Orthosilicic acid is a chemical compound with formula. It has been synthesized using non-aqueous solutions. It is assumed to be present when silicon dioxide dissolves in water at a millimolar concentration level.
Introduction
The term silicic acid has traditionally been used as a synonym for silica, SiO2. Strictly speaking, silica is the anhydride of orthosilicic acid, Si4. 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 in mg kg−1 and T is absolute temperature in K. 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 tetrakissilane, 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, Si3O36 and cyclic tetrasilicic acid, Si4O48 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 Si4. The traditional usage is retained in this article for quotes from cited publication which use it. The derivative Si3F has been characterized in aqueous solutions containing silicic acid and the fluoride ion. A fluorideIon selective electrode was used to determine its stability constant.
Oceanic silicic acid
In the uppermost water column the surface ocean is undersaturated with respect todissolved 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. Theoretical computations indicate that the dissolution of silica in water proceeds begins with the formation of a ·2 complex, which is converted to orthosilicic acid. The biogeochemical silica cycle is regulated by the algae known as the diatoms. These algae polymerise the silicic acid to so-called biogenic silica, which is used to construct their cell walls.
Plants and animals
Outside the marine environment compounds of silicon have very little biological function. Small quantities of silica are absorbed from the soil by some plants, to be then excreted in the form of phytoliths. Subcutaneous injections of orthosilicic acid solutions in mice were found to cause local inflammation and edema. Peritoneal injections of 0.1 mL of freshly prepared acid were often lethal. The toxicity decreased markedly as the solution aged, to the point that after the solution turned to a gel it had no effects other than mechanical ones. The solutions were equally toxic when administed by intravenous injection, but seasoned or gelled solutions were about as toxic as fresh ones. Research concerning the correlation of aluminium and Alzheimer's disease has included the ability of silicic acid in beer to reduce aluminium uptake in the digestive system as well as to increase renal excretion of aluminium. Choline-stabilized orthosilicic acid is a dietary supplement. It has been shown to prevent the loss of tensile strength in human hair; to have a positive effect on the surface and mechanical properties of skin, and on the brittleness of hair and nails; to abate brittle nail syndrome; to partially prevent femoral bone loss in aged ovariectomized rats; to increase the concentration of collagen in calves; and to have a potentially beneficial effect on the formation of collagen in the bones of osteopenic women.
