NMR Studies of Aqueous Electrolyte Solutions. I. Hydration Number of NaCl Determined from Temperature Effects on Proton Shift

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Abstract
Proton chemical shifts of purified water and of aqueous sodium chloride solutions are measured as a function of temperature and concentration. The shifts are referenced with respect to the resonance signal of gaseous ethane, which does not vary with temperature. An increase in temperature causes an upfield shift. Increasing sodium chloride concentration also causes an upfield shift. The shift of the single resonance line observed is interpreted as being a time average of the proton in its different envorinments, namely, in the normal water structure and in a form hydrated to the ions. Although water in the secondary layer may be affected by the ions, such effects are neglected. It is argued that the shift of a proton in the hydrated form does not vary with temperature, whereas the shift of a proton in the normal water structure increases with increasing temperature due to the breaking of hydrogen bonds. From this simple interpretation, the total primary, or so-called ``effective'' hydration number of sodium chloride, is determined to be 4.