A: Transfer 0.1mLof Oral Solution to a small test tube,and add 0.1mLof a freshly prepared mixture (1:1)of sodium alizarinsulfonate solution (1in 1000)and zirconyl nitrate solution (1in 1000)in 7Nhydrochloric acid:a yellow color is produced.

B: If necessary,reduce the volume of a portion of it by heating on a steam bath to a reduced volume containing about 10mg of sodium per mL:the solution so obtained responds to the tests for Sodium á191ñ.

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 an accurately measured volume of Oral Solution,equivalent to about 10mg of fluoride,to a 500-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 response line,determine the concentration,C,in µg per mL,of fluoride ion in the Assay preparation.Calculate the quantity,in mg,of fluoride ion in each mLof the Oral Solution taken by the formula: in which Cis the determined concentration of fluoride,in µg per mL,in the Assay preparation,and Vis the volume,in mL,of Oral Solution taken.Multiply the quantity of fluoride ion by 2.21to obtain the quantity of NaF.