Packaging and storage—
Preserve in well-closed containers.
Labeling—
The labeling states the claimed percentage or range of percentages for the content of calcium oxide and for the content of silicon dioxide.
Identification—
A:
Mix 0.5 g with 10 mL of 3 N hydrochloric acid, filter, and neutralize the filtrate to litmus paper with 6 N ammonium hydroxide: the neutralized filtrate so obtained responds to the tests for
Calcium 
191
.
B:
Prepare a bead by fusing a few crystals of sodium ammonium phosphate on a platinum loop in the flame of a gas burner. Place the hot, transparent bead in contact with the specimen of Calcium Silicate, and again fuse. Silica floats about in the bead, producing, upon cooling, an opaque bead having a web-like structure.
pH 791:
between 8.4 and 10.2, determined in a well-mixed aqueous suspension (1 in 20).
Loss on ignition 733—
Transfer about 1 g, accurately weighed, to a suitable tared crucible, dry at 105
for 2 hours, and ignite at 900
to constant weight: it loses not more than 20.0% of its weight.
Lead 251—
Dissolve 1.0 g in 20 mL of 3 N hydrochloric acid, evaporate on a steam bath to about 10 mL, dilute with water to about 20 mL, and cool: the limit is 0.001%.
Heavy metals 231—
Boil 4.0 g with a mixture of 50 mL of water and 10 mL of hydrochloric acid for 20 minutes, adding water to maintain the volume during the boiling. Add ammonium hydroxide until the mixture is only slightly acid to litmus paper. Filter with the aid of suction, and wash with 15 mL to 20 mL of water, combining the washing with the original filtrate. Add 2 drops of phenolphthalein TS, then add a slight excess of 6 N ammonium hydroxide. Discharge the pink color with dilute hydrochloric acid (1 in 100). Dilute with water to 100 mL, and use 25 mL of the solution for the test: the limit is 20 µg per g.
Limit of fluoride—
NOTE—Store all solutions in plastic containers.
Buffer solution—
Add 800 mL of hot water to 74.4 g of edetate disodium and 24.2 g of tris(hydroxymethyl)aminomethane, and stir until dissolved. Adjust with 5 N sodium hydroxide to a pH of 7.5 to 7.6. Allow the solution to cool, and adjust with 5 N sodium hydroxide to a pH of 8.0. Dilute with water to 1000 mL, and mix.
Electrode system—
Use a fluoride-specific, ion-indicating electrode and a calomel reference electrode connected to a pH meter capable of measuring potentials with a reproducibility of ±0.2 mV (see
pH 791).
Standard stock solution—
Dissolve an accurately weighed quantity of
USP Sodium Fluoride RS quantitatively in water to obtain a solution containing 221 µg per mL. Each mL of this stock solution contains 100 µg of fluoride ion.
Standard solutions—
[NOTE—Prepare on the day of use.] Transfer 10.0 mL of Standard stock solution to a 100-mL volumetric flask, dilute with water to volume, and mix. This solution contains 10 µg of fluoride ion per mL (Standard solution A). Transfer 1.0 mL of Standard stock solution to a second 100-mL volumetric flask, dilute with water to volume, and mix. This solution contains 1.0 µg of fluoride ion per mL (Standard solution B).
Test solution—
Transfer 5.0 g of Calcium Silicate to a 150-mL polytef beaker. Add 40 mL of water and 20 mL of 1 N hydrochloric acid. Heat to near boiling for 1 minute, stirring continuously. Cool in an ice bath, transfer the suspension to a 100-mL volumetric flask, dilute with water to volume, and mix.
Procedure—
Transfer 20.0 mL of Standard solution A, Standard solution B, and the Test solution to separate polytef beakers, add 10.0 mL of Buffer solution to each beaker, and stir with a plastic-coated stirring bar. Concomitantly measure the potentials, in mV, of the solutions. [NOTE—When taking measurements, immerse the electrodes in the solution, 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 ion-specific electrode.] Plot the logarithms 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 response line, determine the concentration, C, in µg per mL, of fluoride ion in the Test solution. Calculate the quantity, in µg of fluoride per g of Calcium Silicate by multiplying C by 20. The limit is 10 µg per g.
Assay for silicon dioxide—
Transfer about 400 mg of Calcium Silicate, accurately weighed, to a beaker, add 5 mL of water and 10 mL of perchloric acid, and heat until dense white fumes of perchloric acid are evolved. Cover the beaker with a watch glass, and continue to heat for 15 minutes longer. Allow to cool, add 30 mL of water, filter, and wash the precipitate with 200 mL of hot water.
[NOTE—Retain the combined filtrate and washings for use in the
Assay for calcium oxide.
] Transfer the filter paper and its contents to a platinum crucible, heat slowly to dryness, then heat sufficiently to char the filter paper. After cooling, add a few drops of sulfuric acid, and ignite at about 1300
to constant weight. Moisten the residue with 5 drops of sulfuric acid, add 15 mL of hydrofluoric acid, heat cautiously on a hot plate until all of the acid is driven off, and ignite at a temperature not lower than 1000
to constant weight. Cool in a desiccator, and weigh: the loss in weight represents the weight of SiO
2. The percentage of silicon dioxide in the Calcium Silicate is between 90.0% and 110.0% of the content stated in the labeling, or within the range of percentages stated in the labeling.
Assay for calcium oxide—
Neutralize the combined filtrate and washings retained from the
Assay for silicon dioxide to litmus with 1 N sodium hydroxide. Add, while stirring, about 30 mL of 0.05 M edetate disodium VS from a 50-mL buret. Add 15 mL of 1 N sodium hydroxide and 300 mg of hydroxy naphthol blue, and continue the titration to a blue endpoint. Each mL of 0.05 M edetate disodium is equivalent to 2.804 mg of CaO. The percentage of CaO in the Calcium Silicate is between 90.0% and 110.0% of the content stated in the labeling, or within the range of percentages stated in the labeling.
Sum of CaO, SiO2, and Loss on ignition—
The sum of the percentages obtained in the three tests is not less than 90.0%.
