Silicic acid

Orthosilicic acid is a chemical compound with formula Si(OH)4. It has been synthesized using non-aqueous solutions. It is assumed to be present when silicon dioxide (silica) SiO2 dissolves in water at a millimolar concentration level.Dissolved silica concentration in the upper pelagic zone.Dissolved silica concentration at 10m depthDissolved silica concentration at 1000m depth Orthosilicic acid is a chemical compound with formula Si(OH)4. It has been synthesized using non-aqueous solutions. It is assumed to be present when silicon dioxide (silica) SiO2 dissolves in water at a millimolar concentration level. The term silicic acid has traditionally been used as a synonym for silica, SiO2. Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH)4. The solubility of silicon dioxide in water strongly depends on its crystal structure. The solubility of amorphous silica at the vapor pressure of solutions from 0 to 250 C is given by the equation where C is the silica concentration /mg kg−1 and T is absolute temperature. This equates to a maximum solubility of about 2 mmol L−1 at ambient temperatures. Attempts to produce more concentrated solutions result in the formation of silica gel. Because the concentration of orthosilicic acid in water is so low, the compounds that are present in solution have not been fully characterized. Linus Pauling predicted that silicic acid would be a very weak acid. The situation changed in 2017, when the orthosilicic acid monomer was obtained by hydrogenolysis of tetrakis(benzoyloxy)silane, (Si(OCH2C6H5)4, in solution in dimethylacetamide or related solvents. The crystal structure of this compound was determined by X-ray crystallography. Neutron diffraction was also used to determine the location of the hydrogen atoms. Di-silicic acid was synthesized by hydrogenation of its hexa-benzoyloxy derivative, R3-SiOSi-R3, R=CH3C6H4O. Cyclic trisilicic acid, Si3O3(OH)6 and cyclic tetrasilicic acid, Si4O4(OH)8 were synthesized by variations of this method. With these new discoveries, the term silicic acid has become ambiguous: in addition to the traditionally use as a synonym for silica, SiO2, it can now be used for the compound Si(OH)4. The traditional usage is retained in this article for quotes from cited publication which use it. The derivative Si(OH)3F has been characterized in aqueous solutions containing silicic acid and the fluoride ion. A fluoride Ion selective electrode was used to determine its stability constant. In the uppermost water column the surface ocean is undersaturated with respect to dissolved silica, except for the Antarctic Circumpolar Current south of 55°S. The dissolved silica concentration increases with increasing water depth, and along the conveyor belt from the Atlantic over the Indian into the Pacific Ocean.