Ion Pairs in Aqueous Magnesium Nitrate Solution by Excess Raman Spectroscopy

Abstract

We study the ion associations in magnesium nitrate solution by Raman spectroscopy.Utilizing the excess spectra and peak decomposition,we analyze the-OH stretching and NO3 symmetric stretching regions.Analysis of the-OH stretching region demonstrates that the amount of water molecules in the first hydration shell of the anion follows a linear relationship in the low concentration range(2.3 mol · kg-1),but deviates from this linear relationship at high concentrations(2.3 mol · kg-1).The contact ion pairs at high concentration result in the nonlinear variation.In the NO3 symmetric stretching region,the contact ion pairs and solvent separated ion pairs exist in the Mg(NO3)2 concentration region of 0.23-4.86 mol · kg-1.From a global fitting of the Raman spectra over the whole concentration region,we deduce that the concentration of ion pairs varies.When the Mg(NO3)2 concentration is below 2.3 mol · kg-1,the relative amounts of all ion pairs increase with the concentration.However,above this concentration,the relative amount of contact ion pairs increases sharply,and solvent separated ion pairs either decrease or increase in their slower rate,until the Mg(NO3)2 concentration is 3.5 mol · kg-1,and above this concentration the relative amount of one kind of solvent separated ion pair does not change.