Dissociated forms of electroactive substances usually undergo charge-transfer reactions with different overpotentials. Electrochemical methods can therefore be used for the determination of the rate of dissociation or recombination of a given substance with proton donors under appropriate experimental conditions for which the rate of protonation is the rate determining step 1. When evaluating the rate constants of chemical reactions coupled with charge-transfer (in the given case the recombination of the anion with H3 O÷) specific effects 2-6 (especially the influence of the double layer and of adsorption--see ref. ) relating to electrochemical techniques must be taken into account.
In this paper some effects which are related to the mechanism of proton transfer in water 7 will be demonstrated. Fluid water is known to maintain a certain degree of structure due to intermolecular hydrogen bonding. There is no commonly accepted model of the structure of fluid water 8. In some work the concept of a continuum model of water structure is used according to which all molecules are bound by H-bonds having various angles and distances 9. Some authors prefer a "mixture" or "two-state" modeP ° supposing a certain fraction of free water molecules, the rest being bound by H-bonds.
The fraction of molecules bound by hydrogen bonds varies with temperature and pressure v' ~ 0 and is also influenced by the presence of dissolved substances ~ 1-~ 5. Increase in temperature and pressure causes the breaking (or distortion) of hydrogen bonds and hence an increase in the number of free monomeric water molecules. However, the influence of electrolytes is not unambiguous. Whether an ion fn solution will act as a water structure breaker or vice versa depends on its charge and its size. It is generally valid that the higher the potential energy on the surface of a simple ion, the stronger is the orientation of water dipoles in its vicinity and an increase in the ordering of the water structure in the bulk of the solution occurs. The classification of ions as "water-structure makers" and "waterstructure breakers" is made using physico-chemical parameters considered to reflect in some way the influence of ions on water structure. One of these parameters is the B-coefficient in the Jones-Dole equation ~7 for the concentration dependence of the relative viscosity ~//~/0 rl/tlo = 1 + Ac~ + Bc .