Hydrolysis reactions aqueous silicates

The hydrolysis of silicate minerals involves chemical reactions with weakly acidic or alkaline aqueous solutions. With natural waters that contain dissolved C02-that is, C02(aq) which can be written as carbonic acid (H2C03)-the hydrolysis of magnesium silicate (olivine) proceeds as... 

On the other hand, hydrolysis with aqueous alkalis is complete, producing hydrogen and silicates. Silyl iodide gives a Wurtz-type reaction with sodium, wliich affords a useful path to disilane ... 

Earlier in this chapter we discussed the sol-gel processing of aqueous silicates and mentioned the preparation of Si02 gels from fine particles made by flame oxidation (2)-(4). The preparation of alumina sols from alkoxides has been characterized by Yoldas (45). Aluminum alkoxides such as aluminum sec-butoxide and aluminum isopropoxide are readily hydrolyzed by water to form hydroxides. Which hydroxide is formed depends on the conditions used in the hydrolysis. The initial hydrolysis reaction of aluminum alkoxides can be written... 

Gelation is initiated in aqueous silicate systems by pH changes, and in alkox-ide precursor systems by addition of water (hydrolysis reactions to generate Si-OH groups). 

Anilines are converted into nitrosoarenes ArNO by the action of hydrogen peroxide in the presence of [Mo(0)(02)2(H20) (HMPA)]224, whereas catalysis of the reaction by titanium silicate and zeolites results in the formation of azoxybenzenes ArN (0)=NAr225. Azo compounds ArN=NAr are formed in 42-99% yields by the phase-transfer assisted potassium permanganate oxidation of primary aromatic amines in aqueous benzene containing a little tetrabutylammonium bromide226. The reaction of arylamines with chromyl chloride gives solid adducts which, on hydrolysis, yield mixtures of azo compounds, p-benzoquinone and p-benzoquinone anils 234227. 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

Silicate and aluminosilicate mineral-weathering reactions involve both H -ion attack on the mineral and hydration or hydrolysis of the product minerals (clays and metal oxyhydroxides) or product aqueous species. If the primary mineral contains aluminum, then clays may result from the weathering. If the primary mineral contains Fe(II) or other reduced metal ions, weathering may involve oxidation as well as hydrolysis. 

A central issue in the attempt to establish a reliable database is the requirement of critically evaluated thermodynamic data for several key species. One such pivotal element is aluminum, which has an extensive literature of solubility and thermochemical data from which to choose, for each of the aqueous species or complexes. The aluminum species are fundamental to the calculation of solubility and reaction state with respect to many silicates and aluminum oxides and hydroxides and are principal components in numerous surface chemical reactions in the environment. Two key chapters in this volume address this fundamental problem Apps and Neil give a critical evaluation of the data for the aluminum system and Hem and Roberson present the kinetic mechanisms for hydrolysis of aluminum species. 

Reactions of bivalent species investigated include the insertion of CCU into SiH bonds and the reactivity of Sip2 towards a series of dienes. In a previous Report we commented on the potential value of Si FouriCT-transform n.m.r. measurements. This technique gives a great deal of information about the species present in aqueous solutions of sodium silicate . In the condensation reactions of monosilicic acid, prepared by the hydrolysis of Si(OMe)4 in dilute hydrochloric acid, it has been shown that the first product of appreciable stability is cyclotrisilicic acid, not disilicic acid. The variation of concentration of higher monocyclic and polycyclic species as a function of pH and concentration has been reported. 

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