For concentrated ionic solutions the hydration of ions must be taken into consideration, as done by Stokes and Robinson in their hydration model from 1948. The activity coefficient of the electrolyte is split into electric and statistical components by E. Glueckauf who modifies the Robinson–Stokes model.
The statistical part includes hydration index number , the number of ions from the dissociation and the ratio  between the apparent molar volume of the electrolyte and the molar volume of water and molality .
Concentrated solution statistical part of the activity coefficient is:
The Stokes–Robinson model has been analyzed and improved by other investigators as well.