A: To 5mLof a solution (1in 100)in a test tube add 2mLof calcium chloride TS:a fine,white precipitate of calcium fluoride is formed.

B: Mix on a spot plate 2drops of a solution (1in 100)with 2drops of silver nitrate TS:a brown-black precipitate is formed.

C: Add 1drop of a solution (1in 100)to 2drops of mercuric chloride TS:a white,silky precipitate is formed.On further addition of the solution (1in 100),a brown-black precipitate is formed.

Rhodamine Bsolution— Dissolve 20mg of rhodamine Bin 200mLof 0.5Nhydrochloric acid.

Standard preparation— Transfer 55.0mg of antimony potassium tartrate,accurately weighed,to a 200-mLvolumetric flask,dissolve in water,dilute with water to volume,and mix.Transfer 5.0mLof this solution to a 500-mLvolumetric flask,add 6Nhydrochloric acid to volume,and mix.

Test preparation— Transfer 1.0g of Stannous Fluoride,accurately weighed,to a 50-mLvolumetric flask,add 6Nhydrochloric acid to volume,and mix.

Procedure— Pipet 5mLeach of the Standard preparationand the Test preparationinto separate 125-mLseparators,add 15mLof hydrochloric acid and 1g of ceric sulfate,and allow to stand for 5minutes,with occasional shaking.Add 500mg of hydroxylamine hydrochloride,and shake for 1minute.Pipet 15mLof isopropyl ether into the mixture,shake for 30seconds,add 7mLof water,and mix.Cool in a water bath at room temperature for 10minutes,shake for 30seconds,allow the layers to separate,and discard the aqueous phase.Add 20mLof Rhodamine Bsolution,shake for 30seconds,and discard the aqueous layer.Decant the ether layer from the top of the separator,and centrifuge,if necessary,to obtain a clear solution.Concomitantly determine the absorbances of the ether solutions from the Test preparationand the Standard preparationat the wavelength of maximum absorbance at about 550nm,with a suitable spectrophotometer,using water as the blank:the absorbance of the Test preparationdoes not exceed that of the Standard preparation(0.005%).

0.1N Potassium iodide-iodate— In a 1000-mLvolumetric flask,dissolve 3.567g of potassium iodate,previously dried at 110to constant weight,in 200mLof oxygen-free water containing 1g of sodium hydroxide and 10g of potassium iodide,dilute with oxygen-free water to volume,and mix.Standardize this solution by titrating a solution prepared from an accurately weighed quantity of reagent tin (Sn)and hydrochloric acid.Each mLof 0.1N Potassium iodide-iodateis equivalent to 5.935mg of Sn.

Procedure— Transfer about 250mg of Stannous Fluoride,accurately weighed,to a 500-mLconical flask,and add 300mLof hot,recently boiled 3Nhydrochloric acid.While passing a stream of an oxygen-free inert gas over the surface of the liquid,swirl the flask to dissolve the Stannous Fluoride,and cool to room temperature.Add 5mLof potassium iodide TS,and titrate in an inert atmosphere with 0.1N Potassium iodide-iodate,adding 3mLof starch TSas the endpoint is approached.Each mLof 0.1N Potassium iodide-iodateis equivalent to 5.935mg of Sn++.

Buffer solution— Dissolve 57mLof glacial acetic acid,58g of sodium chloride,and 4g of (1,2-cyclohexylenedinitrilo)tetraacetic acid in 500mLof water.Adjust with 5Nsodium hydroxide to a pHof 5.25±0.25,dilute with water to 1000mL,and mix.

Standard preparations— Dissolve an accurately weighed quantity of USP Sodium Fluoride RSquantitatively in water to obtain a solution containing 420µg per mL.Each mLof this solution (Standard preparation A)contains 190µg of fluoride ion (10-2M).Transfer 25.0mLof Standard preparation Ato a 250-mLvolumetric flask,dilute with water to volume,and mix.This solution (Standard preparation B)contains 19µg of fluoride ion per mL(10-3M).Transfer 25.0mLof Standard preparation Bto a 250-mLvolumetric flask,dilute with water to volume,and mix.This solution (Standard preparation C)contains 1.9µg of fluoride ion per mL(10-4M).

Assay preparation— Transfer to a 250-mLvolumetric flask about 100mg of Stannous Fluoride,accurately weighed.Add 50mLof water,mix vigorously for 5minutes,dilute with water to volume,and mix.Transfer 10.0mLof this solution to a 50-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Pipet 20mLof each Standard preparationand of the Assay preparationinto separate plastic beakers each containing a plastic-coated stirring bar.Pipet 20mLof Buffer solutioninto each beaker.Concomitantly measure the potentials (see pHá791ñ),in mV,of the solutions from the Standard preparationsand of the solution from the Assay preparation,with a pHmeter capable of a minimum reproducibility of ±0.2mVand equipped with a fluoride-specific ion-indicating electrode and a calomel reference electrode.[NOTE—When taking measurements,immerse the electrodes in the solution,stir on a magnetic stirrer having an insulated top until equilibrium is attained (1to 2minutes),and record the potential.Rinse and dry the electrodes between measurements,taking care to avoid damaging the crystal of the specific-ion electrode.]Plot the logarithms of the fluoride-ion concentrations,in µg per mL,of the Standard preparationsversus potential,in mV.From the measured potential of the Assay preparationand the standard reponse line,determine the concentration,C,in µg per mL,of fluoride ion in the Assay preparation.Calculate the percentage of fluoride (F)in the portion of Stannous Fluoride taken by the formula: in which Cis the determined concentration of fluoride,in µg per mL,in the Assay preparation,and Wis the weight,in mg,of Stannous Fluoride taken.