Orthosilicic acid () is an inorganic compound with the formula . Although rarely observed, it is the key compound of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

and silicates and the

precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...

to other silicic acids . Silicic acids play important roles in

biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...

and technology.N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.

Isolation

Typically orthosilicic acid is assumed to be a product of the hydrolysis of the ortho esters , as is practiced in sol-gel syntheses. These conditions are however too vigorous to allow isolation of the parent acid. Orthosilicic acid can be produced by Pd-catalyzed hydrogenolysis of tetrabenzoxysilicon: : The acid was crystallized from a solution of dimethylacetamide and

tetrabutylammonium chloride Tetrabutylammonium chloride is the organic compound with the formula (C4H9)4NCl. A white water-soluble solid, it is a quaternary ammonium salt of chloride. It is a precursor to other tetrabutylammonium salts. Often tetrabutylammonium bromide ...

. As established by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, the chloride anions interact with the acid via hydrogen bonds. Otherwise, the structure consists of the expected tetrahedral silicon center.

Reactions

Silicic acid readily condenses to give "higher" silicic acids including disilicic and cyclic-tetrasilicic acid: : : These derivatives have also been characterized crystallographically.

Orthosilicic acid in plants

Silicon has been explored as a nutrient for plant growth, with silica comprising up to 10% of plant weight on a dry matter basis. Orthosilicic acid is of particular interest as it is thought to be the form in which plants uptake silicon from the soil, before being deposited as phytoliths throughout the plant, leading to research in the application of orthosilicic acid through foliar sprays to supplement plant growth. Studies have demonstrated that foliar application of stabilized orthosilicic acid can alleviate abiotic stressors such as drought, heavy metal toxicity, and salinity, resulting in increased yields. Additionally, applications of orthosilicic acid have been demonstrated to reduce fungal infections and disease in plants, suggesting the possibility of using stabilized orthosilicic acid as an alternative or complement to existing disease control measures. The mechanisms by which orthosilicic acid alleviates abiotic stress and controls diseases is not well understood; current theories advanced include the activation of plant defense reactions and the precipitation of silica in the apoplast of the plant.

Oceanic silicic acid

''Dissolved silica'' (DSi) is a term used in the field of oceanography to describe the form of water-soluble

, which is assumed to be (orthoslicic acid) or its conjugate bases (orthosilicate anions) such as and . Theoretical computations indicate that the dissolution of silica in water proceeds through the formation of a ·2 complex and then orthosilicic acid. The biogeochemical cycle of

is regulated by the algae known as the diatoms.Siever, R. (1991). Silica in the oceans: biological-geological interplay. In: Schneider, S. H., Boston, P. H. (eds.), ''Scientists On Gaia'', The MIT Press, Cambridge MA, USA, pp. 287-295. These algae

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

ise the silicic acid to so-called

biogenic silica Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plant ...

, used to construct their cell walls (called

frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on ...

s). In the uppermost water column the surface

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

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.

References

{{reflist Oxoacids Silicon compounds