Packaging and storage—
Preserve in tight containers.
Labeling—
Label it to indicate that absorption of atmospheric moisture should be avoided following the opening of the original package, preferably by storage of the remainder of the contents in a tight container.
Identification—
Add 2 g in small portions to 100 mL of water, with intense agitation. Allow to stand for 12 hours to ensure complete hydration. Place 2 mL of the mixture so obtained on a suitable glass slide, and allow to air-dry at room temperature to produce an oriented 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: the largest peak corresponds to a
d value between 15.0 and 17.2 angstrom units. Prepare a random powder specimen of Bentonite, record the X-ray diffraction pattern, and determine the
d values in the region between 1.48 and 1.54 angstrom units: the peak is between 1.492 and 1.504 angstrom units.
pH 791—
Disperse 4.0 g in 200 mL of water, mixing vigorously to facilitate wetting. The pH of this suspension is between 9.5 and 10.5.
Loss on drying 731—
Dry it at 105
for 2 hours: it loses between 5.0% and 8.0% of its weight.
Arsenic 211—
Test preparation—
Transfer 8.0 g to a 250-mL beaker containing 100 mL of dilute hydrochloric acid (1 in 25), mix, cover with a watch glass, and boil gently, with occasional stirring, for 15 minutes without allowing excessive foaming. Filter the hot supernatant through a rapid-flow filter paper into a 200-mL volumetric flask, and wash with four 25-mL portions of hot dilute hydrochloric acid (1 in 25), collecting the washings in the volumetric flask. Cool the combined filtrates to room temperature, add dilute hydrochloric acid (1 in 25) to volume, and mix.
Procedure—
Use a 25-mL aliquot of Test preparation for the Procedure. The absorbance due to any red color from the Test preparation does not exceed that produced by 5.0 mL of Standard preparation (5 µg of As) when treated with the same quantities of the same reagents and in the same manner: the limit is 5 ppm.
Lead—
[NOTE—The
Standard preparation and the
Test preparation may be modified, if necessary, to obtain solutions, of suitable concentrations, adaptable to the linear or working range of the instrument. It may be necessary to correct the value obtained for the
Test preparation for interference from the test specimen matrix.
]
Standard preparation—
On the day of use, dilute 3.0 mL of
Lead Nitrate Stock Solution (see
Heavy Metals 231) with water to 100 mL. Each mL of the
Standard preparation contains the equivalent of 3 µg of lead.
Test preparation—
Transfer 3.75 g to a 250-mL beaker containing 100 mL of dilute hydrochloric acid (1 in 25), stir, cover with a watch glass, and boil for 15 minutes. Cool to room temperature, and filter through a rapid-flow filter paper into a 400-mL beaker. Wash the filter with four 25-mL portions of hot water, collecting the washings in the 400-mL beaker. Concentrate the combined extracts by gentle boiling to approximately 20 mL. If a precipitate appears, add 2 to 3 drops of nitric acid, heat to boiling, and cool to room temperature. Filter the concentrated extracts through a rapid-flow filter paper into a 50-mL volumetric flask. Transfer the remaining contents of the 400-mL beaker through the filter paper and into the flask with water. Dilute with water to volume, and mix.
Procedure—
Determine the absorbances of the Test preparation and the Standard preparation at 284 nm in a suitable atomic absorption spectrophotometer equipped with a lead hollow-cathode lamp deuterium arc background correction, and a single-slot burner, using an oxidizing flame of air and acetylene. The absorbance of the Test preparation is not greater than that of the Standard preparation (0.004%).
Gel formation—
Mix 6 g with 300 mg of magnesium oxide. Add the mixture, in several divided portions, to 200 mL of water contained in a blender of approximately 500-mL capacity. Blend thoroughly for 5 minutes at high speed, transfer 100 mL of the mixture to a 100-mL graduated cylinder, and allow to remain undisturbed for 24 hours: not more than 2 mL of supernatant appears on the surface.
Swelling power—
To 100 mL of water contained in a glass-stoppered cylinder of 100-mL capacity add 2 g in portions, dropping it upon the surface of the water and allowing each portion to settle before adding the next. The mass at the bottom gradually swells until it occupies an apparent volume of not less than 24 mL at the end of a 2-hour period.
Fineness of powder—
Sprinkle 2 g upon 20 mL of water contained in a mortar. Allow to swell, disperse evenly with a pestle, and dilute with water to 100 mL. Pour the suspension through a No. 200 standard sieve, and wash the sieve thoroughly with water. No grit is felt when the fingers are rubbed over the wire mesh of the sieve.
