3D model (JSmol)
|Molar mass||96.11 g·mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Orthosilicic acid is the chemical compound with formula Si(OH)
4. It is assumed to be present in dilute solutions of silicon dioxide (silica) SiO
2 in water. It can be synthesized in non-aqueous solutions.
The term "silicic acid" has traditionally been used as a synonym for silicon dioxide, SiO2, also known as "silica". Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH)4, from which it can be obtained by a dehydration reaction.
- Si(OH)4 → SiO2↓ + 2H2O
The solubility of silica in pure water is about 1.5mM, or less, depending on the solid state structure of the compound. Because the concentration of orthosilicic acid is so low, it has not been fully characterized. It has been predicted to be "a very weak acid". More concentrated solutions of orthosilicic acid are unstable and turn into silica gel and other species.
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: it has been applied traditionally as a synonym for silica, SiO2, but it can now also be used for the compound Si(OH)4. The traditional meaning is retained in this article when it was used as such in a cited publication.
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 (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 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 (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.
Oceanic silicic acid
Dissolved silica (DSi) is a term used in the field of oceanography to describe the form of water-soluble silica, which is assumed to be Si(OH)
4 (orthoslicic acid) or its conjugate bases (orthosilicate anions) such as Si(OH)
2. Theoretical computations indicate that the dissolution of silica in water proceeds through the formation of a SiO
2O complex and then orthosilicic acid. The biogeochemical cycle of silica is regulated by the algae known as the diatoms. These algae polymerise the silicic acid to so-called biogenic silica, used to construct their cell walls (called frustules).
The silicic acids
For many decades it was debated whether orthosilicic acid, Si(OH)4, exists in aqueous solutions at ambient temperature; this uncertainty originated from the poor solubility of silicon dioxide, SiO2.
- Si(OH)4 (aq) SiO2↓ + 2 H2O
The solubility of silicon dioxide in water strongly depends on its crystal structure. The species that are present in a solution in equilibrium with a solid have not been definitively characterised because of their low concentrations. Many authors have speculated that silicic acid may be present, at sub-millimolar concentrations, in rivers, lakes and sea water.
Orthosilicic acid has been synthesized for the first time in the year 2017. It 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.
The conversion of mono-silicic acid to di-silicic acid
- 2 Si(OH)4 → (HO)3SiOSi((OH)3 + H2O
is a dehydration reaction, not an acid-base reaction; in consequence, di-silicic acid cannot be easily obtained from mono-silicic acid. 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.
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. The concentration of silicic acid was maintained below 2.5×10−3 mol dm−3.
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