Coordination numbers and hydration of the ions Li⁺, Na⁺, K⁺, Cl⁻, Br⁻, and I⁻ in aqueous solutions

Summary

• 1. The integral heats of solution of LiCl, NaCl, KCl, NaBr·2H₂O, NaI·2H₂O and N(CH₃)₄Br in aqueous solution of HCl of various concenrations have been measured at 25° in a calorimeter with an isothermal jacket.

• 2. A thermochemical method for the evaluation of coordination numbers of ions in aqueous solutions has been developed. The coordination numbers of Li⁺, Na⁺, K⁺, Cl⁻, Br⁻, I⁻ have been determined at 25°. In dilute solutions they are close to the mean coordination number of the water molecule in water.

• 3. The following expression is proposed as a measure of the hydration of ions;$\rho ' = \frac{n}{{4\pi (r + 1.38)^2 }}$, where$\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{n}$ is the coordination number of the ion in solution, ī and 1.38 are the radii (in A) of the ion and of the water molecule respectively,ρ is the surface density of distribution of water molecules in the first coordination layer of the ions. For ions that undergo hydrationρ' >ρH₂O, and for non-hydrating ions (negative hydration)ρ' <ρ'H₂O.