NUCLEAR MAGNETIC RESONANCE DETERMINATION OF HYDRATION NUMBERS OF ELECTROLYTES IN CONCENTRATED AQUEOUS SOLUTIONS

Total effective hydration numbers have been determined for a number of alkali metal salts and alkaline earth metal salts by studying the temperature-dependent proton chemical shifts of their aqueous solutions. The hydration numbers obtained for the alkali metal halides were rationalized by assuming that the cation was coordinated to four waters whereas the anions contributed little if any to the hydration number obtained. The hydration numbers for the alkaline earth metal halides were found to include both primary and secondary waters which was attributed to their larger charge-size ratio. Also studied was the effect of several anions (Cl-, Br-, ClO4-, and p-toluenesulfonate-) on the effective hydration number of their sodium salts. Both NaClO4 and Na p-toluenesulfonate gave hydration numbers lower than NaCl and NaBr. The decrease in the effective hydration number was explained in terms of ion pairing, which was further supported by 23Na relaxation times.