Packaging and storage—
Preserve in well-closed containers.
Labeling—
Label it to indicate whether it is anhydrous or the dihydrate.
Identification—
A:
Dissolve about 100 mg by warming with a mixture of 5 mL of 3 N hydrochloric acid and 5 mL of water, add 2.5 mL of 6 N ammonium hydroxide dropwise, with shaking, and then add 5 mL of
ammonium oxalate TS: a white precipitate is formed.
B:
To 10 mL of a warm solution (1 in 100) in a slight excess of nitric acid add 10 mL of
ammonium molybdate TS: a yellow precipitate of ammonium phosphomolybdate is formed.
Loss on ignition 
733
—
Ignite it at 800
to 825
to constant weight: anhydrous Dibasic Calcium Phosphate loses between 6.6% and 8.5% of its weight, and the dihydrate form of Dibasic Calcium Phosphate loses between 24.5% and 26.5% of its weight.
Carbonate—
Mix 1.0 g with 5 mL of water, and add 2 mL of hydrochloric acid: no effervescence occurs.
Chloride 221—
To 0.30 g add 10 mL of water and 2 mL of nitric acid, and warm gently, if necessary, until no more dissolves. Dilute to 25 mL, filter, if necessary, and add 1 mL of
silver nitrate TS: the turbidity does not exceed that produced by 1.0 mL of 0.020 N hydrochloric acid (0.25%).
Sulfate 221—
Dissolve 1.0 g in the smallest possible amount of 3 N hydrochloric acid, dilute with water to 100 mL, and filter, if necessary. To 20 mL of the filtrate add 1 mL of
barium chloride TS: the turbidity does not exceed that produced by 1.0 mL of 0.020 N sulfuric acid (0.5%).
Arsenic, Method I 211—
Prepare the
Test Preparation by dissolving 1.0 g in 25 mL of 3 N hydrochloric acid and diluting with water to 55 mL: the resulting solution meets the requirements of the test, the addition of 20 mL of 7 N sulfuric acid specified under
Procedure being omitted. The limit is 3 µg per g.
Barium—
Heat 0.50 g with 10 mL of water, and add hydrochloric acid dropwise, stirring after each addition, until no more dissolves. Filter, and to the filtrate add 2 mL of
potassium sulfate TS: no turbidity is produced within 10 minutes.
Heavy metals, Method I 231—
Warm 1.3 g with 3 mL of 3 N hydrochloric acid until no more dissolves, dilute with water to 50 mL, and filter: the limit is 0.003%.
Limit of acid-insoluble substances—
Heat 5.0 g with a mixture of 40 mL of water and 10 mL of hydrochloric acid until no more dissolves, and dilute with water to 100 mL. If an insoluble residue remains, filter, wash with hot water until the last washing does not give a reaction for chloride, and dry the residue at 105
for 1 hour. The weight of the residue does not exceed 10 mg: not more than 0.2% of acid-insoluble substances is found.
Limit of fluoride—
[NOTE—Prepare and store all solutions in plastic containers.
]
Buffer solution—
Dissolve 73.5 g of sodium citrate dihydrate in water to make 250 mL of solution.
Standard solution—
Dissolve an accurately weighed quantity of
USP Sodium Fluoride RS quantitatively in water to obtain a solution containing 1.1052 mg per mL. Transfer 20.0 mL of the resulting solution to a 100-mL volumetric flask containing 50 mL of
Buffer solution, dilute with water to volume, and mix. Each mL of this solution contains 100 µg of fluoride ion.
Electrode system—
Use a fluoride-specific, ion-indicating electrode and a silver-silver chloride reference electrode connected to a pH meter capable of measuring potentials with a minimum reproducibility of ±0.2 mV (see
pH 791).
Standard response line—
Transfer 50.0 mL of Buffer solution and 2.0 mL of hydrochloric acid to a beaker, and add water to make 100 mL. Add a plastic-coated stirring bar, insert the electrodes into the solution, stir for 15 minutes, and read the potential, in mV. Continue stirring, and at 5-minute intervals add 100 µL, 100 µL, 300 µL, and 500 µL of Standard solution, reading the potential 5 minutes after each addition. Plot the logarithms of the cumulative fluoride ion concentrations (0.1 µg per mL, 0.2 µg per mL, 0.5 µg per mL, and 1.0 µg per mL) versus potential, in mV.
Procedure—
Transfer 2.0 g of the specimen under test to a beaker containing a plastic-coated stirring bar, add 20 mL of water and 2.0 mL of hydrochloric acid, and stir until dissolved. Add 50.0 mL of Buffer solution and sufficient water to make 100 mL of test solution. Rinse and dry the electrodes, insert them into the test solution, stir for 5 minutes, and read the potential, in mV. From the measured potential and the Standard response line determine the concentration, C, in µg per mL, of fluoride ion in the test solution. Calculate the percentage of fluoride in the specimen taken by multiplying C by 0.005: the limit is 0.005%.
Assay—
Dissolve about 250 mg of Dibasic Calcium Phosphate, accurately weighed, with the aid of gentle heat if necessary, in a mixture of hydrochloric acid and water (5:3) contained in a 250-mL beaker equipped with a magnetic stirrer, and cautiously add 125 mL of water. With constant stirring, add, in the order named, 0.5 mL of triethanolamine, 300 mg of hydroxy naphthol blue, and, from a 50-mL buret, about 23 mL of 0.05 M edetate disodium VS. Add sodium hydroxide solution (45 in 100) until the initial red color changes to clear blue. Continue to add it dropwise until the color changes to violet, and add an additional 0.5 mL. The pH is between 12.3 and 12.5. Continue the titration dropwise with the 0.05 M edetate disodium VS to the appearance of a clear blue endpoint that persists for not less than 60 seconds. Each mL of 0.05 M edetate disodium is equivalent to 6.803 mg of CaHPO4 or to 8.604 mg of CaHPO4·2H2O.
