  The existence of an isosbestic point in the OH bond vibration band of H2O
molecules in solutions of strong electrolytes has been shown by many research
groups but its physical meaning has been only recently read as the equilibrium
point between two populations of water molecules where either water-ion or
water-water interactions respectively prevail. Thus the isosbestic point
provides useful information on the bulk displacement of water molecules in
presence of solutes. Herein we show that the isosbestic point occurrence is
accurately described by an equation depending on the balance of the
electrostatic forces of the ion and its counter-ion. Furthermore, its
wavelength is also related to the slope of the density vs. solute
concentration. This support the view that the effect of the solute can be
barely regulated within few hydration shells.
