Water activity is generally considered to affect ionic association in aqueous electrolyte solutions; however, it is usually ignored when the association reactions or constants are discussed. In this work, the effect of water activity on the association reaction (Ni2+(aq) + nCl-(aq) ⇔ NiCl2-nn(aq) was investigated by EXAFS and UV-Vis spectroscopy measurements on NiCl2 aqueous solutions at room temperature with constant Cl-/Ni2+ ratio (~66) and various water activities; the latter were adjusted by adding MgCl2 and Mg(ClO4)2. Both the EXAFS and the UV-Vis spectra measurements indicated that the extent of Ni-Cl association increases in decreasing water activity environments, independent on the Cl-/Ni2+ ratio. Thus, the effect of water activity on the ionic association should not be ignored and more emphasis should be paid on it, especially, when the water activity is very low.
- UV-Vis spectroscopy
- Water activity