|3D model (Jmol)||Interactive image|
|Molar mass||96.11 g·mol−1|
|Acidity (pKa)||9.84, 13.2|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Silicic acid /sɪˈlɪsɪk ˌæsɪd/ is a chemical compound. Silicic acid is the general name for a family of chemical compounds containing the element silicon attached to oxide and hydroxyl groups. This family of compounds have the general formula [SiOx(OH)4-2x]n. Some simple silicic acids have been identified, but only in very dilute aqueous solution, such as metasilicic acid (H2SiO3), orthosilicic acid (H4SiO4, pKa1=9.84, pKa2=13.2 at 25 °C), disilicic acid (H2Si2O5), and pyrosilicic acid (H6Si2O7); however in the solid state these probably condense to form polymeric silicic acids of complex structure.
In general, silicic acid reactions are difficult to control. Partial dehydration to metasilicic acid, for example, is challenging because the reaction typically progresses all the way to silicon dioxide and water.
Naturally occurring silicic acid is produced by a non-biological process called hydration involving water, and quartz, which is known to be common on Earth. The reaction producing silicic acid from quartz can be written as:
2 + Water2 H
2O → Silicic acidH
If the products are left in air dry products are formed.
Silicic acid can be produced by the acidification of sodium silicate in aqueous solution. The main problem in using silicic acids in chemical synthesis is that silicic acids readily lose water to form randomly polymeric silica gel, a form of silicon dioxide. Such conversion involve condensations.
Oceanic silicic acid
Dissolved silica (DSi) is a term used in the field of oceanography to describe the form of water-soluble silica as silicon hydroxide which can be measured by standard analyses (e.g. Strickland and Parsons, 1972). However, the term dissolved silica excludes silicate occurring as silicate minerals, which is a class of minerals forming rings, sheets, chains, and tetrahedrons. Likewise, the term dissolved silica is different from the term silicone, which is organic polymers of silicon. There exist three different species of DSi in natural waters:
DSi (in this context silicic acid) is created near the ocean’s surface, by the process of hydration. It is carried into the abyssopelagic zone by falling water in the poles. Uncontrolled build-up of silicic acid in the ocean is naturally checked – although human influence can upset this natural regulation. It is primarily removed by conversion to silicon dioxide and water. In the oceans, silicon exists primarily as orthosilicic acid (H4SiO4), and its biogeochemical cycle is regulated by the group of algae known as the diatoms. These algae polymerise the silicic acid to so-called biogenic silica, used to construct their cell walls (called frustules).
Marine diatoms transport Si(OH)4.
Silicic acid in health
Continuing research of the correlation of aluminium and Alzheimer’s disease has in the last few years included the use of silicic acid in beverages, due to its abilities to both reduce aluminium uptake in the digestive system as well as cause renal excretion of aluminium.
Choline-stabilized orthosilicic acid is a bioavailable nutritional supplement. It has been shown to prevent the loss of hair tensile strength, have positive effect on skin surface and skin mechanical properties, and on brittleness of hair and nails, abate brittle nail syndrome, partially prevent femoral bone loss in the aged ovariectomized rat model, increase collagen concentration in calves, and have potential beneficial effect on bone collagen formation in osteopenic females.
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- 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.
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