Packaging and storage—
Preserve in well-closed containers.
Identification—
Add 2 g in small portions to 100 mL of water, with vigorous agitation. Allow to stand for at least 12 hours to ensure complete hydration. Place 2 mL of the resulting mixture on a suitable glass slide, and allow to air-dry at room temperature to produce a uniform film. Place the slide in a vacuum desiccator over a free surface of ethylene glycol. Evacuate the desiccator, and close the stopcock so that the ethylene glycol saturates the desiccator chamber. Allow to stand for 12 hours. Record the X-ray diffraction pattern (see
X-Ray Diffraction 
941
), and calculate the
d values: several peaks are observed; the characteristic peak corresponds to a
d value between 10.3 and 10.7 Angstrom units.
pH 791—
Disperse 1.0 g in 10 mL of carbon dioxide-free water, and mix: the pH of the mixed dispersion so obtained is between 7.0 and 9.5.
Loss on drying 731—
Dry it at 105
to constant weight: it loses between 5.0% and 17.0% of its weight.
Loss on ignition 733—
When ignited at 1000
for 1 hour, it loses between 17.0% and 27.0% of its weight.
Volatile matter—
When ignited at 600
for 1 hour, it loses between 7.5% and 12.5% of its weight on the dried basis.
Powder fineness—
Add 50 g to 450 mL of water containing 5 g of sodium pyrophosphate, and stir for 10 minutes. Pour the resulting dispersion slowly through a No. 325 standard sieve (see
Particle Size Distribution Estimation by Analytical Sieving 786), and carefully wash the residue until clean. Dry the residue at 105
to constant weight: the dry weight of the residue so obtained is not more than 0.30% of the weight of the specimen taken.
Acid-soluble matter—
Boil 2.0 g with 100 mL of 0.2 N hydrochloric acid for 5 minutes, and cool. Add water to adjust the volume to 100 mL, and filter. Evaporate 50 mL of the filtrate so obtained to dryness, and ignite the residue at 600
: not more than 0.15 g is found (15%).
Carbonate—
Mix 1.0 g with 15 mL of 0.5 N sulfuric acid: no effervescence occurs.
Arsenic and Lead—
To 5.0 g add 50 mL of 1 N nitric acid, and boil for 30 minutes, adding 1 N nitric acid at times to maintain the volume. Filter into a 100-mL volumetric flask, wash the filter with water, and dilute the combined filtrate and washings with water to volume.
Arsenic—
Determine the arsenic in the solution by atomic absorption spectrometry (see
Spectrophotometry and Light-scattering 851), using a graphite furnace to volatilize the arsenic, as directed by the manufacturer of the instrument used, and measuring the absorbance at 189.0 nm against a standard: not more than 2 ppm is found.
Lead—
Determine the lead in the solution by atomic absorption spectrometry (see
Spectrophotometry and Light-scattering 851), using a graphite furnace to volatilize the lead, as directed by the manufacturer of the instrument used, and measuring the absorbance at 283.3 nm against a standard: not more than 0.001% is found.
Adsorptive capacity—
To 10 mL of a 1 in 10 suspension of the specimen in water add 80 mL of methylene blue solution (1 in 1000), and shake. Add 10 mL of barium chloride solution (1 in 50), and shake. Allow to stand for 15 minutes. Transfer 40 mL of the supernatant to a 50-mL centrifuge tube, and centrifuge. To 5 mL of the clear supernatant add 495 mL of water, and mix: the color of the solution so obtained is not deeper than that of a solution containing 1.5 µg of methylene blue per mL.
Organic volatile impurities, Method IV 467:
meets the requirements.
