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

Procedure— Transfer an accurately weighed quantity of Gel,equivalent to about 80mg of stannous fluoride,to a capped plastic vessel equipped for titration in an inert atmosphere.Add a plastic coated stirring bar,20mLof recently boiled 3Nhydrochloric acid,and 5mLof potassium iodide TS.Close the vessel,purge the system with an oxygen-free inert gas,and titrate immediately with 0.1N Potassium iodide-iodateadding 2mLof starch TSas the endpoint is approached.Calculate the quantity,in mg,of stannous ion (Sn++)in each g of Gel by the formula: in which Vis the volume,in mL,of 0.1Npotassium iodide-iodate consumed;5.935is the Sn++equivalent,in mg,of each mLof 0.1Npotassium iodide-iodate;and Wis 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 1.91g of potassium chloride in water to make 100mLof solution.

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

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

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

Blank— Add 2mLof hydrochloric acid and 1.0mLof Potassium chloride solutionto a 100-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Concomitantly determine the absorbance of the Standard preparations,the Test preparation,and the Blankat the tin emission line of 235.5nm,with an atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering á851ñ)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 preparationsand the Test preparationby subtracting the absorbance of the Blank.Plot the corrected absorbances of the Standard preparationsversus 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 Cis the concentration,in µg per mL,of tin in the Test preparation;and Wis 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 Assayunder Sodium Fluoride Oral Solution.

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

Procedure— Proceed as directed for Procedurein the Assayunder 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 156.71is the molecular weight of stannous fluoride,38.0is twice the atomic weight of fluorine;Cis the determined concentration,in µg per mL,of fluoride in the Assay preparation;and Wis the weight,in g,of the Gel taken.