Useful information on the polymerization of silicic acid is obtained from a combined use of (1) spectrophotometry based on the formation of yellow molybdosilicic acid, (2) trimethylsilylation of silicic acid and determination of trimethylsilyl derivatives by gas-liquid chromatography and (3) gel permeation chromatography of silicic acid. The polymerization of silicic acid is divided into three processes: (1) the formation of polysilicic acid by the reaction between molybdate reactive silicic acids (MRS acid), (2) the particle growth of polysilicic acid by the reaction between MRS acid and polysilicic acid, and (3) the particle growth of polysilicic acid by the reaction between polysilicic acids. The pH value giving maximum rate of polymerization of silicic acid is different among these three processes. Fluoride promotes processes (1) and (2); the effect of fluoride on the process (3) is relatively small.
JapanIn the deposition of siliceous material from geothermal water, a relationship between chemical com positions of geothermal waters and siliceous deposits was investigated. The results indicated that iron and aluminium were concentrated into these siliceous deposits. For the chemical state of the iron in the deposits, some oxides and oxyhydroxides, sulfide and amorphous iron silicate were identified by powder X-ray diffraction and "Fe Mossbauer spectroscopy. For the aluminium, two states were detected by 27A1 MAS NMR: 6-coordinate aluminium (such as aluminium hydroxide) and 4-coordinate aluminium substituting a Si atom site in the deposit. In some deposit samples, both 4 and 6-coordinate aluminium were detected, but in others only the former was found.
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