Single-ion activity: experiment versus theory.

The almost century-old dispute over the validity of the experimentally derived activity of a single ion, a(i), is still unsettled; current interest in this issue is nourished by recent progress in electrochemical cell measurements using ion-specific electrodes (ISEs) and advanced liquid junctions. Ionic solution theories usually give expressions for a(i) values of the positive and negative ions, that is, the respective a(+) and a(-), and combine these expressions to compute the mean ionic activity, a(±), that is indisputably a thermodynamically valid property readily derivable from experiment. Adjusting ion-size parameters optimizes theory's fit with experiment for a(±) through "optimizing" a(+) and a(-). Here I show that theoretical a(i) values thus obtained from the smaller-ion shell treatment of strong electrolyte solutions [Fraenkel, Mol. Phys. 2010, 108, 1435] agree with a(i) values estimated from experiment; however, theoretical a(i) values derived from the primitive model, the basis of most modern ionic theories, do not agree with experiment.