Study on the Salting-out Effect using Silica Species by FAB-MS

The salting-out effect has been characterized on the basis of the relative peak intensity of silica species, observed by FAB-MS (fast atom-bombardment mass spectrometry) in solutions of sodium chloride, sodium nitrate, sodium sulfate, calcium chloride, lithium chloride and magnesium chloride. A critical change in the peak intensity ratios of the linear and cyclic tetramers of silica against the sodium ion (Na+) concentrations was observed at Na+ concentration between 0.1 and 1 mol⋅dm−3. The degrees of the changes of these peak intensity ratios increased in the order NaNO3 < Na2SO4 < NaCl. In CaCl2 solutions, these peak intensity ratios changed significantly at Ca2+ concentrations between 0.05 and 0.5 mol⋅dm−3. The salting-out effect observed is the total change in the concentration of silica brought about by complex factors, such as the changing solubility of silicate complexes, the increases in the concentrations of different kinds of soluble silicate complexes induced by changes in the hydrophobicity and hydrophilicity of the solution, and the contribution of hydrolysis.