Molecular formula. C₁₆H₁₈N₂O₅S

Relative molecular mass. 350.4

Graphic formula.

Chemical name. (2S,5R,6R)-3,3-Dimethyl-7-oxo-6-(2-phenoxyacetamido)-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid; [2S-(2α,5α,6β)]-3,3-dimethyl-7-oxo-6 [(phenoxyacetyl)amino]-4-thia-1-azabicyclo [3.2.0]heptane-2-carboxylic acid; CAS Reg. No. 87-08-1.

Description. A white, fine crystalline powder.

Solubility. Soluble in 1700 parts of water and in 7 parts of ethanol (~750 g/l) TS.

Category. Antibiotic.

Storage. Phenoxymethylpenicillin should be kept in a tightly closed container, protected from light.

Additional information. Even in the absence of light, Phenoxymethylpenicillin is gradually degraded on exposure to a humid atmosphere, the decomposition being faster at higher temperatures.

Requirements

Definition. Phenoxymethylpenicillin contains not less than 95.0% and not more than 102.0% of C₁₆H₁₈N₂O₅S, calculated with reference to the anydrous substance.

Identity tests

• Either tests A and B or tests B and C may be applied.

A. Carry out the examination as described under 1.7 Spectrophotometry in the infrared region. The infrared absorption spectrum is concordant with the spectrum obtained from phenoxymethylpenicillin RS or with the reference spectrum of phenoxymethylpenicillin.

B. To 2 mg in a test-tube add 1 drop of water followed by 2 mL of sulfuric acid (~1760 g/l) TS and mix; the solution is colourless. Immerse the test-tube for 1 minute in a water-bath; the solution remains colourless.

C. Place 2 mg in a test-tube, add 1 drop of water and 2 mL of formaldehyde/sulfuric acid TS and mix; the solution is red. Immerse the test-tube for 1 minute in a water-bath; a red-brown colour is produced.

Specific optical rotation. Use a 10 mg/mL solution in 1-butanol R and calculate with reference to the anhydrous substance; = +186° to +200°.

Water. Determine as described under 2.8 Determination of water by the Karl Fischer method, Method B, using about 0.3 g of the substance; not more than 15 mg/g.

pH value. pH of a 5.0 mg/mL suspension in water, 2.4-4.0.

p -Hydroxyphenoxymethylpenicillin. Dissolve about 0.1 g, accurately weighed, in sufficient sodium hydroxide (0.1 mol/l) VS to produce 100 mL. Measure the absorbance of a 1-cm layer at the maximum at about 306 nm; not more than 0.36 (preferably use 2-cm cells for the measurement and calculate the absorbance of a 1-cm layer).

Ultraviolet absorbance range. Dilute 20 mL of the solution obtained in the test for p-hydroxyphenoxymethylpenicillin to 100 mL with sodium hydroxide (0.1 mol/l) VS. Measure the absorbance of a 1-cm layer at the maximum at about 274 nm; not less than 0.56 and not more than 0.62.

Assay. Dissolve about 50 mg, accurately weighed, in a mixture of 0.6 mL of sodium hydrogen carbonate (40 g/l) TS and 10 mL of water and dilute with sufficient water to produce 1000 mL. Transfer two 2.0-mL aliquots of this solution into separate stoppered tubes. To one tube add 10.0 mL of imidazole/mercuric chloride TS, mix, stopper the tube, and place it in a water-bath at 60°C for exactly 25 minutes. Cool the tube rapidly to 20°C (solution A). To the second tube add 10.0 mL of water and mix (solution B).

Without delay measure the absorbance of a 1-cm layer at the maximum at about 325 nm against a solvent cell containing a mixture of 2.0 mL of water and 10.0 mL of imidazole/mercuric chloride TS for solution A and water for solution B.

From the difference between the absorbance of solution A and that of solution B, calculate the amount of C₁₆H₁₈N₂O₅S in the substance being examined by comparison with phenoxymethylpenicillin potassium RS similarly and concurrently examined, but omitting the addition of sodium hydrogen carbonate (40 g/l) TS; each mg of phenoxymethylpenicillin potassium RS (C₁₆H₁₇KN₂O₅S) is equivalent to 0.902 mg of phenoxymethylpenicillin (C₁₆H₁₈N₂O₅S). In an adequately calibrated spectrophotometer the absorbance of the reference solution should be 0.63 ± 0.03.