The speed of sound c0 in the solution is also measured experimentally by a signal propagation time indicated in the POSITION of the signal maximum. For the determination of the ultrasound speed in the solution under study, we measured the time delay between the primary detected OA signal and its double propagation within the OA cell. This measurement is totally independent on the position of the OA source. All the quartz components of the OA cell were polished and plane-parallel. It made it possible to have the medium thickness fixed with the precision better than few μm for all the investigated solutions. Note that the value of c0 could be inconstant in the course of a chemical reaction due to the formation and decomposition of species affecting the thermophysical parameters of the medium. The relative error of the speed of sound measurement was lower than 0.5%.