Mac Innes

Mac Innes D. A. The Principles of Electrochemistry. New York, Rcinhold Publishing Corporation 1939. 

We have already shown that the experiment only allows us to determine the mean activity coefficients. If we could determine the activity coefficient of a single ion, we could deduce that of all other ions step-by-step. To do this, Mac Innes benefitted from the fact that in potassium chloride, the chloride and the potassium ion have the same charge in absolute value, the same electronic stmcture and therefore the same size and the same mobility but slightly different masses, and therefore worked out that they should have, in the same solution, the same activity coefficient. From the mean activity coefficient of potassium chloride, we can deduce those of individual ions using the relation ... 

Mac Innes, D.A. (1939), The Principles of Electro-Chemistry , Reinhold Publishing Corp, New York. 

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