A critical review of methods for obtaining ionic volumes in solution

The various major methods for obtaining individual or "absolute" ionic standard partial molar volumes V̄°(on) from whole electrolyte data in both aqueous and nonaqueous solutions are critically reviewed. A number of undetected errors in previous analyses are pointed out, and it is demonstrated that the reported agreement amongst the various methods in aqueous solution is largely fortuitous. All methods are shown to be unsatisfactory to varying degrees, with the reference electrolyte approach, using an electrolyte such as tetraphenylarsonium tetraphenylborate, appearing to be the least objectionable of those currently available. It is recommended that, subject to future theoretical and experimental developments, the assumption that, at 25°C: V̄°(Ph₄As⁺) - V̄°(BPh₄) = 8 cm³-mol^-1, or its equivalent: V̄°(Ph₄P⁺) - V̄°(BPh^-₄) = 2 cm³-mol^-1 be used in all solvents.