i . Furthermore, we can split the total electrochemical potential µ i into the ideal gas and the excess contribution, βµ i = ln(Λ 3 i c i ) + βµ ex i , where the mean ionic concentration is c i = N i /V 0 . Note that µ ex i contains all the interactions, including the contribution from the mean field electrostatic potential. The activity a H of hydronium ions is then: