Magnesium Stearate

(Ph Eur monograph 0229)

 

Ph Eur


 

 

Definition

 

Magnesium stearate is a mixture of magnesium salts of different fatty acids consisting mainly of stearic acid [(C17H35COO)2Mg; Mr 591.3] and palmitic acid [(C15H31COO)2 Mg; Mr 535.1] with minor proportions of other fatty acids. It contains not less than 4.0 per cent and not more than 5.0 per cent of Mg (Ar 24.30), calculated with reference to the dried substance. The fatty acid fraction contains not less than 40.0 per cent of stearic acid and the sum of stearic acid and palmitic acid is not less than 90.0 per cent.

 

Characters

 

A white, very fine, light powder, greasy to the touch, practically insoluble in water and in ethanol.

 

Identification

 

First identification

 

C, D.

 

Second identification

 

A, B, D.

 

A. The residue obtained in the preparation of solution S (see Tests) has a freezing point (2.2.18) not lower than 53C.

 

B. The acid value of the fatty acids (2.5.1) is 195 to 210, determined on 0.200 g of the residue obtained in the preparation of solution S dissolved in 25 ml of the prescribed mixture of solvents.

 

C. Examine the chromatograms obtained in the test for fatty acid composition. The retention times of the principal peaks in the chromatogram obtained with the test solution are approximately the same as those of the principal peaks in the chromatogram obtained with the reference solution.

 

D. 1 ml of solution S (see Tests) gives the reaction of magnesium (2.3.1).

 

Tests

 

Solution S

 

To 5.0 g add 50 ml of peroxide-free ether  R, 20 ml of dilute nitric acid R and 20 ml of distilled water  R and heat under a reflux condenser until dissolution is complete. Allow to cool. In a separating funnel, separate the aqueous layer and shake the ether layer with 2 quantities, each of 4 ml, of distilled water  R. Combine the aqueous layers, wash with 15 ml of peroxide-free ether  R and dilute to 50 ml with distilled water  R (solution S). Evaporate the organic layer to dryness and dry the residue at 100-105C. Keep the residue for identification tests A and B.

 

Acidity or alkalinity

 

To 1.0 g add 20 ml of carbon dioxide-free water  R and boil for 1 min with continuous shaking. Cool and filter. To 10 ml of the filtrate add 0.05 ml of bromothymol blue solution R1. Not more than 0.5 ml of 0.01M hydrochloric acid or 0.01M sodium hydroxide is required to change the colour of the indicator.

 

Chlorides (2.4.4)

 

0.5 ml of solution S diluted to 15 ml with water  R complies with the limit test for chlorides (0.1 per cent).

 

Sulphates (2.4.13)

 

0.3 ml of solution S diluted to 15 ml with distilled water  R complies with the limit test for sulphates (0.5 per cent).

 

Cadmium

 

Not more than 3 ppm of Cd, determined by atomic absorption spectrometry (2.2.23, Method II).

 

Test solution

 

Place 50.0 mg of the substance to be examined in a polytetrafluoroethylene digestion bomb and add 0.5 ml of a mixture of 1 volume of hydrochloric acid R and 5 volumes of cadmium- and lead-free nitric acid R. Allow to digest at 170C for 5 h. Allow to cool. Dissolve the residue in water  R and dilute to 5.0 ml with the same solvent.

 

Reference solutions

 

Prepare the reference solutions using cadmium standard solution (10 ppm Cd) R, diluted if necessary with a 1 per cent V/V solution of hydrochloric acid R.

 

Measure the absorbance at 228.8 nm, using a cadmium hollow-cathode lamp as a source of radiation and a graphite furnace as atomic generator.

 

Lead

 

Not more than 10 ppm of Pb, determined by atomic absorption spectrometry (2.2.23, Method II).

 

Test solution

 

Use the solution described in the test for cadmium.

 

Reference solutions

 

Prepare the reference solutions using lead standard solution (10 ppm Pb) R, diluted if necessary with water  R.

 

Measure the absorbance at 283.3 nm, using a lead hollow-cathode lamp as a source of radiation and a graphite furnace as atomic generator, depending on the apparatus the line at 217.0 nm may be used.

 

Nickel

 

Not more than 5 ppm of Ni, determined by atomic absorption spectrometry (2.2.23, Method II).

 

Test solution

 

Use the solution described in the test for cadmium.

 

Reference solutions

 

Prepare the reference solutions using nickel standard solution (10 ppm Ni) R, diluted if necessary with water  R.

 

Measure the absorbance at 232.0 nm, using a nickel hollow-cathode lamp as a source of radiation and a graphite furnace as atomic generator.

 

Loss on drying (2.2.32)

 

Not more than 6.0 per cent, determined on 1.000 g by drying in an oven at 100-105C.

 

Microbial contamination

 

Total viable aerobic count (2.6.12) not more than 103 micro-organisms per gram, determined by plate count. It complies with the test for Escherichia coli (2.6.13).

 

Assay

 

Magnesium

 

To 0.500 g in a 250 ml conical flask add 50 ml of a mixture of equal volumes of butanol  R and ethanol  R, 5 ml of concentrated ammonia R, 3 ml of ammonium chloride buffer solution pH 10.0 R, 30.0 ml of 0.1M sodium edetate and 15 mg of mordant black 11 triturate R. Heat to 45-50C until the solution is clear  and titrate with 0.1M zinc sulphate until the colour changes from blue to violet. Carry out a blank titration.

 

1 ml of 0.1M sodium edetate is equivalent to 2.431 mg of Mg.

 

Fatty acid composition

 

Examine by gas chromatography (2.2.28).

 

Test solution

 

In a conical flask fitted with a reflux condenser, dissolve 0.10 g of the substance to be examined in 5 ml of boron trifluoride-methanol solution R. Boil under a reflux condenser for 10 min. Add 4 ml of heptane R through the condenser and boil again under a reflux condenser for 10 min. Allow to cool. Add 20 ml of a saturated sodium chloride solution R. Shake and allow the layers to separate. Remove about 2 ml of the organic layer and dry over 0.2 g of anhydrous sodium sulphate R. Dilute 1.0 ml of the solution to 10.0 ml with heptane R.

 

Reference solution

 

Prepare the reference solution in the same manner as the test solution using 50.0 mg of palmitic acid CRS and 50.0 mg of stearic acid CRS instead of magnesium stearate.

 

The chromatographic procedure may be carried out using:

 

a fused-silica column 30 m long and 0.32 mm in internal diameter coated with macrogol 20 000 R (film thickness 0.5 mm),

 

helium for chromatography R as the carrier gas at a flow rate of 2.4 ml/min,

 

a flame-ionisation detector,

 

with the following temperature programme:

 

Inject 1 ml of the reference solution. When the chromatogram is recorded in the prescribed conditions, the retention time of methyl palmitate relative to that of methyl stearate is about 0.88. The test is not valid unless, in the chromatogram obtained with the reference solution, the resolution between the peaks corresponding to methyl stearate and methyl palmitate is at least 5.0.

 

Inject 1 ml of the test solution. Calculate the percentage content of stearic acid and palmitic acid from the areas of the peaks in the chromatogram obtained with the test solution by the normalisation procedure, disregarding the peak due to the solvent.

 

 

 


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