pH5.25buffer— Dissolve 110g of sodium chloride and 1g of sodium citrate in 700mLof water in a 2000-mLvolumetric flask.Cautiously add 150g of sodium hydroxide,and dissolve with shaking.Cool to room temperature,and,while stirring,cautiously add 450mLof glacial acetic acid to the cooled solution.Cool,add 600mLof isopropyl alcohol,dilute with water to volume,and mix:the pHof this solution is between 5.0and 5.5.

Standard stock solution— Transfer 221mg of sodium fluoride,previously dried at 150for 4hours,to a 100-mLvolumetric flask,add about 20mLof water,and mix to dissolve.Add 1.0mLof sodium hydroxide solution (1in 2500),dilute with water to volume,and mix.Each mLof this solution contains 1mg of fluoride ions.Store in a tightly closed,plastic container.

Standard solutions— Dilute portions of the Standard stock solutionquantitatively and stepwise with pH5.25bufferto obtain 100-mLsolutions having concentrations of 1,3,5,and 10µg per mL.

Test solution— Shake 25mLwith 25mLof water for 5minutes,and allow the liquids to separate completely.Transfer 5.0mLof the water layer to a 10-mLvolumetric flask,dilute with pH5.25bufferto volume,and mix.

Procedure— Concomitantly measure the potential (see Titrimetry á541ñ),in mV,of the Standard solutionsand the Test solution,with a pHmeter capable of a minimum reproducibility of ±0.2mV,equipped with a glass-sleeved calomel-fluoride specific-ion electrode system.[NOTE—When taking measurements,immerse the electrodes in the solution which has been transferred to a 150-mLbeaker containing a polytef-coated stirring bar.Allow to 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 logarithm of the fluoride-ion concentrations,in µg per mL,of the Standard solutionsversus potential,in mV.From the measured potential of the Test solutionand the standard curve,determine the concentration,in µg per mL,of fluoride ions in the Test solution:not more than 10µg per mLis found.