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I have used Davies equation to find what I believed was single ion activities and multiplied them on concentrations to define the thermodynamic dissociation constants for any kind of buffer to find equilibrium conditions. Thereby I could reproduce major findings in recent publications which apparently used the activity coefficients obtained from Davies equation in a similar way.

Now I hear from a reviewer:

Unfortunately a misunderstanding about calculated "single-ion activity coefficients" is being promoted in the manuscript.

From a mathematical expression for the mean activity coefficients an expression for single ion activity coefficients can be derived. But these single ion activity coefficients have no physical or thermodynamic meaning.

I know a heated debate on this issue has been going on. Wonder if any proposals for armistice could be suggested.

Perhaps to clarify my predicament, what I understand as single-ion activity are such as the "fs" in $$K_{12}=f_1[H^+]f_2[HA^{2-}]/f_1[H_2A^-]$$ to define the thermodynamic dissociation constant of a buffer.

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