Hydrophobic Colloidal Silica
» Hydrophobic Colloidal Silica is prepared by partial alkylation for hydrophobation. It contains not less than 99.0 percent and not more than 101.0 percent of SiO2, calculated on the ignited basis.
Packaging and storage Preserve in well-closed containers. No storage requirements specified.
A: Add about 25 mg of Hydrophobic Colloidal Silica to a platinum crucible, and ignite at 900 for 2 hours. Using a copper wire, mix the ignited substance with about 10 mg of sodium fluoride and a few drops of sulfuric acid to give a thin slurry. Cover the crucible with a thin, transparent plate of plastic under which a drop of water is suspended, and warm gently. Within a short time a white ring is rapidly formed around the drop of water.
B: It meets the requirements under Water-dispersible substances.
Limit of chloride
Test solution To 1 g of Hydrophobic Colloidal Silica add 30 mL of methanol and 20 mL of dilute nitric acid. Heat on a water bath for 15 minutes with frequent stirring. Cool, dilute with water to 50 mL, and filter. Dilute 10 mL of the filtrate with water to 15 mL. Add 1 mL of dilute nitric acid, and pour into a test tube containing 1 mL of silver nitrate TS.
Standard solution Prepare a standard in the following manner: Add 10 mL of 0.15 mM sodium chloride and 5 mL of water. Add 1 mL of dilute nitric acid, and pour into a test tube containing 1 mL of silver nitrate TS.
Procedure Examine the tubes laterally against a black background. After standing for 5 minutes protected from light, any opalescence in the Test solution is not more intense than that in the Standard solution (0.025%).
Limit of lead [noteSelect reagents having as low a lead content as practicable, and store all solutions in containers of borosilicate glass. Rinse thoroughly all glassware with warm, dilute nitric acid (1 in 2) followed by water. ]
Ammonium acetate buffer solution, pH 3.5 Weigh 25.0 g of ammonium acetate, and dissolve in 25 mL of water. Add 38.0 mL of dilute hydrochloric acid. Adjust the pH, if necessary, with dilute hydrochloric acid or weak ammonia solution (containing 460 mL of strong ammonia solution per L). Dilute with water to 100.0 mL.
Thioacetamide solution To 0.2 mL of thioacetamide TS, add 1 mL of a mixture of 5 mL of water, 15 mL of 1 N sodium hydroxide, and 20 mL of 85% glycerol. Heat in a water bath for 20 seconds. [notePrepare immediately before use. ]
Test solution Suspend 2.5 g Hydrophobic Colloidal Silica in 30 mL of methanol, stir, and add 30 mL of weak ammonia solution (containing 460 mL of strong ammonia solution per L). With frequent stirring evaporate on a water bath, and dry the residue in an oven at 140. When the dried substance is white, break up the mass with a glass rod. Reduce the residue to a powder, and add 15 mL of methanol and 25 mL of 1 N hydrochloric acid. Boil gently for 5 minutes, stirring frequently with the glass rod. Centrifuge for 20 minutes, and pass the supernatant through a membrane filter. To the residue in the centrifuge tube, add 3 mL of dilute hydrochloric acid and 9 mL of water, and bring to a boil. Centrifuge for 20 minutes, and pass the supernatant through the same membrane filter. Wash the residue with small quantities of water, combine the filtrates and washings, and dilute with water to 50 mL. To 20 mL of this solution, add 50 mg of ascorbic acid and 1 mL of strong ammonia solution. Neutralize with diluted weak ammonia solution (containing 160 mL of strong ammonia solution per L). Dilute with water to 25 mL.
Reference solution Pipet 10 mL of the Standard Lead Solution (see Special Reagents under Heavy Metals 231), and mix with 2 mL of the Test solution.
Blank solution: a mixture of 10 mL of water and 2 mL of the Test solution.
Procedure To the Test solution, Reference solution, and Blank solution add 2 mL of Ammonium acetate buffer solution, pH 3.5. Mix and add 1.2 mL of Thioacetamide solution. Mix immediately. Examine the solutions after 2 minutes. The test is invalid if the Reference solution does not show a slight brown color compared to the Blank solution. The substance to be examined complies with the test if any brown color in the Test solution is not more intense than that in the Reference solution (0.0025%).
[noteIf the result is difficult to judge, pass the solutions through a membrane filter (pore size 3-µm), and carry out the filtration slowly and uniformly. Compare the spots on the filters obtained with the different solutions. ]
Water-dispersible substances Place about 0.4 g of Hydrophobic Colloidal Silica in a 500-mL separating funnel, add 100 mL of water, and shake for 1 minute. Allow to stand for 1 hour. Allow 90 mL of the aqueous phase to run out dropwise without filtration into a suitable dish dried at 140 and cooled in a desiccator. Evaporate to dryness at 140, starting at a low temperature to avoid splashing. Cool in a desiccator: the weight of the residue does not exceed 12 mg (3.0%).
Loss on ignition 733 Ignite about 0.2 g of Hydrophobic Colloidal Silica in a platinum crucible at 900 for 2 hours. [noteIt is advisable to place the crucible in a cold oven and then to heat up the oven. ] Cool in a desiccator before weighing: it loses not more than 6.0% of its weight.
Assay To the residue obtained in the test for Loss on ignition, add sufficient alcohol to moisten the residue completely, and then add 0.2 mL of sulfuric acid. Add 6 mL of hydrofluoric acid, and evaporate to dryness on a hot plate at about 100, taking care to avoid loss from sputtering. Wash down the sides of the platinum crucible with 6 mL of hydrofluoric acid, and evaporate to dryness. Ignite at 900 for 2 hours, cool in a desiccator, and weigh. The difference between the weight of the residue obtained in the test for Loss on ignition, and the weight of the final residue gives the amount of SiO2 in the quantity of the substance to be examined: not less than 99.0% and not more than 101.0% of SiO2, calculated on the ignited basis.