Continuous Variations Curves and Complex Stoichiometry SIR: The recent paper by Likussar and Boltz ( I ) makes a valuable contribution to the treatment of formation constants and solvent extraction processes in a particularly clear and readable manner. It does seem proper to point out that one piece of earlier work by Klausen and Langmyhr (2), omitted from the references given by Likussar and Boltz, reports the computer production of general theoretical continuous variations curves for several stoichiometries. The plots there presented agree in shape both with those shown for restricted cases in earlier reports by Asmus (3) and Klausen and Langmyhr ( 4 ) , and with the curves now reported by Likussar and Boltz for a somewhat different formulation of the general case. (1) W. Likussar and D. F. Boltz, ANAL.CHEM., 43,1265 (1971). (2) K. S. Klausen and F. J. Langmyhr, A m / . Chim.Acta, 40, 167 (1968). (3) E. Asmus,Z. A m / . Chem., 183, 321,401 (1961). (4) K. S. Klausen and F. J. Langmyhr, A m / . Chim. Acta, 28, 335 (1963).
When portions of the curves for a given stoichiometry, but differing overall concentration or stability constants, are concave upwards near one end, the omitted reference shows an interesting property of the points at which lines from that end are tangent to the curves. In fact, such points lie on a straight line parallel to the limiting terminal tangent at the other end of the family of curves as the stability constant goes to infinity. The intercept of the straight line thus obtained on the composition axis for a general stoichiometry of A,B, is x = (n - l)/(rn n - 1). The same process applied to the curves presented in Reference ( I ) leads to similar results.
+
G . F. ATKINSON Department of Chemistry University of Waterloo Waterloo, Ontario, Canada RECEIVED for review October 22,1971. Accepted February 8, 1972.
Sulfate Ion-Selective Membrane Electrode SIR: Efforts to prepare useful sulfate ion-selective membrane electrodes have been largely unsuccessful. The work of Hirsch-Ayalon ( I ) on BaS04-cellophane membranes, of Pungor and coworkers (2) on precipitate impregnated silicone rubber membranes, and our own work (3) on the Pungor electrodes showed that it is relatively easy to achieve potentiometric response to the sulfate ion but extremely difficult to obtain potentiometric selecriciry for sulfate. Recent progress on crystal membrane electrodes ( 4 ) encouraged us to investigate pressed membranes of various crystal mixtures, and we are now able to report on a new electrode with good response and selectivity for the sulfate ion. The electrode is relatively easy to prepare in the laboratory and has properties which suggest that it should be useful for analytical purposes. EXPERIMENTAL Various inorganic salt mixtures, all based upon the use of AgzS as a membrane matrix (4, were pressed into membranes and evaluated for sulfate response. Typically, finely divided powders of the constituents were carefully mixed in known ratios and then pressed into membranes with a laboratory pellet press under controlled conditions of applied pressure, temperature, and pressing time. The resulting crystal membranes were then sealed into glass or plastic electrode bodies and internally connnected to a silver contact wire. Great care was taken to prevent surface contamination of the membranes. (1) P. Hirsch-Ayalon, Electroc/iini. Acta, 10, 773 (1965). (2) E. Pungor and J. Havas, Acta Chim. Hung., 50, 77 (1966) and
references quoted therein. (3) G. A. Rechnitz, Z. F. Lin, and S. B. Zamochnick, A m l . Lett., 1, 29 (1967). (4) J. W. Ross, Jr., in “Ion-Selective Electrodes,” R. Durst, Ed., National Eureau of Standards Special Publication 314, U.S. Government Printing Office, Washington, D.C., 1969, p 57f. 1098
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Although a number of compositions gave membranes with sulfate response and greater or lesser sulfate selectivities, optimum results were obtained with membranes consisting PbS, 32 mole PbS04, and of 32 mole % Ag& 31 mole 5 mole Cu2S. The most successful membranes were formed by pressing with an applied pressure of 102,000 pounds per square inch for 18 hours at temperatures of up to 300 “C. The elevated temperature appears to be the critical variable and may be related to the fact that Ag2S undergoes a phase transition at 178 “C. The small amount of Cu2S in the pellet appears to improve the response time of the electrode membrane ( 5 ) and may be related t o the fact that Cu2S has semiconductor properties. Conditioning of the electrode surface appears to be important in giving optimum performance. We alternately soaked the electrode in dilute solutions of NazSOI and in mixtures of AgN03 and Pb(N03)? for periods of several hours. Occasionally, after prolonged experimentation under severe conditions, the shiny surface of the electrode became dull and performance was improved by gentle polishing with a wiping tissue or, in extreme cases, emery paper. Evaluation experiments were carried out at room temperatures using conventional (6) equipment and techniques. A double junction reference electrode was employed. RESULTS AND DISCUSSION The principal results obtained with the sulfate membrane electrode are presented in Figure 1. The electrode yields a nearly Nernstian slope of 29 millivolts per decade (29.6 theoretical) over a fairly wide range of sulfate activity and, furthermore, displays appreciable selectivity for sulfate over a wide variety of common univalent anions. Anion activities were calculated using the Davies equation. The electrode ( 5 ) H. Hirata and K. Higashiyama, Bid/. Cheni. SOC.Jap., 44, 242C (1971). (6) G. A. Rechnitz and N. C. Kenny, Alia/. Lett., 3, 259 (1970).
I
I
I
-BOA
-70-
-60E Y
In
-50-
n w Y
-40-
-30-
, I
2
3
Figure 1, Response of sulfate ion-selective membrane electrode to sulfate and univalent anions All measurements were made at 25 "C and pH 6.3 =I= 0.2 using pure alkali metal salts of the respective anions
employed reached stable potential values in less than one minute for S042- concentrations of lOW3Mand above. Good electrode response is also obtained at sulfate concentrations greater than 10--2but, since there is no reliable method for converting these concentrations to sulfate activity, the points are not plotted in Figure 1 . For practical purposes, where empirical calibration curves are constructed or experiments are carried out at constant ionic strength, the electrode should be useful at least over the 10-l to lOP4Msulfate range. Electrodes prepared to date seem to be stable for periods of at least one month under normal use. Although the interference of the three halide ions tested appears to be related to the solubilities of the corresponding silver salts, we feel that it is not meaningful to compute numerical selectivity ratios for the ions tested on the basis of the present data. In our view, the small potential changes observed with large variations in concentrations of the univalent anions indicate that the electrode is essentially insensitive to these ions and is primarily reflecting secondary activity, liquid junction, and membrane solubility effects. Calibration curves were also constructed using sulfate salts with noncomplexing cations of various types and charges. The fact that these calibration curves were identical in slope and potential, indicates that the electrode is not responsive to noncomplexing ( e . g . , alkalimetal or alkaline earth) cations. Preliminary experiments carried out as a function of solution pH showed little pH effect in the pH 3-10 range. At pH values below 2.5, there is a sharp potential change in the direction of apparent less sulfate; this is probably due to the forma-
tion of H S 0 4 - in acid solution. Above pH 10.5, electrode response also deteriorates; we tentatively attribute this to metal hydroxide film formation involving the metal constituents of the membrane. Naturally, SZ-, Agi, Pb2+,and ions which precipitate or strongly complex components of the membrane will interfere with the operation of the electrode. The mechanism of operation of this electrode is still obscure although the observation that best performance is obtained with membranes where the three principal constituents are present in a 1:1 :1 ratio is provocative. While additional studies are being made to further improve the characteristics and more carefully define the limits of these electrodes, it appears that the present membranes already possess sensitivity, selectivity, and response time properties suited to analytical purposes. We plan to report on our detailed studies and selected applications in a full paper to follow. G. A. RECHNITZ G. H. FRICKE M. S. MOHAN Chemistry Department State University of New York at Buffalo Buffalo, N. Y. 14214 RECEIVED for review December 20, 1971. Accepted March 7 , 1972. We gratefully acknowledge support from the Environmental Protection Agency and the National Science Foundation.
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