Effect of change of temperature on the constitution of water in solutions of electroytes

Abstract

In a recent paper an account has been given of the author's investigations on the constitution of water as affected by the solution in it of several typical electrolytes. It was found from a study of the Raman band of water in solutions that the effect of the dissolved substance on the behaviour of the solvent is to cause a shift of the water equilibrium between the single (H 2 O), double (H 2 O) 2 and triple (H 2 O) 3 Molecules supposed to exist in water, as is evidenced from changes in the intensity distribution of the band. These changes were also partly attributed to association of the molecules of the solvent with the ions of the solute to form complex compounds known as hydrates, the extent of hydration being greater fro those substances which are known to crystallize with a large amount of associated water. Different electrolytes, when studied at the same concentration, exhibited marked differences in the structure of their Raman water band, the maximum of the band being shifted by different amounts for the different substances. But, when their water content is equalized, especially with the acids studied, the differences noticed at the same concentration of the electrolytes tended to disappear, the bands becoming increasingly similar. This seemed to indicate that the changes observed in the structure of the band are caused more by changes in the constitution of water itself rather than by differences in the nature of the solute. Nevertheless, even with solutions containing the same amount of water, certain minor differences in the structure of the band continued to persist with the acids, and they were even considerable with the salts. Also, the water-band in solutions of sodium nitrate, for example, was much sharper than that with nitric acid, on the one hand, and with solutions of lithium nitrate and chloride on the other; the latter are known to crystallize in general with a far larger amount of water of crystllization than the sodium salts. These results are taken to show the existence of hydration in solution, the extent and complexity of which varies for different substances. It is proposed to deal in this paper with the effect of a change of temperature on the constitution of water in electrolytic solutions.Ramakrishna Rao, working with water at different temperatures, found that the maximum of the Raman band for water shifted to the side of higher frequency with rise of temperature, thus resulting in a change in the relative distribution of intensity along the band. This he interpreted as due to a change in the proportions of the three types of molecules assumed to exist in liquid water; the number of the triple molecules corresponding to the lower frequency side of the band decreasing while that of the single molecules corresponding to the higher frequency side increasing with rise of temperature. The proportion of the double molecules corresponding to the central component of the band remains more or less constant with change of temperature.