ACTIVITY COEFFICIENTS IK MIXEDAQUEOUS CdC12-HCl SOLUTIONS
July, 1962
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ACTIVITY COEFFICIENTS IN MIXED AQUEOUS CADMIUM CHLORIDEHYDROGEN CHLORIDE SOLUTIONS AT 2.5’ BY THE ELECTROMOTIVE FORCE METHOD BYLESLIELEIFER,~ WILLIAMJ. ARGERSINOER, JR.,AND ARTHURW. DAVIDSON Department of Chemistry, The UniPlersity of Kansas, Lawrence, Kansas Received January $8, 1969
Activity coefficients of aqueous mixtures of cadmium chloride and hydrochloric acid have been measured a t 25’ by means of the electromotive force method. Measurements were made at total ionic strengths of 1.0, 0.5 and 0.2 molal, and at ten ionic strength fractions a t each total ionic strength. The variation of the logarithm of the ackvity coefficient of each of the solute species with ionic strength fraction was found to be non-linear. These measurements comprise one of the few cases so far reported of experimental determinations of activity coefficients of both electrolytes in a mixture. The thermodynamic consistenc of the results was checked by application in two different ways of the cross-differentiation relationship (a In azjbrnl), = In al/&nz)m, (l),in which the subscript 1 refers to hydrochloric qcid and 2 to cadmium chloride. It was found that, within the limits imposed by the paucity of the data, agreement obtained was satisfactory.
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Introduction The recent attempts of numerous investigators from this as well as from other laboratories to apply the concepts of classical thermodynamics to ionexchange processes have emphasized the paucity of available data on activity coefficients in mixed electrolyte solutions. Prior to 1949 there were primarily two methods used for the determination of the activity coefficient of 1one electrolyte in a solution also containing another. They were (a) determination of the solubility of salts in salt solutions, and (b) electromotive force measurements on cells without liquid junction containing a mixture of electrolytes. I n the case of aqueous solutions of metal halides and the corresponding acids, the mean activity coefficient of the acid in the mixed solutions was determined experimentally a t various concentrations from electromotive force measurements.2.8 For many of these aqueous systems at constant total ionic strength, the logarithm of the activity coefficient of the acid was found to be a linear function of the ionic strength of the other solute. The activity coefficient of the salt was then computed from the osmotic coefficients of the solvent in the separate pure solutions at the total ionic strength in question, on the assumption of a similar linear variation of the logarithm of the activity coefficient of the salt with the ionic strength of the acid. This principle of linear variation has come to be known as the Harned rule. However, since the application of the Harned rule to both solute species was admittedly based upon an assumption, it was thought desirable to test the validity of the Harned rule by direct measurements on both electrolytes in their mixed aqueous solutions at constant total ionic strength. A series of measurements of the activity coefficient of hydrochloric acid in its aqueous mixtures with cadmium chloride at constant total ionic strength of 5.0 molal has been pre(1) From part of a thesis submitted by L. Leifer in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Ehnsas, 1959. Presented before the Division of Physical Chemistry, 138th National Meeting of the American Chemical Society, New York, N. y., September, 1960. ( 2 ) H. 8. Harned and B. B. Owen, “The Physioal Chemistry of Electrolytic Solutions,” 3rd ed., Reinhold Puhl. Corp., New York, N. Y., 1958, p. 600. (3) H. S. Harned and R. Gary, J . Am. Chcm. Soc., 76, 5924 (1954): 77, 1994, 4695 (1955).
viously reported4; in this instance, approximate agreement with the Harned rule of linearity was again observed. I n order that the individual activity coefficients in a mixed electrolyte system with a common ion may be determined by means of electromotive force measurements, reversible electrodes must be found for all ions present. I n our measurements an Ag-AgC1 electrode served as a reversible chloride electrode, so that by use of such an electrode together with a hydrogen electrode and a saturated cadmium amalgam electrode for the cations present we could in principle measure the activity coefficients of cadmium chloride and hydrogen chloride in their mixed aqueous solutions at various total ionic strengths. It should be noted that in the present investigation some difficulties might be expected from direct cheniical action of cadmium metal and hydrogen ion at the cadmium amalgam electrode, without the passage of any current, and from the tendency of hydrogen electrodes to become poisoned in solutions containing cadmium, first pointed out in a qualitative statement by Glasstone.6 The results obtained indicated that at least the second possible difficulty did not interfere with the experimental measurements, and it was hoped that the first did not occur to a sufficient extent to constitute a serious source of error. This point is tested and discussed later. In this work cells of the following types were used
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CdHp,/ CdClz(ma), HCl(mi) AgC1-Ag Pti-HZ[ CdClz(m2), HCl(mi) [ AgCI-Ag and
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CdHgxI CdCL(mz), HCl(mi) Hz-Pt The expressions for the electromotive force of these cells are, respectively
El
=
E10
RT - 2.3026 log [mz(mif 2m!J2] 2F 2.3026
3RT log ~2 2F
(2)
(4) € 8.I Harned . and R. Gary, J . PhU8. Chem., 65, 2080 (1959). S. Glasstone, “An Introduction t o Electrochemistry,” D. Van Nostrand Co., New York, N. Y.,1942, p. 352. (5)
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Ez
=
L. LEIFER,W. J. ARGERSISGER, JR.,AND A. W. DAYIDSON
ET Ezo- 2.3026 - log [ml(ml 4-2mz)I F
and
E3
=
RT log m2 E3' - 2.3026 2F m12
I n these equations, y represents the stoichiometric mean ionic molal activity coefficient. Equations 2, 3, and 4 are not all independent because
Experimental Methods Bsiutians.-All solutions used in these investigations were
prepared from demineralized water and Baker's Analyzed reagent grade cadmium chloride and hydrochloric acid. I n order to ensure stoichiometric cadmium chloride, the stock solution of cadmium chloride was analyzed for both cadmium by gravimetric determination as the sulfate) and chloride by precipitation with silver nitrate solution). The mixed electrolyte solutions were prepared by weight from stock solutions, the 0.5 rtnd 0.2 molal solutions being obtained by dilution of the 1.0 moIal solution. Prior to actual measurement, the solutions were deoxygenated by boiling under reduced pressure and bubbling hydrogen through the solution, also under reduced pressure. The original concentrations were corrected for loss of water in this process. Electrodes.-The hydrogen electrodes were prepared by the following procedure. Each platinum electrode was cleaned electrolytically by making i t the anode in a concentrated HCl solution (new electrodes were cleaned in hot alkali). The electrodes were then made the cathode in the electrolysis for about 5 min., a t a current density of 1.35 amp./dm.2, of a 3% solution of chloroplatinic acid containing 0.5 ml. of 0.1 N lead acetate in 100 ml. The electrodes were then rinsed with water, made the cathode in the electrolysis of a dilute sodium hydroxide soIution for a few seconds, washed, made the cathode in the electrolysis of a dilute sulfuric acid solution for one minute, again washed, and stored in deionized water until ready for use. The silver-silver chloride electrodes were prepared by the thermal method used by Rule and LaMer,e Keston,T and Owens in the preparation of Ag-AgC1, Ag-AgBr, and AgAgI electrodes, respectively. The cadmium amalgam electrodes, containing 6-8 weight % (10.7-14.3 atom %) of cadmium, were prepared from Baker and Adamson reagent grade mercury and Mallinckrodt reagent grade cadmium metd. The cadmium, scraped virtually free of any oxide film, was placed in a 60-ml. S uibb separatory funnel and the air was evacuated. dercury was introduced and, after re-evacuation, hydrogen was added. By means of successive evacuations and hydrogen additions, the space was freed of oxygen and then filled with oxygen-free hydrogen a t approximately 0.5 atm. pressure. The funnelwas then warmed in a water bath at 60-80", and agitated until complete solution of the cadmium had taken place. The assembly was then cooled, during which process some solid crystallized out and could be seen floating on the surface, and hydrogen was added to atmospheric pressure. Amalgams prepared in this way remained bright and oxide-free over long periods of time. The electrodes were checked by means both of measurements on cells containing the individual electrolytes and of internal comparisons during the course of each measurement. Where discrepancies greater than 0.1 mv. occurred, the faulty electrode was discarded.
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(0) C. N. Rule and V. I
