The mobility of alkali ions in gases III. The mobility of alkali ions in water vapour

Abstract
It is known that in electrolytes at infinite dilution the mobility of an alkali ion increases with its mass and this has been attributed by some to a decrease in its degree of hydration as the size of the alkali atom increases. In Part I evidence was obtained, at least in helium and neon, that the average number of water molecules which are attached to an alkali ion when water is present as an impurity also decreases as the atomic weight of the ion increases. As a natural corollary to this work a determination of the mobility of the alkali ions in pure water vapour has been undertaken and is here described. The method and apparatus of Part I was used. The nature of the ion from the source was first verified by running it in a pure gas which was then pumped off and water vapour introduced. The results are shown in fig. 1, where the mobility of the ion is plotted with E/p . For the sake of clearness the results for Rb + are excluded from the graph except at low values of E/p . The remainder of the Rb + graph follows more or less that for Na + .

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