Response to: “Comments on ‘Ideal Behavior of Water Solutions of Strong Electrolytes and Non-electrolytes at High Concentrations’ by A.A. Zavitsas {Journal of Solution Chemistry 39, 301–317 (2010)}” by A. Apelblat

Abstract

Apelblat and I agree that “non-ideality is often reduced to deviations from Raoult’s law.” He also agrees that my treatment of the experimental data produces results consistent with Raoult’s Law and “fulfills the purpose very satisfactorily.” He also states “However, if the activity coefficient γ1(x1) is identically equal to unity for any x1, this does not mean that the system is thermodynamically ‘ideal’ but only indicates the possibility of compensation between the enthalpy and entropy terms as often is observed in organic mixtures.” Granting that such fortuitous, but exact, compensation is conceivable so that the activity of water is equal to unity over wide ranges of concentration, one has to consider the probability that such fortuitous exact compensation occurs with every single one of the numerous cases I have treated. It would seem unlikely that such lucky coincidence would occur in every single case. Referring to organic mixtures that can show such compensation occasionally is not relevant to solutions of table salt in water. I certainly agree with Apelblat that “the hydration number is a rather inadequately defined quantity.” This is why very different hydration numbers are obtained by different techniques, e.g., diffraction studies (neutron diffraction, X-ray diffraction, EXAFS, etc.), dielectric relaxation, diffusion coefficients, electrostriction, spectroscopic measurements (IR, NMR), colligative properties, molecular dynamics simulations, ion-water gas-phase clusters, etc. This is emphasized in my article. To avoid confusion, I felt it appropriate to give HT some kind of a different name. I gave it the designation “thermodynamic hydration number” not to indicate that it is “more worthy and certain” than other hydration numbers, but because HT values agree remarkably well with the number of water molecules adhering to the solute by binding energies greater than about 56 kJ·mol−1. There was no attempt on my part to imply that HT is better than “hydration numbers” reported by others, as Apelblat seems to suggest. Apelblat asserts that it would be “strange” if we start calling hydration numbers “ ‘ultrasound’ hydration number” or “ ‘spectroscopic’ hydration number”, etc. Doing