May, 1954
POTENTIOMETRIC DETERMINATION OF CATIONS AND ANIONS
409
THE POTENTIOMETRIC DETERMINATION OF CATIONS AND ANIONS WITH PERMSELECTIVE COLLODION AND PROTAMINE-COLLODION MEMBRANE E L E C T R O D E S BYHARRY P. GREGORAND KARLSOLLNER~ Department of Ph?piologi/,University of Minnesota, Minneapolis, Minn.; the Laboratory of Physical Biologu, National Instiiute of Arthritis and Metabolic Diseases, National Institutes of Health, U.S. Public Health Service, Department of Health, Education and Welfare, Bethesda, Md.; and the Department of Cheinistry, Polytechnic Institute of Brooklyn, Brooklyn, N . Y . Received December 22, 1965
Permselective membranes in concentration chains of numerous electrolytes act electromotively as ideal or nearly ideal reversible electrodes over a fairly wide range of concentrations. Permselective collodion membranes which show an ideal or nearly ideal degree of selective ionic permeability to cations can thus be used as virtually reversible membrane electrodes for cations. The selectively anion permeable, permselective protamine-collodion membranes, notwithstanding a small ‘Lleak’’of cations, can be used in an analogous manner as membrane electrodes for anions. The determination of ion activities by means of such membrane electrodes can be carried out either by the evaluation of e.m.f. measurements according t o the Nernst equation or, better, by the evaluat#ionof e.m.f. data from empirical calibration curves or by the use of a null method involving titration through zero potential. The error of the first exploratory determinations which were made by theRe methods was on the average considerably less than l%,.in no instance more than 2%. Permselective membrane electrodes thus afford a simple means of determining the activities of numerous cations and anions, including many for which other electrometric methods are not available.
The potentiometric determination of activities tively allows the reversible transfer of only a single of the alkali and alkaline earth metal ions in solu- ion species from the one solution to the other tion by means of conventional electrodes is beset gives rise to a potential and acts electromotively with experimental difficulties, and so far has been in a manner strictly analogous to a conventional successful for practical purposes only with more reversible electrode for this ion. The general theory of membrane electrodes has concentrated solutions. The potentiometric determination of the activities of anions is a t present been discussed in some detail by Haber and colrestricted to a limited number of ionic species, laborator~,~*’~ H o r o w i t ~ , ~ ~Tendeloo, J~ l4 Marbecause of the lack of suitable electrodes; the shall,16 and most recently by Scatchard.lG Also activity of many, even of some of the most com- pertinent is the work of Meyer and Sievers” and mon anions cannot be determined electrometrically. the experimental studies on membranes of high Many of these difficulties can be overcome readily ionic selectivity by Michaelis and collaborators, l 8 by the use of permselective membranes as “mem- and Sollner and collaborators.1g-24 brane electrodes,” as was shown in a preliminary The experimental work on the use of glass memnote published several years Since that brane electrodes for the determination of hydrogen time the permselective membrane electrodes have The attempts of Horoproven themselves as useful tools in the hands of ions is well known.10~25~26 \vital2 and Schiller2’ to use glasses of various several investigators, particularly in the study of compositions as membrane electrodes for several the activities of counter ions in colloidal systems. 4--8 This paper presents a description and critical cations did not meet with success. Marshall and collaborator^^^^^^ and later evaluation of the uses of permselective membranes as membrane electrodes in systems which contain WyllieSohave prepared membranes from various zeolitic minerals, and have shown that they can only one species of potential determining ions. The potential usefulness of membrane electrodes serve as reversible electrodes for alkali and alkaline was first recognized by HaberlgJ0 after Nernst (12) K. Horowitz, 2. Phyaik., 16, 369 (1923); et seq. (13) K.Horowitz, 2. physik. Chem., 116, 424 (1925). and Riesenfeld” had shown that any interphase (14) H. J. C. Tendeloo, J . B i d . Chem., 113,333(1936). (membrane) which in a concentration chain selec(15) C. E . Marshall, THISJOURNAL, 43, 1155 (1939): 48, 67 (1944): (1) Based on a portion of the Dissertation of H. P. Gregor, presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry a t the University of Minnesota, May, 1945. (2) Laboratory of Physical Biology, National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland. (3) K. Sollner, J . A m . Chem. SOC..66, 2260 (1943). (4) C. W. Carr, W. F. Johnson and I. M . Kolthoff, THISJOURNAL, 61, 636 (1947). (5) R. C. Chandler and J. W. McBain, ibid., 63,930 (1949). (6) C. W.Carr and L. Topol, ibid.. 64, 176 (1950). (7) C. W.Cam, Arch. Biochem. and Biophys., 40, 286 (1952); 43, 147 (1953); 46, 417, 424 (1953); paper presented before the 103rd Meeting of the Electrochemical Society, April 12-16, 1953, New York (in press). ( 8 ) F. M. Snell, paper presented before the 103rd Meeting of the Electroohemioal Society, April 12-16, 1953, New York. (9) F. Haber and R . Beutner, Ann. Phyeik, [4] 26, 327 (1908): F. Haher, ibid.,141 26, 927 (1908). (10) F. Haber and 2. Klemensiewica, 2. physik. Chem., 67, 385 (1909). (11) W.Nernst and E. H. Riesenfeld, Ann. Physik. 141 8 . 600 (1902).
et aeq. (16) G. Scatchard, J . A m . Chem. SOC.,7 6 , 2883 (1953). (17) K. H. Meyer and J.-F. Sievers, Helu. Chim. Acta, 19, 649 (1936). (18) L. Miohaelis, CoEEoid Symposium Monograph, 6, 135 (1927); KoEEoid Z.,62, 2 (1933). (19) C. W.Carr and K. Sollner, J . Gen. PhysioE., 28, 119 (1944). (20) C. W.Carr, H. P. Gregor and K. Sollner, ibid., 28, 179 (1945). (21) K. Sollner and H. P. Gregor, THIS JOURNAL, 61,209 (1947). (22) K. Sollner and H. P. Gregor, ibid.. 54,330 (1950). (23) K.Sollner, ibid.. 49, 47, 171, 265 (1945). (24) K.Sollner, J. Electrochem. Soc., 97, 139C (1950). (25) D.A. MacInnes, “The Principles of Electrochemistry,” Rein. hold Publ. C o r n . New York. N. Y.. 1939. (26) M. Dole, “The Glass Electrode,” John Wiley and Sonn, Inc., New York. N. Y..1941. (27) H.‘Schiller, Ann. Physik, 74, 105 (1924). 43, 1155 (1939); 48, 67 (1944). (28) C.E.Marshall, THISJOURNAL, , (29) C. E. Marshall and W. E. Bergman, J. A m . Chem. S O C .63, 1911 (1941); 64, 1814 (1942): THISJOURNAL, 46, 52,326 (1942); C. E. Marshall and A. D. Ayers, J . A m . Chem. SOC.7 0 , 1797 (1948); et seq. (30) M. R. G. W y , Science, 108,684 (1048).
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HARRY P. GREGOR AND KARLSOLLNER
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earth cations. These membranes have certain where n is the valence of the critical ion, and al practical drawbacks; they possess high ohmic and a2 its activities in solutions (1) and (Z), reresistances (1-10 megohms), require several hours spectively. Then, if solution (I) is a reference soluor days to attain equilibrium, and are stable for tion where al is known, the determination of E limited periods of time only. allows the calculation of a2 in the unknown soluSollner3 has demonstrated the use of perm- tion. selective collodion and protamine-collodion memThe use of the above expression as applied to branes as membrane electrodes for numerous cat- membrane chains requires that several simplifying ions and anions, respectively, the protamine- assumptions be employed, since measurements of collodion membrane being the first anion re- the electromotive forces of cells with or without sponsive membrane electrode. The permselective transference cannot yield thermodynamically decollodion and protamine-collodion membranes are fined single ion activities, a topic that has been easy to prepare in a reproducible manner, are treated by many authors, notably by Harned and stable over prolonged periods, show low ohmic Owen.89 The same limitations exist as for pH resistances, and in a fairly wide range of concen- measurements. 40 trations rapidly yield stable and highly reproIn the practical determination of such ionic ducible potentials in simple concentration chains. activities, two devices are customarily employed. These potentials closely approach or coincide with First, salt bridges are used which reduce the those calculable from known activity data.19-21,31-a4 magnitude of the liquid junction potentials, the Membranes were prepared from plastic bonded difference between these two potentials usually conventional cation-exchange materials by Wyllie being neglected. This practice has been followed and Patnodea5; Juda and M C R W , M ~ ~a n e ~ k e , ~ ’in this paper. Second, with regard to ionic acBonhoeffer, Miller and Schindewolf38 and others tivities, some arbitrary assumptions must be have also studied some of the electrochemical made. Conventionally, with uni-univalent elecproperties of ion-exchanger membranes. trolytes, single ion activity coefficients are set equal to mean activity coefficients. With bi-univalent Theory and Methods and uni-bivalent electrolytes which contain either The determination of the activity of an ionic the potassium or the chloride ion, the ion activities species in solution by the use of membrane elec- are calculated by assuming that the single ion trodes is based upon the use of a known reference activity coefficients of these ions are identical solution containing the same potential determining with the mean activity coefficient of a potassium ions, which in the literature on membranes are fre- chloride solution of the same ionic strength. The experimental determination of ion activities quently referred t o as the so-called “critical” ions. by means of membrane chains can be carried out Conventionally, chains of the type S.C.E./Sat. KCl//Soln. (Z)/Membrane/Soln. (l)//Sat. KC1/ by: (1) evaluation of e.m.f. measurements acS.C.E. are measured, where S.C.E. refers to the cording to the Nernst equation; (2) evaluation of saturated calomel electrode connected to the solu- e.m.f. data from empirical calibration curves; tions by means of a saturated potassium chloride and (3) the use of a null method, namely, titration bridge. Solutions (1) and (2) contain the poten- through zero potential. The Nernst equation can be used in a straighttial determining ion, the sole ionic species present that can move across the membrane, the “critical” forward manner, without any corrections, only ion being the cation in the case of electronegative, in those cases where the membrane is virtually of acidic membranes, and the anion with electro- an ideal degree of ionic selectivity and where the positive, basic membranes. With membranes of difference in the magnitude of the two liquid juncideal ionic selectivity the nature of the non-critical tion potentials is sufficiently small, well within the ions in solutions (1) and (2) is of no significance limits of the desired accuracy. The applicability except as far as they eo-determine the activity of of this method must be established by the measthe critical ions. (With “leaky” membranes a slight urement of chains containing solutions of known activity. error may be introduced if different non-critical Empirical calibration curves can be utilized ions with a differential tendency to leak are con- where membranes do not show ideal behavior due tained in the two solutions.) to the “leak” across the membrane of ions other When a membrane possesses an ideal degree of than the critical ions, or where liquid junction ionic selectivity, the e.m.f. is given by the Nernst potentials are a source of significant error. With expression this method the use of an unknown solution of approximately the same composition with respect t o ions other than the critical ion is indicated. The null method consists of a potentiometric (31) H.P. Gregor and K. Sollner, THISJOURNAL, 50, 53, 88 (1946). titration where the membrane separates the solu(32) Ioa measured volume of water, to which standard solution is added stepwise from a buret, potential measurements being made at
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each point. The titration is continued considerably past the point where the potential changes in sign. The endpoint (zero potential) is evaluated graphically.
TABLE I1 THE POTENTIOMETRIC DETERMINATION OF ACTIVITIES OF VARIOUSCATIONS BY THE USE OF EMPIRICAL CALIBRATION CURVES Sol. (1) 0.01 N (reference)
Electrolyte in sol. (2) (unknown)
KCI KCI KCI KCl KCI KCI KCI KIOs KIOa , LiCl LiCl KCI KCI 1
