Thermal diffusion in solutions of electrolytes

Abstract

A conductimetric method for following the small concentration changes that occur when a temperature gradient is maintained in an aqueous electrolyte is described. The solution is contained in a Perspex cell between silver end-plates which are faced with platinized platinum and kept at temperatures differing by about 10°C. A further connexion to the cell (a ‘centretap’) is made through a small lateral hole equidistant from the ends. The cell is incorporated in an audio-frequency Wheatstone bridge and movement of solute from one half of the cell to the other is followed by measuring the ratio of their resistances. For a convection-free system, the Soret coefficient (σ) may be derived either from the initial rate of change of the ratio or from its value in the steady state. It is found experimentally that there are discrepancies between the two estimates of σ, and also related anomalies in the rate of change of concentration, which can be ascribed to convection. It can be shown that the initial rate observations should be free from convection errors, and the effect of convection on the steady state can be analysed by dimensional methods. The observed discrepancies are correlated with the relevant properties of the solutions in the manner suggested by this analysis. The Soret coefficients of eighteen 1:1 salts in 0⋅01 m aqueous solution and at mean temperature 25⋅0°C have been determined by this method. Some additional measurements have been made at 34⋅7°C and at other concentrations in the range 0⋅002 to 0⋅02m. Three salts of other valency types (potassium, thallous and cadmium sulphates) have also been studied. The molar heats of transport of the salts ( Q *) have been calculated from the Soret coefficients. The results show that Q * (i) is an additive function of contributions characteristic of the constituent ions in dilute (0⋅01 M) solutions of 1:1 electrolytes, (ii) increases markedly on raising the mean temperatures from 25⋅0 to 34⋅7°C, in agreement with the results of Alexander (1954) and Longsworth (1957) (iii) increases appreciably on dilution below 0⋅01 M, indicating that heats of transport are influenced by long-range inter-ionic forces.