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.
Si(OH)4 ⇌ SiO2↓ + 2H2O
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 and 200 to 1379 bar is given by the equation
log C = −731/T + 4.52
where C is the silica concentration in mg/kg and T is absolute temperature in kelvins. This equates to a maximum solubility of about 2 mmol/L 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.
Si(OH)4 ⇌ Si(OH)3O− + H+
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. Disilicic acid was synthesized by hydrogenation of its hexakis(methylphenoxy) derivative, (CH3C6H4O)3SiOSi(OC6H4CH3)3. 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 traditional 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.
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.
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 (around 1%) 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 administered 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 (ch-OSA) 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.
^Fournier, Robert O.; Rowe, Jack L. (1977). "The solubility of amorphous silica in water at high temperatures and high pressures" (PDF). American Mineralogist. 62: 1052–1056.
^ abGye, W. E.; Purdy, W. J. (1922). "The Poisonous Properties of Colloidal Silica. I: The Effects of the Parenteral Administration of Large Doses". British Journal of Experimental Pathology. 3 (2): 75–85. PMC2047780.
^Pauling, Linus (1960). The nature of the chemical bond (3rd. ed.). Ithaka, New York: Cornell University Press. p. 557.
^The figures here have been drawn using the interactive web site which feeds on annual DSi values from LEVITUS94: World Ocean Atlas 1994, an atlas of objectively analyzed fields of major ocean parameters at the annual, seasonal, and monthly time scales. Superseded by WOA98. Edited by Syd Levitus.
^Information, US Department of Commerce, NOAA National Centers for Environmental. "World Ocean Atlas 2009". www.nodc.noaa.gov. Retrieved 17 April 2018.
^ abThe figures here have been drawn using the interactive web site which feeds on annual DSi values from LEVITUS94: World Ocean Atlas 1994, an atlas of objectively analyzed fields of major ocean parameters at the annual, seasonal, and monthly time scales. Superseded by WOA98. Edited by Syd Levitus.
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^Exley C, Korchazhkina O, Job D, Strekopytov S, Polwart A, Crome P (2006). "Non-invasive therapy to reduce the body burden of aluminium in Alzheimer's disease". J. Alzheimers Dis. 10 (1): 17–24, discussion 29–31. doi:10.3233/jad-2006-10103. PMID16988476. S2CID 10817452.
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