(Ph. Eur. monograph 0467)
DEFINITION
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.
CHARACTERS
Appearance
Very fine, homogeneous, greyish-white powder with a more or less yellowish or pinkish tint.
Solubility
Practically insoluble in water and in aqueous solutions.
It swells with a little water forming a malleable mass.
IDENTIFICATION
A. To 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 this residue add 20 mL of boiling water R, mix and filter. Wash the insoluble residue with 50 mL of water R. To this 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 this filtrate add 3 mL of ammonium chloride solution R. A gelatinous white precipitate is formed.
B. Add 2.0 g in 20 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.
C. 0.25 g gives the reaction of silicates (2.3.1).
TESTS
Alkalinity
To 2 g add 100 mL of carbon dioxide-free water R and shake for 5 min. To 5 mL of this suspension add 0.1 mL of thymolphthalein solution R. The liquid becomes bluish. Add 0.1 mL of 0.1 M hydrochloric acid. The liquid is decolourised within 5 min.
Coarse particles
Maximum 0.5 per cent.
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 the suspension to a wet sieve (75), tared after drying at 100-105 °C. Wash with 3 quantities, each of 500 mL, of water R, ensuring that any agglomerates have been dispersed. Dry the sieve at 100-105 °C and weigh. The
particles on the sieve weigh a maximum of 0.1 g.
Loss on drying (2.2.32)
Maximum 15 per cent, determined on 1.000 g by drying in an oven at 105 °C.
Microbial contamination
TAMC: acceptance criterion 10 CFU/g (2.6.12).
FUNCTIONALITY-RELATED CHARACTERISTICS
This section provides information on characteristics that are recognised as being relevant control parameters for one or more functions of the substance when used as an excipient (see chapter 5.15). Some of the characteristics described in the Functionality-related characteristics section may also be present in the mandatory part of the monograph since they also represent mandatory quality criteria. In such cases, a cross-reference to the tests described in the mandatory part is included in the Functionality-related characteristics section. Control of the characteristics can contribute to the quality of a medicinal product by improving the consistency of the manufacturing process and the performance of the medicinal product during use. Where control methods are cited, they are recognised as being suitable for the purpose, but other methods can also be used. Wherever results for a particular characteristic are reported, the control method must be indicated.
The following characteristics may be relevant for bentonite used as viscosity-increasing agent or suspending agent.
Sedimentation volume
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 this suspension to a graduated cylinder. Allow to stand for 24 h. The volume of the clear supernatant is not greater than 2 mL.
Swelling power with water
See Identification B.
