Ultrasonic study of hydrochloric acid association in N,N-dimethylformamide

Ultrasonic absorption and velocity have been measured in anhydrous N,N-dimethylformamide solutions of hydrogen chloride, over the frequency range 13 to 143 mc/s, and the concentration range 0.05 M to 3.5 M. A large relaxation is observed and ascribed to the association of the acid. The rate constants for this reaction are obtained, and their variations with concentration are discussed. The association rate appears to be entirely diffusion controlled, and the corresponding "reaction distance" is large. This may be related to the very large effect on the sound velocity (and hence on the compressibility) that arises from the introduction of these ions.