Wednesday, 15 July 2009

Monographs: Pharmaceutical substances: Acidum benzoicum - Benzoic acid

Molecular formula. C7H6O2

Relative molecular mass. 122.1

Graphic formula.

Chemical name. Benzenecarboxylic acid; CAS Reg. No. 65-85-0.

Description. Colourless, light, feathery crystals or a white, microcrystalline powder; odour, characteristic, faint.

Solubility. Slightly soluble in water; freely soluble in ethanol (~750 g/l) TS, ether R.

Category. Dermatological agent.

Storage. Benzoic acid should be kept in a well-closed container.


Definition. Benzoic acid contains not less than 99.0% and not more than 100.5% of C7H6O2, calculated with reference to the anhydrous substance.

Identity test

Boil 0.1 g with 0.1 g of calcium carbonate R1 and 5 ml of water, and filter; add a few drops of ferric chloride (25 g/l) TS to the filtrate; a beige coloured precipitate is produced.

Melting range. 121-124°C.

Heavy metals. For the preparation of the test solution use 1.0 g dissolved in 25 ml of acetone R, add 2 ml of water and dilute to 40 ml with acetone R and mix; determine the heavy metal content as described under 2.2.3 Limit test for heavy metals, according to Method A; not more than 20 μg/g.

Chlorinated compounds and chlorides. Dissolve 0.35 g in 5 ml of sodium carbonate (50 g/l) TS, evaporate to dryness, and heat the residue until completely charred, keeping the temperature below 400°C. Extract the residue with a mixture of 10 ml of water and 12 ml of nitric acid (~130 g/l) TS, and filter. Proceed with the filtrate as described under 2.2.1 Limit test for chlorides; the chloride content is not more than 0.7 mg/g.

Sulfated ash. Not more than 1.0 mg/g.

Water. Determine as described under 2.8 Determination of water by the Karl Fischer method, Method A, using about 1 g of the substance and 25 ml of a solution composed of 1 volume of methanol R and 2 volumes of pyridine R as the solvent; the water content is not more than 7.0 mg/g.

Readily oxidizable substances. Add 1.5 ml of sulfuric acid (~1760 g/l) TS to 100 ml of water, heat to boiling, and add, drop by drop, potassium permanganate (0.02 mol/l) VS until the pink colour persists for 30 seconds. Dissolve 1.0 g of the substance being examined in the hot solution and titrate with potassium permanganate (0.02 mol/l) VS to a pink colour that persists for 15 seconds; not more than 0.5 ml of potassium permanganate (0.02 mol/l) VS is required.

Assay. Dissolve about 0.25 g, accurately weighed, in 15 ml of ethanol (~750 g/l) TS, previously neutralized to phenol red/ethanol TS, add 20 ml of water and titrate with sodium hydroxide (0.1 mol/l) VS, using phenol red/ethanol TS as indicator. Repeat the operation without the substance being examined and make any necessary corrections. Each ml of sodium hydroxide (0.1 mol/l) VS is equivalent to 12.21 mg of C7H6O2.

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