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.

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%).

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. [note—Prepare 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%).