The hydration entropy of ions is calculated from their entropy in the ideal gaseous state and the standard entropy of their aqueous solutions. The application of the tetraphenylarsonium tetraphenylborate (TATB) and the cognate tetraphenylphosphonium tetraphenylborate (TPTB) assumptions in obtaining the latter is explored, using newly determined heat capacity data for crystalline tetraphenyl phosphonium and arsonium perchlorates, their redetermined solubility data, both over a temperature interval, and data from the literature. The results are compared with those based on the widely accepted estimate of the standard entropy of the aqueous hydrogen ion based on thermocell data.