pH 5.25 buffer— Dissolve 110 g of sodium chloride and 1 g of sodium citrate in 700 mL of water in a 2000-mL volumetric flask. Cautiously add 150 g of sodium hydroxide, and dissolve with shaking. Cool to room temperature, and, while stirring, cautiously add 450 mL of glacial acetic acid to the cooled solution. Cool, add 600 mL of isopropyl alcohol, dilute with water to volume, and mix: the pH of this solution is between 5.0 and 5.5.

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

Standard solutions— Dilute portions of the Standard stock solution quantitatively and stepwise with pH 5.25 buffer to obtain 100-mL solutions having concentrations of 1, 3, 5, and 10 µg per mL.

Test solution— Shake 25 mL with 25 mL of water for 5 minutes, and allow the liquids to separate completely. Transfer 5.0 mL of the water layer to a 10-mL volumetric flask, dilute with pH 5.25 buffer to volume, and mix.

Procedure— Concomitantly measure the potential (see Titrimetry 541), in mV, of the Standard solutions and the Test solution, with a pH meter capable of a minimum reproducibility of ±0.2 mV, 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-mL beaker containing a polytef-coated stirring bar. Allow to stir on a magnetic stirrer having an insulated top until equilibrium is attained (1 to 2 minutes), 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 solutions versus potential, in mV. From the measured potential of the Test solution and the standard curve, determine the concentration, in µg per mL, of fluoride ions in the Test solution: not more than 10 µg per mL is found.