Sodium Fluoride Gel
Sodium Fluoride Gel contains NLT 90.0% and NMT 110.0% of the labeled amount of NaF, in an aqueous medium containing a suitable viscosity-inducing agent. It may contain a suitable buffering agent.
Sample: Amount of Gel, equivalent to 500 mg of fluoride ion
Analysis: Place the Sample in a platinum crucible in a well-ventilated hood, and add 15 mL of sulfuric acid. Cover the crucible with a piece of clear, polished glass, and heat on a steam bath for 1 h. Remove the glass cover, rinse it in water, and dry.
Acceptance criteria: The glass surface exposed to vapors from the crucible is etched.
[NoteStore all solutions, except the Buffer, in plastic containers. ]
Buffer: Dissolve 57 mL of glacial acetic acid, 58 g of sodium chloride, and 4 g of (1,2-cyclohexylenedinitrilo)tetraacetic acid in 500 mL of water. Adjust with 5 N sodium hydroxide to a pH of 5.25 ± 0.25, and dilute with water to 1000 mL.
Standard solution A: 420 µg/mL of USP Sodium Fluoride RS, equivalent to 190 µg/mL of fluoride ion (102 M)
Standard solution B: 19 µg/mL of fluoride ion (103 M), from Standard solution A
Standard solution C: 1.9 µg/mL of fluoride ion (104 M), from Standard solution B
Sample solution: Equivalent to 20 µg/mL of fluoride from the Gel in water
Samples: Standard solution A, Standard solution B, Standard solution C, and Sample solution
Pipet 20 mL of each Standard solution and the Sample solution into separate plastic beakers each containing a plastic-coated stirring bar. Pipet 20 mL of Buffer into each beaker. Concomitantly measure the potentials (see pH 791), in mV, of the solutions from the Standard solutions and of the solution from the Sample solution, with a pH meter capable of a minimum reproducibility of ±0.2 mV and equipped with a fluoride-specific, ion-indicating electrode and a suitable reference electrode.
[NoteWhen taking measurements, immerse the electrodes in the solution, stir on a magnetic stirrer having an insulated top until equilibrium is attained (12 min), 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/mL, of each diluted Standard solution versus potential, in mV. From the measured potential of the Sample solution and the standard response line, determine the concentration (C), in µg/mL, of fluoride ion in the Sample solution.
Calculate the quantity, as a percentage of label claim, of fluoride ion in the portion of Gel taken:
Result = (C/CU) × 100
Calculate the quantity, as a percentage of label claim, of sodium fluoride in the portion of Gel taken:
Result = (C/CU) × (Mr/Ar) × 100
Acceptance criteria: 90.0%110.0%
• pH 791: 6.08.0. Place about 40 mL in a plastic beaker, and determine the pH using a suitable electrode system.
• Packaging and Storage: Preserve in tight, plastic containers.
• Labeling: Label the Gel in terms of the content of sodium fluoride (NaF) and in terms of the content of fluoride ion.
• USP Reference Standards 11
USP Sodium Fluoride RS
Auxiliary Information Please check for your question in the FAQs before contacting USP.
USP35NF30 Page 4658Pharmacopeial Forum: Volume No. 35(5) Page 1175