Aqueous silicate solutions

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 ... 

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Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present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. 

Silica Gels from Aqueous Silicate Solutions... 

The use of Si NMR to obtain definitive information about the chemical structure and kinetics of aqueous silicate solutions is discussed. Special techniques such as enrichment in si are required. Some typical spectra are shown and results presented. 

The advances reported here in our knowledge of the species present in aqueous silicate solutions at high pH were made by combining four techniques, viz -... 

Table I Species giving singlet Si NMR resonances for an alkaline aqueous silicate solution (K Si = 1.0 concentration. 65M in Si) enriched in Si to 95 3 ...

---