Orthosilicic acid, 132

Orthosilicic acid, Si(OH)4, could be an appropriate precursor, but it cannot be stored in monomeric form at reasonable concentrations in aqueous solutions. It enters easily into polycondensation reactions (1) that result in its polymerization. As a monomer, the orthosilicic add exists in aqueous solutions at a concentration of less than 100 ppm [18]. This is too small to fabricate sol-gel derived materials, although diatoms and sponges have the property of concentrating silica from seawater containing only a few mg per liter [60]. Sol-gel processing in the laboratory can be performed with a rather concentrated solution of orthosilicic add. This requires freshly prepared add the procedure is time consuming which restrids its widespread use. 

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H SiO (aq), which is either a monosilicic or an orthosilicic acid, is also expressed as Si(OH)4 (aq) or H2Si02 (aq). This material has not been isolated (1). 

Orthosilicates are derivatives of orthosilicic acid Si(OH)4. The most common is tetraethyl orthosilicate Si(OC2H04, often called TEOS, an acronym that also fits the alternative way of naming such a compound, tetraethoxysilane. Thus orthosilicates are actually a sub-set of the larger class of XRSi(OR )3 silanes, where the XR group is also an alkoxy group. The XR and OR groups do not have to be the... 

Metasilicic acid, H2Si03, and orthosilicic acid, H4Si04, are weak acids. However, when a solution of sodium orthosilicate is acidified, instead of H4Si04, a gelatinous precipitate of silica is produced ... 

One example is silicate cement where orthosilicic acid, chemically generated in solution, condenses to form a silicic acid gel. Another is refractory cement where a cementitious product is formed by the heat treatment of an acid orthophosphate, a process which again involves condensation to form a polyphosphate. 

Orthosilicic acid at concentrations above 100 ppm in solution condenses to a dimer. At higher concentrations cyclic and polymeric species are formed (Her, 1979 Andersson, Dent Glasser Smith, 1982). These processes are ones where silanol groups condense to form siloxane linkages ... 

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

At the pH and ionic strength of seawater, the dominant dissolved species of silicon is orthosilicic acid [H4Si04(aq) or Si(OH)4(aq)]. The speciation of silicic acid is shown in Figure 5.19. At the pH of seawater, a minor amount of dissociation occurs, such that about 5% of the dissolved silicon is in the form of HjSiO faq). Dissolved organic complexes of silicon do not occur naturally. 

Orthosilicic acid. The dominant dissolved species of silicon in seawater (written as H4Si04(aq) or Si(OH)4(aq . 

Reactions with alcohols yield esters of orthosilicic acid. For example, with ethanol the product is tetraethyl orthosilicate or tetraethoxysilane, Si(OC2H5)4 ... 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

Hydrothermal synthesis of the non-uniform silica gel used for preparative chromatography proceeds in a different fashion. Sodium silicate (IMa2Si03),1 obtained by alkaline fusion of very pure sand, is acidified to yield orthosilicic acid (Si(0H)4). This unstable acid initially dimerises then condenses further to yield a gel with a hydroxylated surface. Under conditions of controlled polymerisation, a hydrogel is obtained which is further calcinated to yield a very dense silica gel (xerogel). Some of the processes involved here are of the same type as those used to produce microspheres for analytical chromatography. 

Silicon is present as a gelatinous orthosilicic acid in the waste acid and is removed during filtration. The resulting filtrate can be recycled to the strip operation, and the solids disposed of as mixed waste. The solids are neutralized by thermally degrading CuC204 to CuO, a less toxic waste. It is desirable to keep the U in solution during precipitation and allow it to concentrate by recycling. 

Polymeric chain, band- and sheet silicate structures have been discussed previously (page 742), and it should not be surprising to learn that cyclic silicate anions, such as [Si3Oy]ft and [Si601B]l2 (Fig. 16.28) are known. These anions are sometimes referred to as metasilicates in line with the older system of nomenclature, which assigned vrtlto to the most fully hydrated species [as in orthosilicic acid." Si(QH)J and metu to the acid (and anion) from which one mole of water has been removed [either in fact or formally for example, "metasilicic acid," OSi(OH)2]. 

Orthosilicic acid - [SILICA- AMORPHOUS SILICA] (Vol 21) - [SILICONCOMPOUNDS - SILICON ESTERS] (Vol 22)... 

In a series of experiments on diatoms one could show that individual diatom populations require a certain level of dissolved silica (orthosilicic acid)529-531) and that declining ambient silica concentrations in natural environments may influence the sequence of seasonal successions512). Silicoborates have also been mentioned as growth factors533. It is conceivable that other silica-depositing organisms have their specific requirement for silica concentration in the environment. 

Disilicates.—The molecules of orthosilicic acid may lose one molecule of water, the remaining atom of oxygen of the two hydroxl groups serving to unite the two molecules... 

Two molecules of Disilicic acid. 1st anhydride, orthosilicic acid. 

The final loss of water yields anhydrous silica. These acids are not known as such but certain minerals may be regarded as their salts. It is probable that okenite is a disilicate, thus CaH4Si207 and also petalite, a derivative of its second anhydride, AlLi(Si2Os)2. Similarly, three molecules of orthosilicic acid, by losing two molecules of water, may unite to form trisilicic acid and it again by successive loss of one, two, and three molecules of water may yield a first, a second, and a third anhydride. The... 

Considering, then, an example of leaching under moderate pH conditions (pH 9, say), such that ionization of orthosilicic acid can be neglected, the balance of silicon species in a well-mixed solution can be written as follows ... 

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