Ionic vacancies created in an electrode reaction freely transfer like gas molecules in free space without collisions of water molecules in the electrolyte solution. Based on the kinetic model of gases, the collision process between the ionic vacancies with opposite charges was theoretically examined, and to calculate the maximum excess heat emitted by their pair annihilation, the diagram of the maximum collision efficiency against vacancy collision-cross-section was established from the previous data. Then, using double and triple bipolar Pt-magnetohydrodynamic (MHD) electrodes as well as a single one, we measured the maximum excess heat of the pair annihilation of monovalent vacancies in a ferricyanide-ferrocyanide redox reaction (FFRR) under a magnetic field of 15 T. The measured excess heats were in good agreement with the excess heats determined from the diagram.