Packaging and storage—
Preserve in tight, plastic containers.
Labeling—
Label Gel in terms of the content of sodium fluoride (NaF) and in terms of the content of fluoride ion.
Identification—
A:
Place a quantity of Gel, equivalent to about 500 mg of fluoride ion, 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 hour. Remove the glass cover, rinse it in water, and dry: the glass surface exposed to vapors from the crucible is etched.
B:
It responds to the tests for
Phosphate 
191
.
Viscosity 911—
Place a quantity of Gel in a suitable plastic container, insert the stopper securely, and allow to stand until the gel is free from air bubbles. Place it in a water bath maintained at a temperature of 25 ± 0.5
until it adjusts to the temperature of the water bath (30 minutes or longer). Do not stir the gel while it is in the bath. Remove the sample from the bath, stir the gel gently for 5 seconds, and without delay, using a rotational viscosimeter, determine the viscosity using the appropriate spindle to obtain a scale reading between 10% and 90% of full scale at a speed of 60 rpm or of 30 rpm. Calculate the viscosity, in centipoises, by multiplying the scale reading by the constant for the spindle and speed used: the viscosity is between 7000 and 20,000 centipoises.
pH 791—
Place about 40 mL in a plastic beaker, add about 250 mg of quinhydrone, and stir for 1 minute, leaving some of the quinhydrone undissolved. Determine the pH using a hydrofluoric acid frit-junction calomel reference electrode and a platinum electrode: the pH is between 3.0 and 4.0.
Assay—
[NOTE—Store all solutions, except
Buffer solution, in plastic containers.
]
Assay preparation—
Transfer a quantity of Gel, equivalent to about 20 mg of fluoride ion, accurately weighed, to a 1000-mL volumetric flask, add water to dissolve, dilute with water to volume, and mix.
Procedure—
Proceed as directed for
Procedure in the
Assay under
Sodium Fluoride Oral Solution. The quantity, in mg, of fluoride ion in the portion of Gel taken is equivalent to
C, the determined concentration of fluoride ion, in µg per mL, in the
Assay preparation.
