Properties of Water Solutions of Electrolytes and Nonelectrolytes

Abstract

Apparent large deviations of water solutions from ideal behavior are eliminated by taking account of the number of water molecules binding to solute sufficiently strongly (13.0 ± 1.5 kcal mol -1 ) as to be removed from the “bulk” solvent. Freezing point, boiling point, vapor pressure, and osmotic pressure measurements of electrolyte solutions of chlorides, bromides, and iodides are treated successfully, as are those of nonelectrolytes, for up to 50 wt % solute and concentrations hundreds of time greater than those over which Debye−Hückel theory applies. D−H theory focused on the behavior of ions; this work focuses on the nature of water. The nature of “free” water as solvent is changed by electrolytes. No evidence is found for ion pairs up to eutectic points. Hydration numbers ( h ) obtained are in agreement with many from more elaborate techniques. From freezing points, h values are as follows: H +, 6.7 ± 0.7; Li +, 6.6 ± 0.6; Na +, 3.9 ± 0.5; K +, 1.7 ± 0.5; NH 4 +, 1.8 ± 0.5; Mg 2+, 13 ± 2; Ca 2+, 12 ± 2; Sr 2+, 12 ± 2; Ba 2+, 10.5 ± 1.5; Al 3+, 22 ± 2; Fe 3+, 18 ± 2; Cl -, 0; Br -, 0; I -, 0; methanol, 1 ± 0.3; ethylene glycol, 1.8 ± 0.3; glycerol, 2 ± 0.5; glucose, 2.8 ± 0.5; sucrose, 5 ± 0.5; H 2 O 2, 1.2 ± 0.2; NH 3, 1.81 ± 0.05; and urea, 0 ± 0.5. From vapor pressures, h values are as follows: Rb +, 1.8 ± 1; Cs +, 0.6 ± 1. Cl -, Br -, and I - are solvated and not hydrated. Sequential binding energies of water to gas-phase ion−water clusters correlate with h, as do cluster distributions. Some h values show temperature dependence.