The ultrasonic absorption of solutions of alkali halides in dimethyl sulphoxide is studied by the pulse technique over the frequency range from 3.5 to 60 MHz. A relaxation is observed between 2 and 10 MHz for each salt studied. This relaxation is assigned to the second equilibrium in the two step formation of a contact ion pair from free ions in the solution.
[M+]sol.+X-k12<->K21[M+ DMSO X-]sol.k23<->K32[M+X-]sol.
The rate constants for this second step have an upper limit of 10^7 sec^-1. ΔVs for this step is found to be about 2 cm^3mole^-1.
There is evidence that a relaxation at a frequency higher than the range of the apparatus is occuring. This may be due to the first step which is diffusion controlled and much faster.