Hydrogen ion mobility in normal and heavy water solutions of electrolytes

Proton and deuteron mobility in normal and heavy water solutions of some alkali halides and tetraalkylammonium bromides has been measured polarographically, and the diffusion coefficients of the proton and deuteron calculated using a corrected form of the Ilkovic equation. The value of the constant in the correction factor of the equation was found to be 17, in agreement with other experimental and theoretical work. In all cases, proton and deuteron diffusion was retarded as concentration of supporting electrolyte increased. This was also found to occur when cadmium ion diffusion was measured in some nitrate solutions. The main difference was the rapid decrease for hydrogen and deuterium ion diffusion in the 0 - 1 m concentration range. The similarity of both diffusion current and half-wave potential vs. concentration plots for hydrogen and cadmium ions at higher concentrations seemed. to point to a complete reduction of any abnormal diffusion mechanism for the hydrogen ion, in agreement with the conclusions drawn by some other workers. When, however, results were analysed in terms of the effect of electrolyte concentration on the excess or Grotthuss mobility of the hydrogen and deuterium ions, it was found that some of the Grotthuss component was present even up to concentrations of supporting electrolyte of about 4 m. These results also raise some questions about the accepted mechanism for the transport of normal and heavy hydrogen ions in electrolyte solutions. It appears that it is the field-induced orientation of water molecules or hydrogen ions that is the rate determining step in proton and deuteron transport. This exceeds the rate of thermal orientation, is slower than proton or deuteron tunnelling and has been justified theoretically. The thermal orientation of water molecules appears to be no guide to the field-induced orientation in the presence of electrolytes. The lower diffusion currents in heavy water solutions are attributed to the more extensive deuterium bonding as compared with the hydrogen bonding in normal water. The greater reduction of proton and deuteron diffusion in solutions of electrolytes considered to be structure-makers is, therefore, in agreement with this. Energy of activation measurements support generally the above conclusions. When hydrogen ion diffusion coefficients obtained in this study were compared with literature values there appeared some discrepancies, but some values agreed with present ones. It is thought that diffusion through a glass diaphragm cell may not give reliable results for the hydrogen ion because water structure becomes modified when water moves through very fine glass capillaries or pores, and this would affect the abnormal Grotthuss component of mobility.