Orthosilicic acid

Orthosilicic acid (/ˌɔːrθəsɪˈlɪsɪk/) is an inorganic compound with the formula Si(OH)4. Although rarely observed, it is the key compound of silica and silicates and the precursor to other silicic acids [H2xSiOx+2]n. Silicic acids play important roles in biomineralization and technology.[1][2][3]

Orthosilicic acid
IUPAC name
Orthosilicic acid
Other names
Silicic acid
3D model (JSmol)
ECHA InfoCard 100.030.421
EC Number
  • 233-477-0
  • InChI=1S/H4O4Si/c1-5(2,3)4/h1-4H Y
  • InChI=1S/H4O4Si/c1-5(2,3)4/h1-4H
  • InChI=1/H4O4Si/c1-5(2,3)4/h1-4H
  • O[Si](O)(O)O
Molar mass 96.113 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is YN ?)
Infobox references


Structure of Si(OH)4 stabilized by two chloride anions.

Typically orthosilicic acid is assumed to be a product of the hydrolysis of the ortho esters Si(OCR)4, as is practiced in sol-gel syntheses.[1] These conditions are however too vigorous to allow isolation of the parent acid.

Orthosilicic acid can be produced by Pd-catalyzed hydrogenolysis of tetrabenzoxysilicon:[4]

Si(OCH2Ph)4 + 4 H2 → Si(OH)4 + 4 CH3Ph

The acid was crystallized from a solution of dimethylacetamide and tetrabutylammonium chloride. As established by X-ray crystallography, the chloride anions interact with the acid via hydrogen bonds. Otherwise, the structure consists of the expected tetrahedral silicon center.


Chemical structure of Si4O4(OH)8.

Silicic acid readily condenses to give "higher" silicic acids including disilicic and cyclic-tetrasilicic acid:[4]

2 Si(OH)4 → O(Si(OH)3)2 + H2O
4 Si(OH)4 → (OSi(OH)2)4 + 4 H2O

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.[5] Orthosilicic acid is of particular interest as it is thought to be the form in which plants uptake silicon from the soil,[6][7] before being deposited as phytoliths throughout the plant, leading to research in the application of orthosilicic acid through foliar sprays to supplement plant growth.[8] Studies have demonstrated that foliar application of stabilized orthosilicic acid can alleviate abiotic stressors such as drought,[9][10] heavy metal toxicity,[11][12] and salinity,[13] resulting in increased yields.[14] Additionally, applications of orthosilicic acid have been demonstrated to reduce fungal infections and disease in plants,[15] 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[16] and the precipitation of silica in the apoplast of the plant.[17]

Oceanic silicic acid

2009 silicic acid concentration in the upper pelagic zone.[18]

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)
(orthoslicic acid) or its conjugate bases (orthosilicate anions) such as Si(OH)
and Si(OH)
. Theoretical computations indicate that the dissolution of silica in water proceeds through the formation of a SiO
complex and then orthosilicic acid.[19] The biogeochemical cycle of silica is regulated by the algae known as the diatoms.[20][21] These algae polymerise the silicic acid to so-called biogenic silica, used to construct their cell walls (called frustules).[22]

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.[23][24]


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  23. 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.
  24. "World Ocean Atlas 1994".
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