Estimation of Silica Species’ Concentrations in Lithium and Magnesium Chloride Solutions from the Peak Intensities Observed by FAB-MS

Abstract

The concentrations of dissolved silica species in electrolyte solutions were derived from the relative intensities of silica species, obtained from FAB-MS measurements (fast atom bombardment mass spectrometry), and the total concentration of dissolved silica. Generally, silica species in aqueous solutions form various complexes with cations such as sodium (Na+) or calcium (Ca2+), and it has been difficult to determine the concentration of each species. From the observed results from FAB-MS, the chemical species of silica dissolved in lithium chloride (LiCl) and magnesium chloride (MgCl2) solutions do not include complexes with these cations, and thus Li+ and Mg2+ do not replace protons of the silanol groups in silica. Therefore, in LiCl and MgCl2 solutions, all of the simple structures of silicate species can be identified. The concentration of each silica species was estimated on the basis of its mass spectra peak intensities and the total concentration of silica as determined by colorimetry. This study yields the concentration of each silica species within small errors, whereas conventional methods (such as 29Si-NMR) have not yielded the concentrations of individual silica species. From these results, dimers and cyclic tetramers are concluded to be the main species in silica solutions with concentrations of at most 0.1 to 0.2 μmol⋅dm−3. This tendency should also occur in NaCl and CaCl2 solutions, which are major electrolytes in natural waters.