Silicic Acid and Silica

Vitreous Si02 oeeurs as teetites, obsidian and the rare mineral leehatelierite. Synthetie forms inelude keatite and W-siliea. Opals are an exeeedingly eomplex erystalline aggregate of partly hydrated siliea. 

The a-form of each of the three minerals can thus be obtained at room temperature and, because of the sluggishness of the reconstructive interconversions of the -forms, it is even possible to melt -quartz (1550 ) and -tridymite (1703 ) if they arc heated sufficiently rapidly. The bp of Si02 is not accurately known but is about 2800 C. 

Other forms of SiOj can be made at high pressure (Fig. 9.2). Coesite was first made 

Finally, a very low-density form of fibrous silica, W-Si02 has been made by the disproportionation of (metastable) crystalline SiO  

Density of the main forms of Si02 (room temperature) 

Vitreous Si02 occurs as tectites, obsidian and the rare mineral lechatelierite. Synthetic forms include keatite and W-silica. Opals are an exceedingly complex crystalline aggregate of partly hydrated silica. 

Mehner, H. J. Gocke, S. Schlinck and H. SCHNOCm.. Z. anorg. allg. Chem. 580, 121-30 (1990). 

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An extensive review on the knowledge of the structure and chemistry of silica and silicic acid up to 1955 was given by Her 167). Unfortunately, there is no recent edition of this book covering research in the last decade. 

The SiCl4 formed as a byproduct is flushed out and further processed to pyrogenic silica and silicic acid esters. 

Flux Any substance or mixture used to promote fusion, especially the fusion of metals or minerals. Common metallurgical fluxes are silica and silicates (acidic), lime and limestone (basic), and fluorite (neutral). Also any substance applied to surfaces to be joined by soldering or welding. Just prior to or during the operation, to clean and free them from oxide, thus promoting their union. 

Typical gels are prepared from aqusous solutions of reactants such as sodium aluminate, sodium hydroxide, and sodium silicate other reactants include alumina trihydraie (Al O, 3H,0). colloidial silica, and silicic acid. The temperature strongly influences the crystallization time. 

In conclusion, it seems that many particular aspects of the stability of silica hydrosols could be explained not only by the low value of the Hamaker constant but also by the relative importance of the static term. Thus, Si02 is a valuable system for the experimental study of the structural forces. It is likely that the unique chemical properties of silica and silicic acid might also give rise to surprising results. 

Aqueous hydrogen fluoride is a weak acid (see above) and dissolves silica and silicates to form hexafluorosilicic acid hence glass is etched by the acid, which must be kept in polythene bottles. 

Hydrogen-hydroxide cycle Provides the most favorable operating cycle and results in near neutral water of very high quality. The (hydrogen form) cation resin converts silica to silicic acid (H2Si03), which is then retained by the (hydroxide form) anion resin but breaks through first due to its very weak nature. Poor... 

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

Wilson, Groffman Kuhn (1985) calculated that only about 4-5% of the total fluoride is available for release, and Meryon Smith (1984) found that the amount released was not related to the fluoride content of the cement. Wilson, Groffman Kuhn (1985) observed that release of fluoride was accompanied by the release of sodium, necessary to maintain electroneutrality, and silica (as silicic acid). The release of these species could also be fitted to equation (5.1). 

For conventional or normal, in contrast with reversed-phase, LLPC, many materials have been used as the solid support for the stationary liquid. In addition to silica gel, which was the first and is still the most popular material, a variety of other adsorbents that adsorb the polar solvent such as cellulose powder, starch, alumina, and silicic acid have been used. The more recent practice of HPLC has greatly simplified the technique in providing column stability for repeated use and for treatment of large volumes. 

In order to obtain satisfactory absorption spectra of the substance under investigation it is essential that the beam pass through a large number of adsorbed monomolecular layers. In practice this is most satisfactorily achieved by adsorption on transparent microporous solids with a high surface area to mass ratio (200-600 m2/g). The solids found most suitable have been silica gel, silicic acid, and microporous glass. 

Only nonpolar solvents, e.g., cyclohexane (most commonly employed with silica gel and silicic acid), methylcyclohexane, methylpentane, and carbon tetrachloride, can be utilized in these slurries since solvents of greater polarity will compete with the intended adsorbate for available binding sites and will result in incomplete sustrate adsorption. 

Silica and silicates are soluble in aqueous alkali.9,15-17 The silicate anions produced in solution may be regarded as derived from silicic acid [H4Si04 or Si(OH)4], which has a first pK of 9.47 at 0.6 mol L"1 ionic strength ... 

The way in which the proton is associated with the alumina-silica catalyst is a matter of some doubt. Thomas (78) assumes the aluminium to be tetrahedral when linked with tetrahedral silicon, the extra valence electron being supplied by hydrogen from water contained in the catalyst (Fig. 21a). Both aluminium hydroxide and silicic acid are very weak acids because of the affinity of oxygen for the hydrogen (83), and a coordination of aluminium with the hydroxyl oxygen contained in the catalysts... 

In one case, therefore, triple silicates are the proximate constituents of the glass in the others, double silicates. In foot, the proportions of the three essential constituents, namely, silica, potassa, and oxide of lead, vary according as the melting furnace is heated with wood or coal. In the latter case, the proportion of oxide of lead is increased. The following analysis by Dumas has been already given at page 192, but is here repeated, to show the proportion of oxygen in the bases, as compared with that of the silica or silicic acid.—... 

Polymer structures that hold silanols at the interface. Good examples of hydrolytically stable crosslinked structures are silica and silicate rocks. Although every oxane bond in these structures is hydrolyzable, a silicate rock is quite resistant to water. Each silicon is bonded to four oxygens under equilibrium conditions with a favorable equilibrium constant for bond retention. The probability that all four bonds to silicon can hydrolyze simultaneously to release soluble silicic acid is extremely remote. With sensitive enough analytical techniques it is possible to identify soluble silica as it -leaches from rocks, but an individual rock will survive in water for thousands of years. 

English red consists essentially of anhydrous ferric oxide in very fine powder, more or less deep red in colour. It usually contains small quantities of silica and silicates, alumina, lime and magnesia, and often sulphates, chlorides and free sulphuric acid it may also contain manganese and copper. It is often sold mixed with considerable proportions of gypsum (Venetian red), barium sulphate and chalk sometimes its colour is heightened or modified by addition of artificial organic colours. 

Another part of the pigment or the portion insoluble in water is treated with concentrated hydrochloric acid effervescence indicates carbonates (chalk). When effervescence ceases, the liquid is heated for a long time until the ferric oxide dissolves completdy, nitric acid being also added if necessary. The liquid is evaporated to dryness and the residue taken up in hydrochloric acid and hot water and filtered. The insoluble residue is tested by the ordinary methods to see if it consists solely of silica and silicates or if barium sulphate is present. 

Silicon is next to oxygen the most abundant element in the lithosphere the average content amounts about 30% by weight. Inorganic silicon compounds such as silica and silicates form the basis of most of the rocks forming the earth s crust. In the atmosphere, there is no silicon present, except as dust of cosmic and terrestrial origin. The silicon content in the hydrosphere, mainly in form of dissolved silica (silicic acid), is also very small. 

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