Analytical Sciences

Abstract − Analytical Sciences, 15(12), 1241 (1999).

The Identification of Chemical Species of Silica in Sodium Hydroxide, Potassium Hydroxide and Sodium Chloride Solutions by FAB-MS
Miho TANAKA* + and Kazuya TAKAHASHI**
*The Tokyo University of Fisheries, Konan, Minato, Tokyo 108-8477, Japan
**The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-0198, Japan
The chemical species produced by dissolving silica in sodium hydroxide (NaOH) and potassium hydroxide (KOH) solu-tions 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. (Keywords: Dissolution of silica, FAB-MS, sodium chloride solution, identification of chemical species)