(Ph Eur monograph 0467)
Bentonite is a natural clay containing a high proportion of
montmorillonite, a native hydrated aluminium silicate in which some aluminium and silicon
atoms may be replaced by other atoms such as magnesium and iron.
A very fine, homogeneous, greyish-white powder with a more or
less yellowish or pinkish tint, practically insoluble in water and in aqueous solutions.
It swells with a little water forming a malleable mass.
0.5 g in a metal crucible add 1 g of potassium nitrate R and 3 g of sodium
carbonate R and heat until the mixture melts. Allow to cool. To the residue add 20 ml
of boiling water R, mix and filter.
Wash the residue with 50 ml of water R.
To the residue add 1 ml of hydrochloric acid R and 5 ml of water R. Filter. To the filtrate add 1 ml of strong
sodium hydroxide solution R and filter. To the filtrate add 3 ml of ammonium
chloride solution R. A gelatinous white precipitate is formed.
complies with the test for swelling power with water (see Tests).
g gives the reaction of silicates (2.3.1).
To 2 g add 100 ml of carbon dioxide-free water R and shake for 5 min. To 5 ml of the
suspension add 0.1 ml of thymolphthalein solution R. The liquid becomes bluish. Add
0.1 ml of 0.1M hydrochloric
acid. The liquid is decolourised within 5 min.
To 20 g add 1000 ml of water
R and mix for 15 min using a high-speed mixer capable of operating at not less
than 5000 r/min. Transfer to a wet sieve (75), tared after drying at 100C to 105C.
Wash with three quantities, each of 500 ml, of water
R, ensuring that any agglomerates have been dispersed. Dry at 100C to 105C
and weigh. The particles on the sieve weigh not more than 0.1 g (0.5 per cent).
To 6.0 g add 200 ml of water
R and mix for 20 min using a high-speed mixer capable of operating at 10 000
r/min. Transfer 100 ml of the suspension to a graduated cylinder. Allow to stand for 24 h.
The volume of the clear supernatant liquid is
not greater than 2 ml.
Swelling power with water
Add 2.0 g in twenty portions to 100 ml of a 10 g/l solution of
sodium laurilsulfate R in a 100 ml graduated cylinder about 30 mm in diameter.
Allow 2 min between additions for each portion to settle. Allow to stand for 2 h. The
apparent volume of the sediment is not less than 22 ml.
Heavy metals (2.4.8)
To 5.0 g add 7.5 ml of dilute hydrochloric acid R and
27.5 ml of water R. Boil for 5 min.
Centrifuge and filter the supernatant liquid. Wash the centrifugation residue with water R and filter. Dilute the combined filtrates to
50.0 ml with water R. To 5 ml of the
solution add 5 ml of water R, 10 ml of
hydrochloric acid R and 25 ml of methyl isobutyl ketone R and shake for 2
min. Separate the layers. Evaporate the aqueous layer to dryness on a water-bath. Dissolve
the residue in 1 ml of acetic acid R, dilute to 25 ml with water R and filter. 12 ml of the filtrate complies
with limit test A for heavy metals (50 ppm). Prepare the standard using lead standard
solution (1 ppm Pb) R.
Loss on drying (2.2.32)
Not more than 15 per cent, determined on 1.000 g by drying in
an oven at 100C to 105C.
Total viable aerobic count (2.6.12) not more than 103 micro-organisms
per gram, determined by plate-count.