470
KARL SOLLKER AND HARRY P. GREGOR
cent nickel oxide samples rendered dificult an accurate measurement of their positions . SUMMARY
The following is a brief summary of the results of this investigation: 1. Mixed dual gels of nickel and aluminum oxides precipitated and dried a t room temperature exhibit a mutual protective action against crystallization, in confirmation of similar result’s previously obtained in the system cupric oxideferric oxide. 2. The mutual protective a’ction js still pronounced in dual gels heated a.t 500-1000°C. 3. Equimolar mixtures of nickel and alum.inum- oxides react at 1000°C. to form nicltel aluminate, N O .AI2O3. 4. Kickel aluminate forms solid solutions with each cf the pure oxides. REFERENCES (1) CAIRNSA N D OTT:J. Am. Cliem. SOC.66, 527 (1933). Z. anorg. a.llgeni. Chem. 204, 378 (1032). (2) HOLGERSSON: (3) KRAUSEA X D THIEL: Z. anorg. allgem. Chem. 203, 120 (1032). (4) MILLIOAN A N D HOLIIES: J. Am. Chein. SOC.63, 149 (1941). (5) NATTA:Atti accncl. Lincei [61 2, 495 (1026). (6) WEISER A N D MILLICAN:J . Phys. Chem. 40, 1075 (1936). (7) WEISER A N D MILLIGAN: Chem. Rev. 26, 7 (1939). 11 (8) WEISERAP;D MILLICAN: Adrances i n Colloid Science, Vol. I, p. 228. Interscience gublishers, Inc., New York City (1042).
THE ELECTROCHEMISTRY OF PERMSELECTIVE COLLODIOK M E R I B R A S E S . I
R.LTESTUDIES
O N THE ESTABLlSHMENT
OF THE COMCEiVTRATION
POTENTIAL ACROSS V L k R I O c S TYPESO F PERMSELECTIVE
COLLODION MEMBRANES KARL SOLLNER A N D HARRY P. GREGORl Department o j Physiology, Cniversity of dfinnesota, Minneapolis, Alinneso!a Received M a y 28, 1946
I In a preceding paper (4) improved and reproducible methods of preparation of various types of (elect,ronegative) permselective collodion membkanes were described. In addition to data on their water content, these memhranes were characterized by (a) the “characteristic concentration potential” (‘5), i.e., the potential of the chain 0.1 6f IW1 I membrane 10.01 Af K C I ; ( b ) the diffusion potential or “bi-ionic potential” (B.I.P.) across the membrane i f the chain 0.1 llf IiCl 1 membrane 1 0.1 64 LiC1; and (c) the electrical resistance ilf the mem-
* Present address: Polytechnic
Institute of Brooklyn, Brooklyn, New York.
ELECTROCHEMISTRY OF COLLODION MEMBRANES.
I
47 1
branes in contact with 0.1 M potassium chloride solution. Also data on the rate of cation exchange across a few of the membranes were presented. These parameters, though useful in the general characterization of the permselective membranes, give only an outline of their electrochemical behavior. They are also insufficient to be used as the basis of a discussion of the intricacies of the geometrical and electrical structure of such membranes. Any further investigation on permselective membranes should attempt a fuller and more detailed description of their electromotive actions under various conditions and also of their absolute permeability. These studies will be useful in the elucidation of the electrochemical and geometric membrane structure ; they will also be helpful in circumscribing the conditions under which the membranes may be used for various physicochemicalinvestigations, particularly those in which the membranes are used as ideal (or nearly ideal) electrochemical machines,-namely, the study of the Gibbs-Donnan membrane equilibrium (3, 10, 11, 15) and the electrometric determination of various cations (3,7, 16). Accordingly, the primary experimental objective of the several parts of the present investigation is the measurement of the concentration potentials and the bi-ionic potentials (B.I.P.) as well as of the ohmic resistances of several types of membranes with various electrolytes a t several concentration levels. Wiih these three phenomena we have also studied in some detail the concow Itant rate effects, the time required until equilibrium or a well-defined steady state has been reached. Only electrolytes with univalent critical ions (cations) are discussed. Preliniinary experiments with electrolytes having bivalent cations have shown that in these instances it is difficult to obtain fully meaningful data; a separate investigation will be necessary for the adequate treatment of this problem. The present investigation was divided into several parts in order to facilitate proper presentation: Part I is concerned with the rate of establishment of final stable concentration potentials across the various membranes; Part I1 will present data on the h a 1 stable concentration potentials obtained withvariouselectrolytes at several concentration levels; Part I11 will attempt to evaluate the empirical data of Part I1 from the theoretical viewpoint and to arrive at an objective quantitative measure of ionic membrane “selectivity.” Further parts will deal with the bi-ionic potential (B.I.P.) and the ohmic resistance at various concentration levels.
I1 The experimental problem of the present paper is to investigate the rate at which h a 1 stable Concentration potentials are reached across several types of permielective collodion membranes with various electrolytes at different concentration levels. In this connection one must keep in mind that with membranes of porous character a true equilibrium in the strict thermodynamic sense cannot be reached under the experimental conditions under which the concentration potential i8 conventionally measured (even with membranes of ideal ionic selectivity), for the water permeability of these membranes is not zero. The
472
KARL SOLLNER AND HARRY P. GREGOR
differences in osmotic activity of the two solutions which are separated by the membrane have t o be considered. For the establishment of a true thermodynamic equilibrium it would be necessary to compensate for this differencein the osmotic activities of the solutions by applying a hydrostatic pressure of appropriate magnitude to the more concentrated solution or t o incorporate the proper quantity of an osmotically active non-electrolyte in the more dilute solution. In other words, for the establishment of a true equilibrium the conditions for a Gibbs-Donnan membrane equilibrium must be fulfilled. For practical purposes, however, this factor is of little significance with permselective collodion membranes. The water permeability of these membranes is so low that the stable potential is established before a detectable movement of water occurs.2 When considering the rate of the establishment of final stable concentration potentials across the permselective membranes, two fundamentally different cases must be distinguished. On the one hand, one may investigate the time effects which arise if a membrane saturated with critical ions of one type, e.g., hydrogen ions, is brought into contact with the solution of an electrolyte having some other critical ion. I n this case cation exchange must take place throughout the pore system of the membranes (8, 9, 10, 12, 13, 14) before final potentials can be obtained. Concomitant with the base exchange some electrolyte, both anion and cation, enters the membrane, the functional significance of which has not yet been elucidated in full. On the other hand, the membrane may already have undergone base exchange previously, its anionic (acidic) wall groups therefore being compensated for by cations of the same species as the cations in solution. In this case a cation exchange between the solution and the membrane does not occur, for the critical spots in the membrane are already occupied by ions of the proper kind (8,9, 10). The only process which must occur to establish the h a 1 stable state across the membrane is the equilibration between the concentration of the electrolyte in the wider parts of the pores of the membrane and the solutions. The first of these two possibilities is of far greater interest than the second, for any differences in the time effect with various electrolytes and a t different concentration levels are bound to give hints as to the electrochemical and geometric structure of the membranes and thus should contribute to a better understanding of the mechanism of their action. In addition, such experiments give some information useful in the application of the membranes for practical purposes. The second of the alternatives mentioned, the investigation of the time effect with membranes which are already saturated with the critical species of ions, is less likely to yield much information of interest from a broader point of 'view, though it gives information directly applicable t o the practical use of the membranes. 2 The osmotic water movement across these membranes is actually so slow that in cationexchange studies across these membranes, ion-exchange equilibrium is reached before a significant volume change has occurred (1). It was also shown that the addition of a n OSmotically equilibrating non-electtolyte has no detectable effect (3, 15).
ELECTROCHEMISTRY O F COLLODION MEMBRANES.
I
473
In conformity with the general purpose of this paper the first of the two POSsibilities outlined,-namely, the investigation of membranes which had not exchanged beforehand with the critical ion species,-was emphasized. As the standard condition of the membranes the acidic state was chosen, in which hydrogen ions are the counter ions of the acidic (anionic) surface groups (9, 12, 13, 14).
I11 The technique used in the measurements of concentration potential was the conventional one (5) employing the chain: saturated calomel electrode I potassium chloride saturated I saturated potassium chloride-agar bridge I electrolyte c2 I membrane I electrolyte c1 1 saturated potassium chloride-agar bridge1 potassium chloride saturated] saturated calomel electrode. To avoid contamination of the solutions by potassium chloride diffusing from the bridges, the latter were dipped in the electrolyte solutions only for the duration of the actual measurements, their tips being otherwise kept in a saturated potassium chloride solution. I n some instances an appreciable leak of electrolyte across the membrane may occur while the final state is being reached. To avoid any possible error due to. this factor or to contamination of the solutions with potassium chloride from the bridges, the two solutions separated by the membranes were renewed periodically as required, while the measurements were repeated until the concentration potential no longer changed with time. All data presented in this paper on the rate effect are given as measured, without correction for the asymmetries in the liquid-junction potentials which arise between the potassium chloride-agar bridges and the two solutions of different concentration. Membranes can be brought into the standard acidic state either by treatment with dilute acid followed by thorough washing with double-distilled water, or by electrodialysis, or simply by prolonged immersion in several changes of doubledistilled water (9, 12, 13, 14). Membranes kept in distilled water for at least 24 hr. prior t o each experiment were used throughout. They were always returned to the acidic state before use with a solution of different concentration of the same electrolyte or the solution of another electrolyte. The membranes originally had been aged by about 3 days’ immersion in 0.1 M potassium chloride solution. Such prolonged contact with electrolyte solutions reduces the electrochemical activity of some of the membranes perceptibly (4) but makes them less liable t o further small changes on contact with electrolyte solutions. For the present study three types of membranes were selected out of the unlimited number of types which can be prepared (4), the system of designation of the membranes following the previously established convention. As an example of a membrane which shows maximum characteristic concentration potential, high bi-ionic potential (B.I.P.), and relatively high resistance membrane Ox 8 -Hum 43 was selected; membrane Ox 12 - Hum 43 was taken because it combines maximum characteristic concentration potential with medium B.I.P. and low resistance, thus being for many purposes the generally
474
KARL SOLLNER AND HARRY P. GREGOR
most useful type of permselective membrane (4). As an example of alcoholswollen membranes the type Ox 10 - Hum 43 -Ale 65 was used, having a high, although not maximum, characteristic concentration potential, a low B.I.P., and very low resistance. The latter type of membrane is of considerable interest and usefulness when one is dealing with electrolytes having bi- or polyvalent anions. 0.01 N/O.OOI N
O.IN/O.OIN
+58 t 56
t 54 t 52 t 50
s t48 9 f58
-
E $56 .-E -.- t 5 4 t
1 t52 a 0
;+SO
e
+48
+58
+ 56 + 54 t 52
+ 50 t 48 0
20 4 0 b 0 80 100 120 140 160 Tma
0 in
20 40 60 80 100 I20 14Q 160 180
minutes
FIG.1. The rate of establishment of steady concentration potentials C2:C1 = 1O:l of several electrolytes across various permselective collodion membranes.
'
I n order t o assure that relatively small differences in the behavior of different electrolytes may not be overshadowed by minor differences between different membrane specimens of the same type, all experiments on the time effect reported for one type of membrane mere performed with the same membrane specimen. Figure 1 shows for three types of membranes the rate of establishment of the concentration potential (at 25.00"C.f 0.05"C.)with potassium chloride, lithium chloride, and potassium sulfate for two concentration ratios, 0.1 N/0.01 N and 0.01 N / 0.001 N.
ELECTROCHEMISTRY OF COLLODION MBMBRANES.
I
475
The accuracy of the measurements represented in figure 1is fi 0.10 millivolt for the values in the final state. IV Figure 1 shows that the rate a t which the stable state is established across the various permselective collodion membranes depends on the nature of the electrolyte and the nature of the membrane, and to some extent also upon the absolute concentration of the solutions used. I n some instances stable potentials are reached within a few minutes; in other instances a few hours may be required. One arrives at the following conclusions: (a) The denser the membranes the more slowly are the final steady potentials established. (b) With lithium chloride, i.e., with an electrolyte having a large cation, equilibrium is reached very much more slowly than with potassium chloride, which has a cation of smaller hydrated size. ( c ) The rate a t which equilibrium is establiskied with potassium chloride and potassium sulfate is identical within the limits of significance of the experimental data. In other words, the rate at which the h a 1 stable concentration potentials are reached seems t o be practically independent of the nature (size and valency) of the anion. (d) With more dilute solutions (0.01N/O.OOl N ) equilibrium is reached somewhat faster than with more concentrated (0.1 N/0.01 N ) ones. New and unexpected is the observation that the nature and valency of the anion does not play an important r61e with regard to the rate a t which the final stable concentration potential is reached. The full meaning of this fact cannot be evaluated a t present. Studies on the conductance of the membranes in contact with various electrolytes, to be reported in a later part of this investigation, will throw some light on this point. A determination of the quantities of electrolyte, i.e., of cations and anions, which are contained in membranes in equilibrium with different concentrations of various electrolytes (2) will be equally important; such studies have not been carried out as yet but are planned for the future. One may mention briefly the fact that 6he rate a t which stableconcentration potentials are reached with permselective membranes is very much greater than that observed by Michaelis and collaborators (G), owing to the much more open structure of the permselective membranes. It is necessary to compare the rate at which stable concentration potentials are established across the membranes with the rate of base exchange observed in previous studies (13, 14). There it was found that the base exchange of fibrous collodion,-and the same is true for membranes,-in many instances requires as much as 24 hr. In other words, a final and stable electrical state across the membrane is reached before all the groups capable of base exchange are saturated with the critical cation. As mas pointed out previously (13, 14),this situation must be expected on account of the irregularity of the structure of these membranes. Evidently a large number of the active groups lie in extremely narrow dead-end cavities or in the interior of micelles. These inaccessible groups are therefore located in places which are not operative in the typical electrochemical functions of the membranes. Functionally important are only those groups which lie along the various possible pathways which are involved in the penetration of ions I across the membrane. Concerning the rate of establishment of the concentration potential across membranes which are already saturated with the critical ion species by prior ion ex-
476
KARL SOLLNER AND HARRY P. GREGOR
change, it suffices here to report that these rates were found to be much higher and less characteristic for the different ions. From the practical point of view, however, it is interesting to note that with such membranes final stable potential readings can frequently be made almost instantaneously, in most instances at least within 2 min.; adaptation periods of more than 10 min. are rare. SUMMARY
1. A study was made of the rates a t which final stable concentration potentials are established with three electrolytes-potassium chloride, lithium chloride, potassium sulfate-across three types of permselective collodion membranes being originally in the acidic state with their fixed anionic wall groups compensated for electrically by hydrogen ions. 2. The rates at Ghich stable concentration potentials across these membranes are established depend on the Aature of the membrane, the nature of the electrolyte, and the absolute concentration; they vary from several minutes up t o several hours. SFecifically it was found that: (a) the denser the membranes the more slowly are the final steady potentials established; ( b ) with lithium chloride, i.e., with an electrolyte having a large cation, equilibrium is reached very much more slowly than with potassium chloride, an electrolyte having a cation of smaller hydrated size; (e) the rates a t which equilibrium is established with potassium chloride and potassium sulfate are identical within the limits of the significance of the experimental data. In other words, the rate at which the final stable concentratioh potentials are reached seems to be practically independent of the nature (size and valency) of the anion; (d) with more dilute solutions (0.01 N/0.001 Nj equilibrium is reached somewhat faster than with more concentrated (0.1 N/0.01 N ) ones. 3. The establishment of stable concentration potentials across membranes which are already saturated with the critical ion species by prior ion exchange occurs much faster than in the cases where ion exchange must occur; stable potentials are frequently obtained nearly instantaneously, adaptation periods of more than 10 min. being rare in this case. t REFERENCES (1) CARR,C. W.,AND SOLLNER, K.: J. Gen. Physiol. 28, 119 (1914). (2) GREEN,A. A., WEECH,A. A., A N D MICHAELIS,L.: J. Gen. Physiol. 12, 473 (1929). (3) GREGOR,H.P.: Ph. D. Thesis, University of Minnesota, 1945. K.: J. Phys. Chem. 60,53 (1946). (4) GREGOR,H.P., AND SOLLNER, (5) MICHAELIS,L.: Bull. Natl. Research Council, No. 69 (1929); Ilolloid-Z. 62, 2 (1933). (6) MICHAELIS,L., ELLSWORTH, R . McL., AND WEECH,A . A . : J. Gen. Physiol. 10, 671 (1927). (7) SOLLNER, K.: J. Am. Chem. SOC. 66, 2260 (1943). (8) SOLLNER, K.:J. Phys. Chem. 49, 47 (1945). (9) SOLLNER, K.:J. Phys. Chem. 49, 171 (1945). (10) SOLLNER, K.:J. Phys. Chem. 49, 265 (1945). (11) SOLLNER, K.:J. Am. Chem. SOC.68, 156 (1946). (12) SOLLNER, K., ABRAMS,I., AND CARR,c. w.: J. Gen. Physiol. 26, 411 (1942). (13) SOLLNER, K.,AND ANDERMAX, J.: J. Gen. Physiol. 27, 433 (1943). (14) SOLLNER, K.,AND CARR,C. W.: J. Gen. Physiol. 28, 1 (1944). (15) SOLLNER, K,., AND GREGOR,H. P.: J. Am. Chem. SOC.67, 346 (1945). (16) SOLLNER, K.,AND GREGOR, H . P . : In preparation
