In chemistry, a silicic acid (/sɪˈlɪsɪk/) is any chemical compound containing the element silicon attached to oxide (=O) and hydroxyl (−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 in aqueous solutions, including seawater, and play a role in biomineralization.[3] They are typically colorless weak acids that are sparingly soluble in water. Like the silicate anions, which are their better known conjugate bases, silicic acids are proposed to be oligomeric or polymeric. No simple silicic acid has ever been identified, since these species being 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.
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[citation needed] 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 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 hydrated to a pair of 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 molybdate anions, yielding yellow silicomolybdate complexes. This reaction has been used to titrate 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 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 freezing point of the solution (cryoscopy).[8]
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]
^Greenwood, N. N.; Earnshaw, A. (2012). Chemistry of the Elements. Elsevier Science. p. 346. ISBN978-0-08-050109-3. Retrieved July 27, 2022.
^Iler, R. K. (1979). The Chemistry of Silica. New York: Wiley. ISBN0-471-02404-X.
^Frank Wigglesworth Clarke (1914). "The Silicic Acids" (PDF). The Constitution of the Natural Silicates. pp. 10–19.
^ abW. E. Gye and W. J. Purdy (1922): "The Poisonous Properties of Colloidal Silica. I: The Effects of the Parenteral Administration of Large Doses". British Journal of Experimental Pathology, volume 3, issue 2, pages 75–85. PMC 2047780.
^Katsumi Goto (1956): "Effect of pH on Polymerization of Silicic Acid". Journal of Physical Chemistry, volume 60, issue 7, pages 1007–1008. doi:10.1021/j150541a046.
^Hans Ramberg (1952): "Chemical Bonds and Distribution of Cations in Silicates". The Journal of Geology, volume 60, issue 4, pages 331–355. doi:10.1086/625982.
^Bhaskar Mondal, Deepanwita Ghosh, and Abhijit K. Das (2009): "Thermochemistry for silicic acid formation reaction: Prediction of new reaction pathway". Chemical Physics Letters, volume 478, issues 4–6, pages 115–119. doi:10.1016/j.cplett.2009.07.063.
^ abG. B. Alexander (1953): "The Reaction of Low Molecular Weight Silicic Acids with Molybdic Acid". Journal of the American Chemical Society, volume 75, issue 22, pages 5655–5657. doi:10.1021/ja01118a054.
^Robert Schwarz; Ernst Menner (1924), "Zur Kenntnis der Kieselsäuren", Ber. Dtsch. Chem. Ges. (in German), 57 (8): 1477–1481, doi:10.1002/cber.19240570850.
silicic, acid, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, newspapers, books, scholar, jstor, july, 2022, . This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Silicic acid news newspapers books scholar JSTOR July 2022 Learn how and when to remove this template message In chemistry a silicic acid s ɪ ˈ l ɪ s ɪ k is any chemical compound containing the element silicon attached to oxide O and hydroxyl 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 in aqueous solutions including seawater and play a role in biomineralization 3 They are typically colorless weak acids that are sparingly soluble in water Like the silicate anions which are their better known conjugate bases silicic acids are proposed to be oligomeric or polymeric No simple silicic acid has ever been identified since these species being 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 Contents 1 Examples 2 Reactions 3 History 4 Preparation 5 See also 6 ReferencesExamples EditFormula Name Computed StructureH4 SiO4 or Si OH 4 orthosilicic acid H2 SiO3 or SiO OH 2 metasilicic acid H6 Si2 O7 or O Si OH 3 2 pyrosilicic acid H2 Si2 O5 or Si2 O3 OH 2 disilicic acid Reactions EditSilicic 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 citation needed 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 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 hydrated to a pair of hydroxyl OH groups 4 Si O H 2 O Si OH 2 displaystyle ce Si O H2O lt gt Si OH 2 For example H 2 SiO 3 metasilicic acid H 2 O H 4 SiO 4 orthosilicic acid displaystyle ce overset metasilicic acid H2SiO3 H2O lt gt overset orthosilicic acid H4SiO4 or H 2 Si 2 O 5 disilicic acid 2 H 2 O HO 3 Si O Si OH 3 pyrosilicic acid displaystyle ce overset disilicic acid H2Si2O5 2H2O lt gt overset pyrosilicic acid HO 3Si O Si OH 3 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 Si O Si H 2 O Si OH HO Si displaystyle ce Si O Si H2O lt gt Si OH HO Si 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 molybdate anions yielding yellow silicomolybdate complexes This reaction has been used to titrate 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 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 freezing point of the solution cryoscopy 8 History EditSilicic acid was invoked by Jons Jacob Berzelius in the early 19th century to explain the dissolution of silicon dioxide silica quartz in water namely through the hydration reaction SiO 2 silica H 2 O water H 2 SiO 3 silicic acid displaystyle ce overset silica SiO2 overset water H2O lt gt overset silicic acid H2SiO3 Based on the vapor pressure curves for silica gel Reinout Willem Van Bemellen 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 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 EditCrystalline 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 EditBoric acidReferences Edit Greenwood N N Earnshaw A 2012 Chemistry of the Elements Elsevier Science p 346 ISBN 978 0 08 050109 3 Retrieved July 27 2022 Iler R K 1979 The Chemistry of Silica New York Wiley ISBN 0 471 02404 X Frank Wigglesworth Clarke 1914 The Silicic Acids PDF The Constitution of the Natural Silicates pp 10 19 a b W E Gye and W J Purdy 1922 The Poisonous Properties of Colloidal Silica I The Effects of the Parenteral Administration of Large Doses British Journal of Experimental Pathology volume 3 issue 2 pages 75 85 PMC 2047780 Katsumi Goto 1956 Effect of pH on Polymerization of Silicic Acid Journal of Physical Chemistry volume 60 issue 7 pages 1007 1008 doi 10 1021 j150541a046 Hans Ramberg 1952 Chemical Bonds and Distribution of Cations in Silicates The Journal of Geology volume 60 issue 4 pages 331 355 doi 10 1086 625982 Bhaskar Mondal Deepanwita Ghosh and Abhijit K Das 2009 Thermochemistry for silicic acid formation reaction Prediction of new reaction pathway Chemical Physics Letters volume 478 issues 4 6 pages 115 119 doi 10 1016 j cplett 2009 07 063 a b G B Alexander 1953 The Reaction of Low Molecular Weight Silicic Acids with Molybdic Acid Journal of the American Chemical Society volume 75 issue 22 pages 5655 5657 doi 10 1021 ja01118a054 Robert Schwarz Ernst Menner 1924 Zur Kenntnis der Kieselsauren Ber Dtsch Chem Ges in German 57 8 1477 1481 doi 10 1002 cber 19240570850 a b Silicates Ullmann s Encyclopedia of Industrial Chemistry 7th ed 2007 doi 10 1002 14356007 a23 661 Retrieved from https en wikipedia org w index php title Silicic acid amp oldid 1135915945, wikipedia, wiki, book, books, library,
