Silicic acid solution, aging

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. 

The complex of tetraethylurea was prepared as follows. To 500 parts of a silicic acid solution prepared as above, aged for about 2 hr. were added 45 parts of tetraethylurea and 155 parts of sodium chloride. The mixture was stirred for 5 min and then centrifuged. A layer lighter than the aqueous phase formed and collected at the upper part of the solution 43 parts of this liquid layer was recovered, to which was added 12.5 parts of 95% alcohol. This resulting solution was analyzed and found to contain 21.4% SiOj by weight. The liquid complex was soluble in alcohol and in excess tetraethylurea. and as originally separated must have contained about 28% SiO,. 

In semi-batch operation, the SCISR is first filled with a solution of sodium silicate with certain concentration, and then a sulfuric acid solution of a given concentration is dripped at a certain rate into the reactor to react with the sodium silicate at a controlled temperature. The reaction continues for a certain interval of time after the dripping has finished. Stirring is then stopped for ageing of the precipitate for a term, and then the precipitate is sampled and the sample is measured with a laser particle-measuring instrument of FAM type to obtain the sizes and size distribution of the particles in the wet product. 

Add 158.9 g of silicic acid to solution (1) in portions under stirring shake the resulting mixture for 1 h at ambient temperature followed by aging of the mixture at 100 °C for 16 h ... 

From AlCI and SifEtO) 0.002 M silicic acid was obtained by hydrolysis of Si(EtO)4. 0.1 M aqueous AICI3 was added to produce the desired Al Si ratio. The solution was titrated with 0.1 M NaOH at 7.5 em /min until opalescence appeared. It was then aged for 2 h, and titration was continued to reach the desired Hnal pH. It was then refluxed, flocculated and dialyzed. 

Aging of silicic acid sols To characterize the silicate species in sodium silicate solutions, the silicic acid liberated therefrom must be examined promptly. At a silica concentration of lOOmM Si02 at pH 1.7, 25 C, measurable change begins in about 30 minutes. The sol must be examined within this time. 

Figure 6, Monosilicic acid, reacted with molybdic acid in 2 min, in silicic acid sols made from concentrated ("5 M Si02) sodium silicate solutions. Conditions pH 1.7 116.6 mM Si02 aged 5 min (A) and 10 d (B). Silicate ratios indicated.

Figure 8. Silicomolybdate reaction data for silicic acid sols made from diluted silicate solutions. Silicic acids of 16.6 mM S/Oj pH 1.7 25°C aged less than 15 min. Silicate solutions diluted from 5-1 M SiOz, aged 3 mo at 25°C. Dashed lines are for polymeric silica extracted into THF within 15 min.

Figure 10. Silicomolybdate reaction rate constant k vs. SiOitNa O ratio of silicates used. Key A, silicic acid, 16.6 wM SiO, pH 1.7, aged 1 min at 25°C, made from sodium silicate solutions diluted from 5-1 M 5/0, aged 3 mo at 25°C and B, colloidal silica in THF extracted from A.

Her examined the pH-tltration behavior of silicic acid in the presence of 2-hydroxypyridine 1-oxide by titrating 16 mA/ (1000 ppm SiO ) silicic acid silica from pH 10.5 to 3.0 in the presence and absence of a 43 mA/ concentration of the N-oxide. At no point did the titration curves differ, indicating that no complex had formed. In another experiment, a solution of Si(OH) containing 100 ppm as SiOj was mixed with a 200-fold excess of the above -oxidc at pH 6.15 and aged for a few hours. Tests with molybdic acid showed that the reaction rate with silica monomer was the same as a control, indicating either that no complex was formed at this pH or that it dissociated very rapidly. However, the rate of dissolution of monomer from colloidal silica particles at pH 1.4 was apparently doubled in the presence of a 20 mA/ concentration of the yV-oxide, indicating some type of interaction at low pH. -... 

Polysilicic acids of different molecular weights can be separated and molecular weights estimated by gel chromatography on Sephadex columns, using 0.1 M NaCl solution adjusted to pH 2 with HCl as the eluent. A blue dextran 2000 in 0.2% solution was used as a standard. Tarutani (83) made silicic acid at a concentration of 500 ppm by neutralizing the monomeric solution of sodium metasilicate with acid to pH 7. This solution was aged for various lengths of time and then acidified to pH 2 to stop polymerization. 

He also aged a silicic acid sol containing 2000 ppm SiO, for 6 days at different pH values from 7 to 10 and then measured the rate at which the particles depolymerized in I g l" Na2COj solution at pH 10.8. Particles formed at pH 7 depolymerized faster and were thus concluded to be smaller than those formed at pH 10. 

The preparation of silica gel plates for the cultivation of certain types of bacteria has been described by Muller and Holm (391). A silicic acid sol is made by adding a solution of sodium silicate to hydrochloric acid buffered at about pH 4.5. The gel is permitted to set in Petri dishes, washed, soaked in nutrient solution, and then autoclaved. For a purer medium, the intermediate sol may be dialyzed before it is permitted to set to gel (392). An improved procedure for incorporating nutrients directly into the acid, forming the gel under sterile conditions and eliminating dialysis, was developed by Sterges (393). A simplified method (394) is described as follows. Seven hundred milliliters of 0.5 N HCl is mixed with 300 ml of the mineral nutrient solution to be employed. To I volume of 0.5 N sodium silicate is added 1 volume of lime water, and to this 1 volume of the acid nutrient is added. The mixture is poured into Petri dishes, aged 2 hr, and placed in an oven at lOO C for 1 hr. The use of tetramethyl ester of orthosilicic acid is advocated by Ingelman and co-... 

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