The Identification of Chemical Species of Silica in Sodium Hydroxide, Potassium Hydroxide and Sodium Chloride Solutions by FAB-MS

Abstract

The chemical species produced by dissolving silica in sodium hydroxide (NaOH) and potassium hydroxide (KOH) solutions were measured by FAB-MS (fast atom bombardment mass spectrometry). As a system for a comparison, silica gel was dissolved in a sodium chloride (NaCl) solution. The pH of the NaCl solution was controlled and a NaCl solution containing silica was measured by FAB-MS. NaOH and KOH solutions contain the monomer (Si(OH)₃O^-; m/z=95), monomer-Na⁺ and K⁺ complexes (Si(OH)₂O₂Na^-; m/z=117, Si(OH)₂O₂K^-; m/z=133), dimer (Si₂(OH)₅O₂^-; m/z=173), dimer-Na⁺ and K⁺ complexes (Si₂(OH)₄O₃Na^-; m/z=195, Si₂(OH)₄O₂K^-; m/z=211), cyclic tetramer (Si₄(OH)₇O₅^-; m/z=311), linear tetramer (Si₄(OH)₉O₄^-; m/z=329) and cyclic tetramer-Na⁺ and K⁺ complexes (Si₄(OH)₆O₆Na^-; m/z=333, Si₄(OH)₆O₆K^-; m/z=349). The results for the monomer, dimer and cyclic tetramer silica observed in our study are consistent with the results of a 29-Si NMR measurement. The relative-intensity ratios of these species vary with the pH. It is notable that the production rate of the dimer is superior to those of other chemical species, such as the monomer, monomer-Na⁺, cyclic tetramer and linear tetramer in NaOH solution. In a KOH solution, the dimer concentration increases initially and becomes constant when the cyclic tetramer concentration starts to increase with increasing pH. In a NaCl solution, monomer-Na⁺ complexes (Si(OH)₂O₂Na^-; m/z=117, Si(OH)O₃Na₂^-; m/z=139), dimer (Si₂(OH)₅O₂^-; m/z=173), dimer-Na⁺ complexes (Si₂(OH)₄O₃Na^-; m/z=195, Si₂(OH)₃O₄Na₂^-; m/z=217, Si₂(OH)₂O₅Na₃^-; m/z=239), trimer (Si₃(OH)₇O₃^-; m/z=251), trimer-Na⁺ complexes (Si₃(OH)₆O₄Na^-; m/z=273, (Si₃(OH)₅O₅Na₂^-; m/z=295), cyclic tetramer (Si₄(OH)₇O₅^-; m/z=311), and cyclic tetramer-Na⁺ complexes (Si₄(OH)₆O₆Na^-; m/z=333, Si₄(OH)₅O₇Na₂^-; m/z=355) of silica are observed. NaCl solution contains monomer-Na⁺ (m/z=117), dimer (m/z=173), dimer-Na⁺ (m/z=195), cyclic tetramer (m/z=311) and cyclic tetramer-Na⁺ (m/z=333) complexes, which are in chemical equilibrium, and their relative intensity ratios are almost constant regardless of the pH of the solution.