Silicates, aqueous

This is an acid-base reaction, in which the base is the oxide ion (p. 89) the acidic oxide SiOj displaces the weaker acidic oxide CO2 in the fused mixture. But in aqueous solution, where the 0 ion cannot function as a strong basefp. 89),carbon dioxide displaces silica, which, therefore, precipitates when the gas is passed through the aqueous silicate solution. In a fused mixture of silica and a nitrate or phosphate, the silica again displaces the weaker acidic oxides N2O5 and P4OJ0 ... 

Silicate solution species are controlled by the pH of the medium and silicon concentration.14 Figure 1.5 displays the aqueous silicate species as a function of pH for concentrations of 0.01 and 105 M of silicon. 

Porous silica is one of the different forms of amorphous silica. It can be prepared by acidification of basic aqueous silicate solutions, and when reaction conditions are properly adjusted, porous silica gels are obtained [150], If water is evaporated from the pores of silica hydrogels prepared in this fashion, porous xerogels are obtained [153],... 

Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer is dominant in the aqueous silicate solutions. This indicates that the use of a silica source with tetra-functionality is required for the selective structure formation with the aid of organic quaternary ammonium ions. 

The method of Lentz (35,36) was adopted for trimethylsilylation of the aqueous silicate solutions. The mixture of cone, hydrochloric acid, water, 2-propanol and hexamethyldisiloxane was used as the trimethylsilylating reagent. Trimethylsilylated derivatives obtained were adaptable to gas-phase analysis. The distribution of silicate species in solutions, which was analyzed quantitatively by the trimethylsilylation technique combined with gas-liquid chromatography, was expressed as the Si(>2 recovery, that is, the percentage of silica as a silicate species in total silica component in the solutions. 

P.W.J.G. Wijnen, A spectroscopic study of silica gel formation from aqueous silicate solutions, Ph.D. Thesis, TU Eindhoven, 1990. 

Fumed silica is comparatively expensive but exhibits a high purity [109]. Silica gel is synthesized by sol-gel methods using aqueous silicate solutions [110]... 

For silicon, the metal-coordinating properties, as found in AnEiyt, should, in principle, be shared by each of the monosaccharides. A particular monosaccharide is expected to act as a good ligand if its cA-furanose form is of considerable stability, so that the stability constant of the complex is not charged with the isomerization energy of the ligand. In fact, almost all of the monosaccharides enrich alkaline aqueous silicate solutions with five- and six-coordinate silicon species, to some extent. However, the determination of the compositions and stmctures of the involved species is largely complicated by the mere number of the species in equilibrium. This has recently been demonstrated for D-ribose solutions by Kinrade et al., who detected a vast amount of various five- and six-coordinate species in such solutions [94]. For monosaccharides other than D-ribose, the situation is not any better. 

This product is by far the most frequently used sorbent in TLC and is prepared by dehydration of aqueous silicic acid generated by addition of a strong acid to a silicate solution. 

Aqueous silicate product forms with widely varying properties such as leach rate, hardness, and volume can be produced at various costs by using the three alternative process routes. Typical leach rates for the products in water range from g/cm -day. In general, the volume... 

J.7.2.5 Recipe from [2315,2316] Aqueous silicic acid was obtained from sodium silicate by ion exchange. AICI3 was dissolved in aqueous silicic acid, and the solution was sprayed into dilute ammonia. The product contains 72.5% of alumina by mass. 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

Figure 2. Polymerization pathway of aqueous silicates according to Iler (2). Stages of growth recognized by Iler polymerization of monomer to form particles, growth of particles, and linking of particles together into branched chains, networks, and, finally, gels. (Reproduced with permission from reference 2. Copyright 1978.)...

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