Equations Silicic acid

In these equations silicic acid is written as Si(OH)4 (rather than as H4Si04> in order to emphasize that metalloids (Si, B, Ge) similar to metal ions have a tendency to coordinate with hydroxo and 0x0 ligands. Like multivalent metal ions, such metalloids tend to form multinuclear species. 

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). 

A convenient method to introduce organic functions into inorganic networks is the use of substituted siliceous acid esters and the sol-gel route. The basic reactions, therefore, are given in Equations 1 and 2. Equation 1 describes the sol-gel reaction to a borosilicate glass. 

The unstable cycloadducts 207, obtained from the dihydropyridines 205 (R = Me or Bn) and the benzoyl nitroso compound 206, undergo a hetero-Cope rearrangement in the presence of silicic acid to yield fused dioxazines 208 (equation 114)106. Adding the racemic hydroxamic acid 209 (R = r-Bu, cyclohexyl or Ph) to a two-phase mixture of... 

The reaction in equation (6) is irreversible since glass cannot precipitate from solution. Equations (4) or (5) affect the overall rate at which equation (6) proceeds because they impact the rate at which Si-OH groups are produced. However, three silanol groups must be formed before the last anchoring bond is broken, releasing a silicic acid (FLiSKTO molecule into solution. 

Hydrogel of Silicic Acid. Pour 5 ml of a concentrated solution of soluble glass into a test tube, rapidly pour 3 ml of a 24% hydrochloric acid solution into it, and stir with a glass rod. What is observed Write the equation of the reaction. How can you explain the formation of a gel ... 

Preparation of Silicic Acid Anhydride. Dissolve a small amount of potassium silicate in a minimum amount of water, filter the solution if necessary, and add a 37 % hydrochloric acid solution up to a strongly acidic reaction. Transfer the solution into a porcelain bowl and evaporate it until dry in a sand bath. Wash the dry residue with warm water until the chloride ions are removed, then roast it and see how it reacts with solutions of an alkali and hydrofluoric acid in a lead bowl in a jum cupboard ). Write the equations of the reactions. What is obtained when silicic acid anhydride is melted with alkalies ... 

The model involves four basic assumptions. First, it is postulated that an adsorbed phase of silicic acid exists on the particle surface, the equilibrium of which can be expressed by a Langmuir isotherm (Equation 1, symbols are defined in table of nomenclature). 

There are several explanations for these deviations. As Figure 11 indicates, it takes some time to establish the adsorption equilibrium. Thus, the theoretical assumption concerning the adsorption equilibrium and resulting in Equation 3 may not be satisfied adequately. Here a higher initial silicic acid concentration would result. On the other hand, at high silicic acid concentrations the Langmuir isotherm is an idealization which will not properly represent experimental data of extensive... 

The value of pKo is determined by extrapolating curves plotted on coordinates of pK vs. a to a zero value of a (uncharged surface). The value found for pKo = 5.3 proved to be considerably lower than pK of free silicic acid in the solution. When an ion-permeable gel layer is present, Gouy s equation to calculate xp0 from adsorption data is not valid. Detailed measurements and calculations on y-Al203 showed that the calculated values of V>o agree well with values of t/h calculated according to Gouy s equation ... 

Equation 3.18 can be used to construct an activity-ratio diagram in respect to A1 solubility as influenced by the leaching of silicic acid (H4SiOj). The equations for log[(solid)/(Al3 )] are as follows ... 

Where is the total concentration of acid species and A, and K2 are the first and second stepwise dissociation constants of the acids. This equation can be used to compute the buffer index of a polyprotic acid as long as successive dissociation constants differ by at least 20 times (this assures a calculation error of 5% or less). In other words, for a diprotic acid K2fK should be less than 0.05 (cf. Butler 1964). Thus, for example, Eq. (5.114) may be used to compute the buffer index due to species of carbonic acid, for which A = 10 and K2 = 10" °, or/ for species of silicic acid, for which a , = lO- and K, = 10... 

Although the solubilities of quartz, its polymorphs, and amorphous silica are fixed and pH-in-dependent in most natural waters, the dissociation of silicic acid at alkaline pH s leads to substantial increases in their solubilities above pH 9 to 10. The following calculations show how we can predict this effect. First, the solubility of any silica solid must equal the sum of the concentrations of all species of silica in solution at equilibrium. This summation is given by the mass-balance equation... 

The second equation represents one of the ways that silica gel is produced. Note that in the second reaction the first product is silicic acid, H4Si04. However, this acid rapidly polymerizes and looses H2O, forming a network of very strong Si-0 bonds. 

Strelko and co-workers (71-73) investigated the properties of the siloxane bond and the polymerization mechanism for silicic acid on the basis of the difference in the electronic structure of the Si-O bonds in silanol and the siloxane groups of silica. A kinetic equation was proposed to describe the polymerization of silicic acid throughout the entire pH range. The authors believe that the molecular mechanisms for the formation of globular skeletons in silica gels are based on the polymerization and depolymerization of silicic acids. 

Trichlorosilanethiol is a colorless liquid boiling at 95.5 to 95.6° at 759 mm. pressure. It hydrolyzes vigorously to hydrogen chloride, hydrogen sulfide, and silicic acid, and slowly deposits sulfur on exposure to air. The pure compound decomposes at 700° in approximate accord with the equation ... 

The adsorption of silicic acid on a-FeOOH tends to release protons (a) and causes a decrease in surface charge (b). The extent of adsorption as a function of pH can be predicted by an equilibrium model that considers the equilibrium constants given in Equations 22 and 23 and the acidity constant of H SiO and =FeOH, (IS). 

Although esters of type Si(OR)4 (equation 14.66) are known, no well-defined silicic acid (H4Si04) has been established. 

Equation 19.5 demonstrates why the presence of bicarbonate ions and molecular silicic acid in the streams of Taylor Valley is evidence that chemical weathering of feldspar and other silicate minerals is occurring in spite of the harsh climatic conditions. The only prerequisite to chemical weathering of minerals on the surface of the Earth as well as in the subsurface is the presence of liquid water. The work of Nezat et al. (2001) has demonstrated that silicic acid and bicarbonate ions are present in all meltwater streams in the watersheds of Lake Bonney, Lake Hoare, and Lake Fryxell. 

A general theory of polycondensation equilibrium in silicic acid solutions was proposed by Stbber (94). From it he deduced the concentration of monomer in equilibrium with polymers of different degrees of condensation. Further data are needed to check the validity of the complex equations that were developed. 

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