Silicic acid

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In

conjugate bases, silicic acids are proposed to be oligomeric or polymeric. No simple
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 by X-ray crystallography.

Examples

Formula Name Computed Structure
H
4SiO
4 or Si(OH)
4		 orthosilicic acid
H
2SiO
3 or SiO(OH)
2		 metasilicic acid
H
6Si
2O
7 or O(Si(OH)
3)
2		 pyrosilicic acid
H
2Si
2O
5 or Si
2O
3(OH)
2		 disilicic acid

Reactions

Silicic acids can be seen as hydrated forms of silica, namely 2 H2xSiOx+2 = 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.[4][5][6] 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[

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 SiO2·2H2O complex and then orthosilicic acid.[7]

The silicon–oxygen double bond of metasilicic acid, implied by the formula H2SiO3, is hypothetical or highly unstable. Such double bonds can be

hydroxyl (−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 polymers HO[−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 by hydrolysis of the Si−O−Si bridges, or such bridges may be created by condensation:

Like organic silanols, silicic acids are weak acids. Orthosilicic acid has calculated dissociation potentials pKa1 = 9.84, pKa2 = 13.2 at 25 °C.

Silicic acids and silicates in solution react with

dimeric pyrosilicic acid in 10 minutes, and higher oligomers in considerably longer time. The reaction is not observed with colloidal silica.[8]

The degree of polymerization of silicic acids in water solution can be determined by its effect on the

cryoscopy).[8]

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:

Based on the

natural silicate gels.[9]

The first crystalline silicic acid was prepared from the

phyllosilicate natrosilite (Na2Si2O5) in 1924. More than 15 crystalline acids are known and comprise at least six modifications of H2Si2O5. Some acids can adsorb and intercalate organic molecules, and therefore are interesting alternatives to silica.[10]

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.[10]

See also

References

  1. ISBN 978-0-08-050109-3
    . Retrieved July 27, 2022.
  2. ISBN 0-471-02404-X
    .
  3. ^ Frank Wigglesworth Clarke (1914). "The Silicic Acids" (PDF). The Constitution of the Natural Silicates. pp. 10–19.
  4. ^
    PMC 2047780
    .
  5. doi:10.1021/j150541a046
    .
  6. doi:10.1086/625982
    .
  7. doi:10.1016/j.cplett.2009.07.063
    .
  8. ^
    doi:10.1021/ja01118a054
    .
  9. doi:10.1002/cber.19240570850
    .
  10. ^
    doi:10.1002/14356007.a23_661
    .
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