The behaviour of electrolytes in mixed solvents. Part IV.—The free energy of zinc chloride in water-alcohol solutions

Abstract

In Part I of this series were described measurements of the free energy and heat content of hydrogen chloride in a series of water-alcohol mixtures. It appeared to be very desirable to find if the behaviour shown by hydrogen chloride is typical of that of strong electrolytes in general. For this purpose an attempt was made in the first place to determine the free energies of lithium chloride in these solvents, using amalgam electrodes. Preliminary measurements showed that even Under rigidly air-free conditions reproducible values of the electromotive forces of cells containing lithium amalgams could not be obtained in alcohol solutions. Similar experiences with sodium amalgams have been reported from the Balliol College Laboratory, while Scatchard has also experienced similar difficulties with calcium amalgams. It appeared that zinc chloride might be suitable, for when proper precautions are taken for the with zinc amalgam in aqueous solutions. The conductivities show that zinc chloride is a strong electrolyte in aqueous solution, and although several investigators have found that their product hydrolysed to give turbid solutions in water, we had no difficulty in obtaining a salt which gave perfectly clear solutions at all dilutions. The conductivity measurements of Getman and Gibbons, which show that in alcoholic solution zinc chloride is an extremely weak electrolyte, escaped our notice at the beginning of this work. But although on this account the behaviour of zinc chloride does not extend our knowledge of the properties of completely ionised substances in mixed solvents, it has proved to be an interesting case showing in the range of solvents investigated the transition from a strong to an exceedingly weak electrolyte. The free energies were determined by measuring the electromotive force of cells of the type Zn 1 percent. amalgam | ZnCl 2 , ( m ) Alcohol, N mols. per cent. AgCl( s ) | Ag The free energy change in the cell reaction, viz., Zn (Hg) + 2AgCl( s ) = ZnCl 2 ( m , N) + 2Ag, is given by ∆F = - 2 EF = - 46148E calories, where E is the electromotive force and F the electrochemical equivalent. The electromotive forces were determined in solvents containing 0, 25, 50, 90 and 100 mols. per cent, alcohol, each for a range of salt concentrations from about m = 0⋅01 to m = 1, at 15 ± 0⋅02°. It was originally intended to make a similar series of measurements also at 25°, in order to evaluate the heat content changes, but the reproducibility of the results in solutions containing large proportions of alcohol did not warrant this extension.