Abstract
The electrical conductances of aqueous solutions of magnesium and manganese (II) sulphates and benzenedisulphonates have been measured at several frequencies from 1 KHz to 50 MHz, using two radio-frequency transformer ratio-arm bridges and an audio-frequency Wheatstone bridge. The radio-frequency conductances were determined using a relative method with potassium chloride as a reference electrolyte. The conductance data were analysed by extrapolation and minimisation techniques using the theoretical conductance equations of Fuoss-Hsia[(13,15)], Pitts[(7,14)], Falkenhagen-Leist-Kelbg[(4)], and Murphy-Cohen[(17)]. It was found that the Murphy-Cohen equation was not able to yield a satisfactory interpretation of the audio-frequency data of the bi-bivalent electrolytes examined in this work. The findings of Atkinson et al.[(43, 44)] that magnesium and manganese benzenedisulphonates could be treated as essentially unassociated were not confirmed by the measurements and analyses of the present work. The increases in conductance brought about by the high-frequency field could be explained in terms of ion atmosphere relaxation plus the effect of the relaxation of the ion-pair equilibrium as calculated by Gilkerson[(30)]. All four electrolytes studied showed variations in the values of their association constants with frequency which were attributed to relaxation of the ion-pair equilibrium. This effect, however, was not significant below about 10 MHz. Rate constants for the ion pair dissociation rate have been estimated from the Gilkerson theory. The conductance dispersion maximum at 2 - 3 MHz for manganese sulphate found by Rance[(41)] has not been detected in the present work. It is suggested that the result of Rance was due to inconsistencies in the measurement apparatus then used.