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Inchemistry, asilicic acid (/sɪˈlɪsɪk/) is anychemical compound containing the elementsilicon attached tooxide (=O) andhydroxyl (−OH) groups, with the general formula[H2xSiOx+2]n or, equivalently,[SiOx(OH)4−2x]n.[1][2]Orthosilicic acid is a representative example. Silicic acids are rarely observed in isolation, but are thought to exist inaqueous solutions, includingseawater, and play a role inbiomineralization.[3] They are typically colorless weak acids that are sparingly soluble in water. Like thesilicate anions, which are their better knownconjugate bases, silicic acids are proposed to beoligomeric orpolymeric. Nosimple silicic acid has ever been identified, since these species are primarily of theoretical interest.
Depending on the number of silicon atoms present, there are mono- and polysilicic (di-, tri-, tetrasilicic, etc.) acids. Well defined silicic acids have not been obtained in a form that has been characterized byX-ray crystallography.
| Formula | Name | Computed Structure |
|---|---|---|
| H4SiO4 orSi(OH)4 | orthosilicic acid |  |
| H2SiO3 orSiO(OH)2 | metasilicic acid |  |
| H6Si2O7 orO(Si(OH)3)2 | pyrosilicic acid |  |
| H2Si2O5 orSi2O3(OH)2 | disilicic acid |  |
Silicic acids can be seen as hydrated forms of silica, namely2 H2xSiOx+2 = SiO2·(H2O)x. Indeed, in concentrated solutions, silicic acids generally polymerize and condense, and ultimately degrade to silicon dioxide andwater. The intermediate stages may be very thick liquids orgel-like solids.[4][5][6] Dehydrating the latter yields a hard translucent form of silica with atomic-scale pores, calledsilica gel, which is widely used as water absorbent anddrying agent.
Silica dissolves very sparingly in water[citation needed] and is present in seawater at concentrations below 100parts 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 aSiO2·2H2O complex and then orthosilicic acid.[7]
The silicon–oxygen double bond of metasilicic acid, implied by the formulaH2SiO3, is hypothetical or highly unstable. Such double bonds can behydrated to a pair ofhydroxyl (−OH) groups:[4]
For example,
or
Alternatively, metasilicic acid is liable to form cyclic polymers[−SiO(OH)2−]n, which can be opened by hydration to chain polymersHO[−SiO(OH)2−]nH. Similarly, disilicic acid is liable to form complex polymers with a tetravalent unit,[=Si2O3(OH)2=]n. Conversely,oligomeric and polymeric acids may depolymerize byhydrolysis of theSi−O−Si bridges, or such bridges may be created bycondensation:
Like organicsilanols, silicic acids are weak acids. Orthosilicic acid has calculateddissociation potentials pKa1 = 9.84, pKa2 = 13.2 at 25 °C.
Silicic acids and silicates in solution react withmolybdate anions, yielding yellowsilicomolybdate complexes. This reaction has been used totitrate the content of silicon in water solutions and determine their nature. In a typical preparation,monomeric orthosilicic acid was found to react completely in 75 seconds,dimeric pyrosilicic acid in 10 minutes, and higheroligomers in considerably longer time. The reaction is not observed withcolloidal silica.[8]
The degree of polymerization of silicic acids in water solution can be determined by its effect on thefreezing point of the solution (cryoscopy).[8]
Silicic acid was invoked byJöns Jacob Berzelius in the early 19th century to explain the dissolution ofsilicon dioxide (silica, quartz) in water, namely through thehydration reaction:
Based on thevapor pressure curves forsilica gel,Reinout Willem Van Bemmelen argued that nosilica hydrates existed, only silica gel. On the other hand,Gustav Tschermak von Seysenegg believed that he had observed different silicic acids as decomposition products ofnatural silicate gels.[9]
The first crystalline silicic acid was prepared from thephyllosilicatenatrosilite (Na2Si2O5) in 1924. More than 15 crystalline acids are known and comprise at least six modifications ofH2Si2O5. Some acids canadsorb andintercalate organic molecules, and therefore are interesting alternatives to silica.[10]
Crystalline silicic acids can be prepared by removing the sodium cations from solutions ofsodium silicates with anion-exchange resin, or by treatingsodium silicates with concentratedsulfuric acid.[10]
