Monosilicic acid solubility

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Vitreous sihca does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubihty is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 sihca dissolves mainly as soluble sihcates. Solubihty also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10 300 atm), for example, the solubihty, S, of Si02 in g/kg H2O can be expressed as foUows, where d ls the density of the vapor phase and T is the absolute temperature in Kelvin. 

Richardson and Waddams (31) found the same behavior for silica. Ground quartz particles agitated with water release some monosilicic acid molecules which form a true solution and, in addition, some very small quartz crystals which form a dispersion in water until a hydrated equilibrium surface is established. Analogous to the ferric oxide, heating the quartz to a temperature above 600° C. regenerates the anomalous solubility behavior. 

Roy et al. also investigated the use of dilute HF in the clean sequence. Because the solubility of monosilicic acid (Si(OH)4) de-... 

It is shown that the solubility of siliceous rocks was 18.6 times higher than the solubility of quartz but 2.3 times lower than the solubility of amorphous silica. The soluble form of silica, monosilicic acid, is unstable. First the concentration of silica in solution increases and reaches a maximum then it decreases because of processes of sorption and polycondensation of soluble forms. 

We determined that vicinal dihydroxy organic reagents stabilize the soluble forms of silica. The stability of monosilicic acid solution is determined by the structure of the stabilizer ethylene glycol and humic acids do not stabilize soluble forms of silica enough, but glycerin and catechol stabilize the silica solution when they are added at 5 - 7% to the solution. This fact is connected with the formation of hydrogen bonds and stable penta- and hexacoordinated compounds, preventing the processes of polycondensation of silica in solution. 

Figure 4.8 (A) Variations in silica solubility with pH (after Williams and Crerar, 1985 Dove and Rimstidt, 1994). (B) The release and sorption of monosilicic acid by a black earth soil under varying pH (after Beckwith...

Beckwith, R.S. Reeve, R. (1964) Studies on soluble silica in soils. II. The release of monosilicic acid from soils. Australian Journal of Soil Research 2, 33-45. 

A) Variations in silica solubility with pH. (B) The release and sorption of monosilicic acid by a black... 

In alkaline solutions, silica exists in the form of silicate ions e.g., Si03 ). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. 

Soluble monosilicic acid reacts with molybdic acid at pH 1-2, in the presence of at least a 0.05 M excess of molybdenum, to form the yellow soluble (l-molybdosilicic acid. The yellow colour is the basis of a rather insensitive spectrophotometric method for silicon [12-15]. The absorption maximum of the complex is in the ultraviolet. At 400 nm, the molar absorptivity is 2.2-10 (a - 0.08) (in the presence of acetone). 

Silica is converted into monosilicic acid by fusing with sodium carbonate and acidifying the alkaline solution produced when the melt is dissolved in water. Alternatively, an acidic sample solution may be made alkaline with sodium hydroxide and heated to convert colloidal silica into silicate. Soluble monosilicic acid is formed after appropriate dilution and acidification. 

On heating with dilute hydrofluoric acid, silica is transformed into the soluble hexafluorosilicic acid, H2Sip6. Aluminium chloride or boric acid introduced subsequently mask the excess of HF and decompose H2S1F6 to monosilicic acid (the more stable AlFe or BF4 complexes being produced). 

The product of this reaction, soluble monosilicic acid, Si(OH)5, is readily leached out of the soil surface, representing export of some of the strong acidity in a weak acid form that is environmentally innocuous. At the same time, part of the acidity accumulates in the soil in more toxic forms—soluble and exchangeable AF+. 

This dissolution reaction converts strong acids (H" ) into less strongly acidic forms (AP+ and monosilicic acid). Because monosilicic acid is a very weak acid, it can leach through the soil in the undissodated (Si(OH)4) form. In reality, however, the A1 component is usually conserved as a solid phase in the soil that is, soluble A1 is typically so low that equation 6.2 is replaced by a more realistic one in which aF predpitates as gibbsite, kaolinite, or aUophane, for example. 

The solubility of silica can be characterized by the following equilibria at 25 C. Monosilicic acid has been written H2Si02(0H)2, rather than Si(OH) or H SiO in order to emphasize its diabasic character, and the tendency of silicon, like other metalloids, to coordinate with hydroxo and oxo ligands. 

Stumm(52) used these equilibria to construct the diagram in Figure 3 which describes the speciation of silica in aqueous solution. His data indicate that at normal environmental pH values (pH 9) dissolved silica exists exclusively as mono-silicic acid. This conclusion is supported by the finding that soluble silica has a diffusion coefficient of 0.53 indicating a molecular size about equivalent to monosilicic acid(53). 

Fig. 14 shows the elution curves for silicic acid in solutions of pH 9.5 on a Sephadex G-lOO column. The peaks on the right are due to monosilicic acid and those on the left to polysilicic acid. The elution curves for the polysilicic acids indicate a symmetrical distribution of particle sizes, and the elution volume of the polymers obtained after standing the sample solution for 250 h is almost the same as that obtained after 100 h. This suggests that when the concentration of monosilicic acid is close to the solubility of amorphous silica, the growth rate of the particles of the polymers becomes very low. It was thus assumed that the growth of the particles is mainly due to the polymerization between monomer and polymer, and the polymerization between polymer species... 

The soluble form of silica is monomeric, containing only one silicon atom and generally formulated as Si(OH)4. This is often called monosilicic acid or orthosilicic acid. The state of hydration is not known, although at high pressure there is some indication that one water molecule is linked to each OH group, probably by hydrogen bonding, so the hydrated molecule is represented by Willey (20) as Si(OH OH,>4. 

The preparation and reactions, for example, polymerization, of dilute solutions of monosilicic acid are further described in Chapter 3. Meanwhile, some of its characteristics are noted, as follows, prior to discussing solubility ... 

Concentrations of silica of around 2 ppm were reached in dilute salt solution with mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was enriched with soluble silica to 25 ppm SiOa, it remained at that level for a year in the absence of these minerals, but when the latter were then added, the silica was removed from solution down to the 2-15 ppm level that was reached when the minerals alone were added. Since many ocean waters contain 2-10 ppm SiOj, it is possible that this value is reached as the equilibrium solubility of colloidal aluminosilicate in suspension. The above experiment is consistent with the fact that in pure water, pure a morphous silica dissolves to give a concentration of monosilicic acid of 100-110 ppm, but in the presence of polyvalent metal cations such as iron, aluminum, and other metals, colloidal silicates are formed with a much lower solubility with respect to monosilicic acid. Her (37) has shown that soluble aluminum reduces the solubility of amorphous silica from about 110 to less than 10 ppm. 

Soluble silica is found in essentially all plants and animals. For e. ample, human blood contains I ppm. Ingested monosilicic acid as an undersaturated solution rapidly penetrates all tissues and body fluids and is excreted apparently without any effect (48). Plants, especially grasses, including the grains and rice, take up silica and deposit it in the tissues as characteristic microscopic amorphous opaline particles, which are later found in the soil and in the intestinal tracts of grazing animals (49). The widespread occurrence and possible role of silica in living systems are more fully discussed in Chapter 7. 

Stober investigated the solubility behavior of several modifeations of silica and developed a theory and equation for the behavior of adsorbed monosilicic acid in retarding or preventing approach to true solubility equilibrium (144). 

Although monosilicic acid, Si(OH)4, is believed to be the material actually deposited, it is possible to use active silica in the form of low molecular weight polysilicic acids (including extremely small colloidal particles) as a source of silica. Such small particles are highly soluble and are in equilibrium with a concentration of Si(OH>4 that is highly supersaturated with respect to larger particles or a flat surface. 

As sources of the silicic acids, crystalline acid-soluble satis of monosilicic, disilicic, and cyclic tri-, tetra-, and hexasilicic acids were dissolved rapidly in metha-nolic HCl, in which the silicic acids are more rapidly dissolved yet are more stable against further polymerization than in water. The liberated silicic acids were reacted at once with molybdic acid reagent at 20°C. 

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