Fluoride ions—
[NOTE—All glassware and/or plasticware used in this test should be scrupulously clean and even free from trace amounts of fluoride. The use of plasticware to contain the solutions while the potential is measured is recommended.
]
Buffer solution—
To 110 g of sodium chloride in a 2-L volumetric flask add 1 g of sodium citrate and 700 mL of water, and dissolve with shaking. Carefully 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. Cool to room temperature, 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—
Accurately weigh 2.211 g of sodium fluoride, previously dried at 150
for 4 hours, into a 1-L volumetric flask, and dissolve in about 200 mL of water. Add 1.0 mL of sodium hydroxide solution (1 in 250), dilute with water to volume, and mix. Each mL of this solution contains 1 mg of fluoride ion. Store the solution in a closed plastic container.
Standard preparations—
Dilute a portion of Standard stock solution quantitatively and stepwise with Buffer solution to obtain a Standard preparation having a fluoride concentration of 1 µg per mL. Prepare the final dilution in a 100-mL volumetric flask. In the same manner, prepare additional Standard preparations having fluoride concentrations of 3 µg per mL, 5 µg per mL, and 10 µg per mL, respectively.
Test preparation—
Place 1 g of Flucytosine, accurately weighed, in a 100-mL volumetric flask, and dissolve in and dilute with Buffer solution to volume.
Procedure—
Concomitantly measure the potential (see
Titrimetry 
541
), in mV, of the
Standard preparations and the
Test preparation, with a suitable pH meter equipped with a fluoride-specific ion electrode and a glass-sleeved calomel reference electrode that has been modified in the following manner. Mix 70 mL of freshly prepared saturated potassium chloride solution with 30 mL of isopropyl alcohol, fill the electrode with the clear supernatant, and allow the electrode to remain in the mixture for not less than 2 hours prior to use, or preferably overnight.
When taking the measurements, transfer the solution to a 150-mL beaker, and immerse the electrodes. Insert a polytef-coated stirring bar into the beaker, place the beaker on a magnetic stirrer having an insulated top, and allow to stir until equilibrium is attained (about 1 to 2 minutes). Rinse and dry the electrodes between measurements, taking care not to scratch the crystal in the specific ion electrode.
Measure the potential of each Standard preparation, and plot the fluoride concentration, in mg per 100 mL, versus the potential, in mV, on semilogarithmic paper. Measure the potential of the Test preparation, and determine from the standard curve the fluoride concentration, in mg per 100 mL. Calculate the percentage of fluoride in the portion of Flucytosine taken by the formula:
C / 10,
in which C is the fluoride concentration, in mg per 100 mL, from the standard curve: not more than 0.05% of fluoride is found.
Fluorouracil—
Dissolve 250 mg in 10 mL of a mixture of glacial acetic acid and water (4:1). Apply 20 µL of this solution to a thin-layer chromatographic plate (see
Chromatography 621) coated with a 0.5-mm layer of chromatographic silica gel mixture. To the same plate apply 20 µL, in 10-µL increments, of a 0.025 mg per mL solution of
USP Fluorouracil RS in a mixture of glacial acetic acid and water (4:1). Develop the chromatogram in a mixture of chloroform and glacial acetic acid (13:7) until the solvent front has moved not less than 14 cm from the origin. Remove the plate from the developing chamber, and allow the solvent to evaporate. Locate the spots on the plate by observing under short-wavelength UV radiation: any spot from the solution under test is not greater in size and intensity than the spot at the respective
RF produced by the Standard solution, corresponding to not more than 0.1% of fluorouracil.
;)
