated and their selectivity ratios were 103 or more, where the selectivity ratio is defined as the ratio of tolerable concentration of diverse ion to the concentration of lead ion. More than l e aM of C1-, Br-, I-, or Cre+, Fe3+, and Mn7+interfered, because halides precipitated with lead ion, and other ions developed reduction-oxidation potentials. Cu'f, Ag+, and Hg2+could not coexist, because they deposited on the surface of the membrane as the solubility products of PbS are larger than those of CuS, AgS, and HgS. SOn* could not coexist as it precipitated as PbS04. Influence of pH. The potentials of PbS-impregnated silicone rubber membrane electrode were constant in pH range from 2.8 to 7. They increased with decrease of pH at pH below 2.8 because of the dissolution of PbS, and they decreased sharply with increase of pH at pH above l because of the precipitation of Ph(OH)*. Effect of Temperature. Lead sulfide-impregnated silicone rubber membrane electrode could be used in the temperature range from 10 to 70 OC with the correction on temperature coefficient. Soaking Effett. Just like CwS-impregnated membrane (4), PhS-impregnated silicone rubber membrane did not develop the stable potentials just after the preparation. The membrane with 15% wiw of silicone rubher had to he soaked (10) J. Kielland, J. Amer. Chem. SOC.,59,1675 (1937). 280
in 10-2Msolution of lead ion for 4 days to develop the stable potentials. The larger the portion of silicone rubber was, the longer the required soaking time. The potentials were unstable and could not be measured when silicone rubber was more than 50%. Considering the adhesive force of silicone rubber, the optimum portion was 25%, and the preferable soaking time was a week in 10-2M solution of Pb(NO& at 25 OC. Once the membrane was soaked, the potentials were stable and steady even if it was exposed to air for a month. When the temperature of the soaking solution [10-2 M Pb(NO& was raised from 25 to 70 OC, the soaking time required for obtaining the stable potentials developed by PbSimpregnated silicone rubber (25 %) membrane decreased from 6 days to less than 3 days. The less the concentration of Ph(NO& solution was, the longer the required soaking time was, though the difference of the time between 10-1 and l W 4 M was only 1 day. When the soaking solution was converted from Pb(NO& solution to buffer solution which had the same pH value as each Ph(NO& solution, the potential variation with the soaking time did not change so much. When the membrane with-25% of silicone rubber was soaked in pure water or 1(F'M solution of KN03,the required soaking time was 8 days. surfaces of PhS-impregnated silicone rubber (25 Z) membranes which were soaked in 10-1 M solution of Ph(NO& for 8 days and in pure water for 8 days, and that of unsoaked membrane. The exposed species on the surfaces of soaked membranes are supposed to be PbS by X-ray microanalysis. The particles of PbS seem to be in contact with each other within the membrane with less than 50% of silicone rubber from electron microscopy. The following considerations are derived from the above results: the surface of the membrane is almost covered with a thin layer of silicone rubber during the polymerization. When the membrane is soaked in aqueous solution, the thin layer is gradually etched, and the particles of PbS which are enough to develop the stable potentials by ion-exchange process are exposed. The speed of the etching depends on the temperature and pH value of the soaking solution. Consid-
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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 ~
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(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).
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