Solutions of Electrolytes

In order to explain the abnormal freezing point depressions, etc., of solutions of electrolytes, Arrhenius supposed that the ions formed by dissociation had the same effect on the behaviour of the solutions as independent molecules. Then if the degree of dissocia tion is 7, the fraction of the electrolyte remaining undissociated is 1 —7. If a molecule of an electrolyte y dissociated into k ions, the number of ions formed is proportional to ky, so that the In solutions of weak electrolytes the limiting value of the molec ular conductivity, corresponding to complete dissociation, cannot be determined directly, because at the greatest dilution at which measurements can be made these substances are only partially dissociated. It can, however, be calculated by means of a relation given by Kohlrausch; and from it the degrees of dissociation are obtained in the same way as with strong electrolytes.

Difficulties of Arrhenius' Theory.

The theory of Arrhenius met at first with vigorous opposition. The more conservative chemists refused to accept the view that common salt, for example, is largely dissociated in aqueous solutions into sodium ions and chloride ions. This opposition was largely based on a misconception of the nature of ions and in course of time died down. Nevertheless, although the Arrhenius theory provided a new basis for our knowledge of salt solutions and served to inspire a great number of investigations for more than 20 years, there remained outstanding difficulties.

Thus it was found that, in some of their properties, the be haviour of salt solutions was more uniform than the varying de grees of dissociation calculated from conductivity measurements, according to the Arrhenius theory, would lead us to expect. For example, A. A. Noyes found in 1904 that the optical powers of 0-bromocamphoric acid and its metallic salts in solu tions of equivalent strengths were almost identical, although con ductivity measurements indicated degrees of dissociation varying from 7o to 93%. Noyes remarked, "If there were not other evidence to the contrary these facts would almost warrant the conclusion that the salts are completely ionized up to the con centration in question." Again, it was found by Ostwald that the variation of the degree of dissociation of weak electrolytes with their concentration was in accordance with the ordinary laws of chemical equilibrium, but no such relation could be obtained with strong electrolytes. Although attempts were made to introduce

the effect of the electrical charges of the ions on the equilibrium between undissociated salt and its ions, no satisfactory explana tion was found. In the earlier stages the methods of measure ments available were not very accurate and it was only possible to show the approximate equality of the degree of dissociation as given by conductivity measurements with the value corresponding to the van't Hoff factor i. In later years many exact measure ments were made of the two quantities, but these only confirmed or accentuated the differences between them.

The Theory of Complete Dissociation.

As we have seen, Arrhenius made the initial assumption that the speeds of ions in a given electric field were independent of the concentration, at any rate in dilute solutions ; so that changes in the molecular conductivities were due to changes in the degree of dissociation. In view of the difficulties mentioned above, the correctness of Arrhenius' assumption began to be questioned. Might not a strong electrolyte be completely dissociated at all concentrations, the changes in the molecular conductivities being due to changes in I the mobility of the ions? This view, which had been tentatively suggested earlier by W. Sutherland, S. R. Milner and others, was put in a definite form by N. B jerrum in 1918.

The early prejudice against the idea of electrolytic dissociation had now disappeared. In the light of the greater insight into the structure of salts afforded by studies in atomic physics, the disso ciation of salts into ions appeared, not as something abnormal and astonishing, but as a natural consequence of their structure. Thus the work of Sir William Bragg and his son, W. L. Bragg, on crystal structures has shown that crystals of sodium chloride, for example, are built up of sodium ions and chloride ions arranged alternately on a cubic lattice. Each sodium ion is surrounded at equal dis tances by six chloride ions and there is no reason for believing that any of these is associated with it by a special bond. It is thus reasonable to suppose, unless evidence to the contrary is found, that there is no tendency for the ions to "pair off" into molecules in solution. It was therefore necessary to see if the properties of salt solutions could be explained in any other way than as due to variations in the degree of dissociation.