Dilute silicic acids from concentrated silicates

For comparison with the above silicic acids from concentrated silicates, samples of the same silicates were diluted from 5 M to 1.0 M and aged at 23 C for 3 months. 

Hydrated magnesium silicate (Talc, 3Mg0 4Si02 H20, CAS No. 14807-96-6) is a magnesium silicate commonly referred to as "soapstone". It is obtained from natural sources and may contain a small amount of aluminum silicate. It is composed of MgO (31.7%), Si02 (63.5%), and H2O (4.8%). It is a crystalline nonhygroscopic, odorless, tasteless powder which is practically insoluble in water, dilute mineral acids, dilute solutions of alkali halides, and alkaline hydroxides but is soluble in hot concentrated sulfuric acid. 

Conversion to silicic acids The general method has been described (10, 19). The sample of concentrated silicate solution (about 5 M Si02) in a syringe was injected suddenly into the vortex of a cooled (0-5 C) dilute solution of H2S0 in a blender. The volume and concentration of acid was such as to result in a sol at pH 1.7 and at a concentration of not over 100 mM Si02(6000 ppm). This requires enough acid to convert the alkali to NaHSO and leave 0.02 N free acid. Unlike monomer which is most stable at about pH 3, silicic acid from these commerical silicates polymerizes least rapidly at pH... 

It is believed that the fresh dilute silicic acid solution at pH 1.7, prepared by the described method, represents the silicate species found in the silicate solution from which it was made. In the following experiments concentrated silicate solutions were compared with diluted and aged solutions as sources of silicic acids. Also silicic acids prepared at two different concentrations were compared. As a matter of side interest, the changes that were observed in silicic acid solutions as they were aged at 25 C were also noted. 

Dilute silicic acids (16.6 mM SiO ) from concentrated silicates (5M Si) ))... 

The reaction data curves A, B and C in Figure 8 for the sols from diluted silicates are entirely different from the counterpart curves A, C and E in Figure 3 for sols made from concentrated silicates. Obviously the silicic acids from diluted silicates reacted much more rapidly with molybdic acid and thus consist of smaller particles. 

Extraction experiments were carried out on (a) the dilute silicic acid sols (16.6 mM Si02) made from diluted silicates (Figures 8, 9, 10) and on (b) the more concentrated sols (100 mM Si02) made from concentrated silicates (Figures 11, 12). 

The various methods of preparing monosilicic acid may be summarized as follows. A saturated solution of monosilicic acid, SifOH), containing about 0.01% SiOj, is obtained when pure amorphous silica is equilibrated with water at room temperature. A more concentrated (supersaturated) solution can be obtained only indirectly by liberating monosilicic acid from its compounds under carefully controlled conditions at low temperature and low pH, dilute solutions remain supersaturated with respect to amorphous silica for appreciable periods. For example, at pH 3 and 0°C, solutions of monosilicic acid up to O.l (0.6% SiOj) can be prepared by spontaneous hydrolysis of monomeric silicon compounds, sich as silicon tetrachloride or methyl orthosilicate, and also by reacting monomeric silicates, such as sodium or magnesium orthosilicates or hydrated crystalline sodium metasilicate, with dilute acid. 

As in the case of gelatin, in very dilute solutions where the concentration of the colloidal silicic acid is about one-half of one per cent, floccu-lent precipitates are formed which contain large spaces filled with water. The constituents of the gel are no longer able to form a mass in which the ultramicrons are uniformly distributed. As the water content becomes less and less the spaces filled with water shrink, the particles of the colloid draw closer together, and it is thus clear why the gels from concentrated solutions exhibit a denser structure than those from more dilute solutions. That this is true to fact is shown by observations under the ultramicroscope during the formation of a gel, and also from the polarization effects. 

The Shell Chlorine Process. The catalyst developed by Shell consists of a mixture of copper(II) chloride and other metallic chlorides on a silicate carrier [202]. The reaction of the stoichiometric mixture of hydrogen chloride and air takes place in a fluidized-bed reactor at ca. 365 °C and 0.1-0.2 MPa. The yield is 75%. The water condenses out from the gas stream, and the hydrogen chloride is removed by washing with dilute hydrochloric acid. After the residual gas has been dried with concentrated sulfuric acid, the chlorine is selectively absorbed, e.g., by disulfur dichloride. After desorption and liquefaction, the chlorine has a purity > 99.95 %. 

Wang and Li have reported the determination of procaine hydrochloride injections, and the quality control of 4-aminobenzoic acid [144]. The column packing used for this work consisted of 8 g of silanized siliceous earth support with 5 mL of hexanol as the stationary phase, previously percolated with 20 mL of 0.05 M sodium carbonate. The drug injection solution (containing 10 mg of procaine hydrochloride) was applied to the column, and eluted with 30 mL of 0.05 M sodium carbonate. The eluent was diluted to 50 mL with water, and 4-aminobenzoic acid was determined by an absorbance measurement at 266 nm. Procaine was then eluted from the column using 60 mL of 0.1 M hydrochloric acid. This eluent was treated with 10 mL of acetate buffer (pH 6), and diluted to 100 ml with water. The analyte was determined on the basis of its absorbance at 290 nm. Equations for the computation of procaine and 4-aminobenzoic acid concentrations were presented. 

Anti-acids, astringents and antiseptic agents may contain a variety of aluminium salts. Organic salts, alumina, the hydroxide and phosphates may be attacked with concentrated hydrochloric acid and diluted to bring the aluminium concentration into the range 10—50 pg ml"1. Alternative procedures for antacids using hydrochloric/nitric acid [67] and extraction with 4M hydrochloric acid [95] have been proposed. For silicates, the sample is best taken up in perchloric/hydrofluoric acid, evaporated to dryness to remove silica, and then the residue dissolved in warm hydrochloric acid [87], In each case the nitrous oxide/acetylene flame is the preferred atom cell, and the method of standard additions may be used to minimise any errors arising from lateral diffusion. 

Both concentrated and somewhat diluted solutions of sodium silicates of different ratios were examined by suddenly injecting samples into rapidly stirred dilute H2S0 to convert the ionized species to the corresponding silicic acids. These were at once characterized by reaction with molybdic acid. Also colloidal species were separated from monomer and oligomers by extraction into tetrahydrofuran (THF). 

The degree of syneresis often depends upon the concentration of the gel. Aqueous cellulose xanthate solutions, which are allowed to stand, set to a gel as a result of the spontaneous decomposition of the xanthate. Initially, the gel occupies the same volume as the original solution. If the experiment is carried out with solutions of various concentration, it is seen that beyond a certain dilution the gel contracts and the system, hence, separates in two phases, the gel and the liquid expelled from it. The phenomenon is the more pronounced the lower the concentration of the original solution. A similar behaviour was observed in silicic acid gels. 

Ceramic filter media are manufactured from crushed and screened quartz or chamotte, which is then thoroughly mixed with a binder (for example, with silicate glass) and sintered. Quartz media are resistant to concentrated mineral acids but not resistant to low-concentration alkalies or neutral water solutions of salts. Chamotte media are resistant to dilute and concentrated mineral acids and water solutions of their salts, but have poor resistance to alkali liquids. 

Sodium hydroxide is used, either in the molten condition (m.p. 318X) or as a concentrated solution at about 200°C, for the breakdown of refractory ores such as phosphates or silicates. Usually, the oxide, hydrated oxide or hydroxide of the metal is produced and this can be dissolved in dilute acid for further purification, after first washing free from the other... 

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. 

The nature of the silica in silicate ions in any alkaline solution cannot be determined by a chemical measurement that involves any change in the concentration of silica or alkali, electrolyte content, or temperature because these all shift the equilibrium between monomeric and various polymeric ion species. However, if a sample is simultaneously and instantaneously diluted and acidified to pH 2 al less than 30 C, the resulting silicic acid is sufficiently stable to permit characterization. The problem is to ensure that acidification is so sudden that the various silica species do not have time to polymerize or depolymerize as the pH is dropped from the usual region of... 

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