Silicic acid

Silicic acids can be seen as hydrated forms of silica, namely {{chem2|2 H_{2x}SiO_{x+2} \d SiO2*(H2O)_{x} }}. Indeed, in concentrated solutions, silicic acids generally polymerize and condense, and ultimately degrade to silicon dioxide and water. The intermediate stages may be very thick liquids or gel-like solids. Dehydrating the latter yields a hard translucent form of silica with atomic-scale pores, called silica gel, which is widely used as water absorbent and drying agent. Silica dissolves very sparingly in water and is present in seawater at concentrations below 100 parts per million. In such dilute solutions, silica is assumed to exist as orthosilicic acid. Theoretical computations indicate that the dissolution of silica in water proceeds through the formation of a complex and then orthosilicic acid. The silicon–oxygen double bond of metasilicic acid, implied by the formula , is hypothetical or highly unstable. Such double bonds can be hydrated to a pair of hydroxyl () groups: The degree of polymerization of silicic acids in water solution can be determined by its effect on the freezing point of the solution (cryoscopy). == History ==

Silicic acid was invoked by Jöns Jacob Berzelius in the early 19th century to explain the dissolution of silicon dioxide (silica, quartz) in water, namely through the hydration reaction: :\overset{silica}{SiO2} + \overset{water}{H2O} \overset{silicic\ acid}{H2SiO3} Based on the vapor pressure curves for silica gel, Reinout Willem Van Bemmelen argued that no silica hydrates existed, only silica gel. On the other hand, Gustav Tschermak von Seysenegg believed that he had observed different silicic acids as decomposition products of natural silicate gels. The first crystalline silicic acid was prepared from the phyllosilicate natrosilite () in 1924. More than 15 crystalline acids are known and comprise at least six modifications of . Some acids can adsorb and intercalate organic molecules, and therefore are interesting alternatives to silica. == Preparation ==

Crystalline silicic acids can be prepared by removing the sodium cations from solutions of sodium silicates with an ion-exchange resin, or by treating sodium silicates with concentrated sulfuric acid. == See also ==