Study of Various Inorganic Phosphate Salts Impregnated in Silicone Rubber as Potential Indicating Electrodes for Phosphate Ion G. G. Guilbault and P. J. Brignac, Jr. Department of Chemistry, Louisiana State University in New Orleans, Lakefront Campus, New Orleans, La. 70122
THEBODY of literature on selective ion electrodes has become very large within the last few years. An excellent literature survey reasonably complete through July 1968 was published recently (1). A survey of the field of ion selective electrodes was provided by Rechnitz (2). The theory and application of anion selective membrane electrodes was published by Pungor (3). Highly useful and selective electrodes for I-, C1-, and Br- have been developed by the use of a precipitate impregnated membrane electrode using silicone rubber as the inert matrix (4-9). Sulfate and phosphate sensitive membranes prepared by Pungor, were studied by Rechnitz, Lin, and Zamochnick (IO). The sulfate electrode consisted of fine BaS04 in silicone rubber. The phosphate electrode consisted of BiP04 in silicone rubber. Neither the sulfate nor the phosphate showed remarkable selectivity to the ion of primary interest. The phosphate electrode had potential drifts even after extensive soaking of 1 mV per 10 min. Crude calibration plots were obtained for phosphate in the range of to 10-4M (IO). Buchanan and Seago have studied impregnated silicone rubber membranes consisting of Co3(P04)2for their response to transition metal cations. These electrodes were not selective to the cobalt(I1) ion. The crystalline form, the degree of hydration, and the associated anion of the embedded material had no pronounced effect on the membranes response. It was concluded that these membranes are not specific in their response to any one kind of cation ( I I ) . In an attempt to construct a very selective phosphate electrode, a study was undertaken using a host of inorganic salts embedded in silicone rubber. The salts tried were: A1P04,CrP04,FePO4, co3(P04)2, LaP04, Co(HP04),Co(en)3PO4, and Mg2P207. Also, an ion exchange material, Dowex 1-X8, was placed in silicone rubber and converted to the phosphate form. The results seem to indicate that selectivity could not be achieved using any of the salts tried. However, the potential drifts were not so pronounced as in the case of the BiP04 electrode studied by Rechnitz (IO). Rapid re(1) T. S. Light and J. L. Swartz, “Selective Ion Electrodes, Litera-
ture Survey,” Foxboro Company Research Center, Foxboro, Mass. 02035 (2) G. A. Rechnitz, Chem. Eng. News, 45 (25), 146 (1967). 39 (13), 29A (1967). (3) E. Pungor, ANAL.CHEM., (4) G. A. Rechnitz, M. R. Kresz, and S. B. Zamochnick, ANAL. CHEM., 38,973 (1966). (5) E. Pungor, J. Havas, and K. Toth, 2.Chem. 5 , 9 (1965). (6) E. Pungor, J. Havas, and G. Madarasz, Fr. Patent No. 1,402,343 (1965). (7) E. Pungor, J. Havas, and K. Toth, Acta. Chim. Hung. Tomus, 41,239 (1964). (8) E. Pungor and J. Havas, Acta. Chim. Acad. Sci. Hung., 50, 77 (1966). (9) G. A. Rechnitz and M. R. Kresz, ANAL.CHEM., 38,1786 (1966). (10) G. A. Rechnitz, Z.F. Lin, and S. B. Zamochnick, Anal. Lett., 1,29 (1967). (11) E. B. Buchanan, Jr. and J. L. Seago, ANAL.CHEM.,40, 517 (1968). 1136
ANALYTICAL CHEMISTRY
-
STIRRER
Figure 1. Membrane cell assembly
sponses of less than 1 minute were observed in the cases studied, and a Nernstian response was obtained over the concentration range of lo-’ to 10-5M. EXPERIMENTAL
Apparatus. A Keithley 610-B multirange electrometer coupled with a Sargent Recorder Model SRG were used in all measurements. The temperature was held at room temperature, about 23 “C. The membrane cell assembly is shown in Figure 1. Reagents. c03(Po4)2‘8H20,FeP04.H20,AlP04.H 2 0 ,and CrP04.4Hz0were purchased from Alfa Inorganics. LaP04. HzO was prepared by adding excess Hap04 to a solution of La(N03)3. Mg,P207 was prepared by adding excess Na4P207 . Hprepared ~O by to a solution of MgC12. C O ( ~ ~ ) ~ P O ~was taking a solution of [ C ~ ( e n ) ~ ] Cand l ~ adding excess KzHP04(12). C ~ ( e n ) ~ Cwas l ~ prepared by a method listed in Inorganic Syntheses (13). CoHPO4.H2Owas prepared by adding disodium hydrogen phosphate to a solution of Co(I1) ion (14). Silicone rubber, (Dow Corning) Silastic clear sealer was used in all studies. Anion exchange resin, Dowex 1-X8, was used. Membrane Preparation. All of the inorganic salts were soaked in excess KHZPO4for 2 hours, were filtered, then dried at 110 “C for 2 hours. The salts were then ground very fine with a mortar and pestle. A 50% mixture by weight of the inorganic salt to silicone rubber was then mixed to an even consistency. (A 50% mixture was used in all studies since this mixture has been shown by Pungor (15) to be best for good particle contact throughout the membrane). The mixture was placed between two sheets of glassine paper and pressed between two pieces of wood to give a thin sheet. The mixture was allowed to polymerize overnight. The sheets
(12) H. W. McCune and G. J. Arquette, ANAL.CHEM.,27, 401 (1955). (13) J. B. Work, “Inorganic Synthesis,” Vol. 11, McGraw-Hill, New York, 1946, p 221. (14) J. Dale and Charles Banks, “Treatise on Analytical Chemistry,” Vol. 11, Interscience Publishers, New York, 1965, p 321. (15) E. Pungor, K. Toth, and J. Havas, “Membrane Electrodes in Chemical Analysis,” Paper presented by Prof. E. Pungor at the Achema, Frankfurt (Main), 1964, published by Hungarian
Scientific Instruments.
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Figure 2. Response of electrodes containing inorganic phosphates in silicone rubber. Reference solution 1 X 10-3M NaH2P04 Slope = 32 mV/p [HzPOP-] Slope = 39 mV/p [HzPO~-] Slope = 35 mV/p [HzPOd-]
A . AIPOu B. CrPO4. C. FePOc
(MOLARITY 1 Figure 4. An electrode containing FePO, in silicone rubber (response to other anions). Reference solution 1 X lO-'MKHzPOi A . NaCl; E. KNOj
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Pk2~po4I Figure 3. Response of electrodes containing inorganic salts in silicone rubber. Reference solution 1 X 10-IM KHzPOi A . CO~(PO&. B. MgzP207. C . LaPOa.
D. Co(HPO4). E, Dowex 1-XS.
F. C0(en)~P0~.
Slope = 17 mV/p [HP04$-] Slope = 20 mV/p [HP042-] Slope = 24 mV/p [HPOa2-] Slope = 33 mV/p [HP042-1 Slope = 36 mV/p [HPO4*-I Slope = 45 mV/p [HP042-]
B. NaC104; C. K a P O 4 ; D. KH2P04;
of glassine paper were then removed, the polymerized membrane was cut into a small circle, and was glued to a glass tube by means of silicone rubber (Figure 1). The glass tube was then filled with an internal reference solution, either 0.1M KzHP04 or KHzP04. The electrode was completed by addition of a standard calomel electrode. The cell potential was taken as the potential difference between two matched SCE. The SCE in the reference compartment was connected to the negative terminal and the SCE in the test solution was connected to the positive terminal. The membranes were allowed to soak in 0.1M K2HP04 for about a week before evaluation. The anion exchange material, Dowex 1-X8 was finely ground with mortar and pestle, and a membrane was constructed as above. The resin was converted to the phosphate form by soaking in 0.1M K2HPO4. Initially, the membranes were soaked in 0.1M KzHP04 raised to a pH of 13.0 with NaOH. After soaking the electrodes, take for example FeP04, the precipitate membrane turned a dark brown, indicating the formation of ferric hydroxide. Therefore, it was decided to soak the electrodes in 0.1MKzHP04(pH 9) to prevent this hydroxide formation. The resistances of these membranes was measured using a Leeds and Northrup conductivity bridge. The resistances of the membranes ranged from 1300 to 10,000 fi, Membranes of pure silicone rubber were found to have an extremely high resistance (over 11,000 M a). RESULTS AND DISCUSSION Response Characteristics. The responses of three silicone rubber electrodes impregnated with A1P04, CrP04, and FeP04 to (HzP04-) are shown in Figure 2. The reference solution was 1 X lO+M NaHzP04. The results of varying KZHPO4 with silicone rubber membrane electrodes impregnated with C O ~ ( P O 8H20, ~ ) ~ + Mg,P,O7, LaP04, CoVOL. 41, NO. 8,JULY 1969
0
1137
(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).
