0.1 N Potassium iodide-iodate— Prepare as directed in the Assay for stannous ion under Stannous Fluoride.

Procedure— Transfer an accurately weighed quantity of Gel, equivalent to about 80 mg of stannous fluoride, to a capped plastic vessel equipped for titration in an inert atmosphere. Add a plastic coated stirring bar, 20 mL of recently boiled 3 N hydrochloric acid, and 5 mL of potassium iodide TS. Close the vessel, purge the system with an oxygen-free inert gas, and titrate immediately with 0.1 N Potassium iodide-iodate adding 2 mL of starch TS as the endpoint is approached. Calculate the quantity, in mg, of stannous ion (Sn++) in each g of Gel by the formula: in which V is the volume, in mL, of 0.1 N potassium iodide-iodate consumed; 5.935 is the Sn++ equivalent, in mg, of each mL of 0.1 N potassium iodide-iodate; and W is the weight, in g, of Gel taken. The quantity, in mg, of stannous ion in each g of Gel is not less than 68.2% of the quantity, in mg, of stannous fluoride in each g of Gel as determined in the Assay, and is not less than 90.0% of the quantity, in mg, of total tin in each g of Gel as determined in the test for Total tin content.

Potassium chloride solution— Dissolve 1.91 g of potassium chloride in water to make 100 mL of solution.

Tin stock standard solution— Transfer 1.000 g of tin, accurately weighed, to a 1000-mL volumetric flask, add 200 mL of hydrochloric acid, and swirl to dissolve. Add 200 mL of water, allow to cool, dilute with water to volume, and mix.

Standard preparations— Transfer 5.0, 10.0, and 15.0 mL of Tin stock standard solution to separate 100-mL volumetric flasks, add 1.0 mL of Potassium chloride solution to each flask, dilute with water to volume, and mix. The Standard preparations contain, respectively, 50.0, 100.0, and 150.0 µg of tin per mL.

Test preparation— Transfer an accurately weighed quantity of Gel, equivalent to about 132 mg of stannous fluoride, to a plastic beaker. Add 80 mL of water and 20 mL of hydrochloric acid, and mix. Transfer this mixture to a 1000-mL volumetric flask, add 10.0 mL of Potassium chloride solution, dilute with water to volume, and mix.

Blank— Add 2 mL of hydrochloric acid and 1.0 mL of Potassium chloride solution to a 100-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbance of the Standard preparations, the Test preparation, and the Blank at the tin emission line of 235.5 nm, with an atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a tin hollow-cathode lamp and a nitrous oxide-acetylene oxidizing flame, using water to adjust the instrument to zero. Aspirate water into the spectrophotometer before and after each Standard preparation, the Test preparation, and the Blank. Correct the absorbances of the Standard preparations and the Test preparation by subtracting the absorbance of the Blank. Plot the corrected absorbances of the Standard preparations versus concentration, in µg per mL, of tin, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration, in µg per mL, of tin in the Test preparation. Calculate the quantity, in mg, of tin in each g of Gel taken by the formula: in which C is the concentration, in µg per mL, of tin in the Test preparation; and W is the quantity, in g, of Gel taken to prepare the Test preparation. Use this value to calculate the percentage of stannous ion in the test for Stannous ion content.

Buffer solution and Standard preparations—Prepare as directed in the Assay under Sodium Fluoride Oral Solution.

Assay preparation— Transfer an accurately weighed quantity of Gel, equivalent to about 8 mg of stannous fluoride, to a 100-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Sodium Fluoride Oral Solution. Calculate the quantity, in mg, of stannous fluoride (SnF2) in each g of the Gel taken by the formula: in which 156.71 is the molecular weight of stannous fluoride, 38.0 is twice the atomic weight of fluorine; C is the determined concentration, in µg per mL, of fluoride in the Assay preparation; and W is the weight, in g, of the Gel taken.