Determination of Methoxyl Group : Pharmaceutical Guidelines

Determination of Methoxyl Group

Learn how to determine the Methoxyl group in Pharmaceutical ingredients.


The apparatus consists of a 50-ml round-bottomed boiling flask into which is sealed a capillary side arm of 1mm diameter to provide an inlet for a stream of carbon dioxide or nitrogen. The flask is also fitted with an upright air condenser about 25 cm long and about 9 mm in diameter, bent through 1800 at the top and terminating in a glass capillary of 2 mm diameter dipping into a small scrubber containing about 2 ml of water. The outlet from the scrubber is a tube of about 7 mm diameter which dips below the surface of the liquid in the first of two receivers connected in series.


Weigh accurately a quantity of the substance under examination containing approximately 50 mg of methyl iodide and place it in the boiling flask. Add a little pumice, 2.5 ml of melted phenol and 5 ml of hydriodic acid and connect the flask with the remainder of the apparatus. The first receiver contains about 6 ml and the second receiver about 4 ml of a 10 percent w/v solution of potassium acetate in glacial acetic acid to which 0.2 ml of bromine has been added. Pass a slow uniform stream of carbon dioxide or nitrogen through the side arm of the boiling flask and gently heat the liquid by means of a mantled micro-burner at such a rate that the vapours of the boiling liquid rise half-way up the condenser. For most substances 30 minutes is sufficient to complete the reaction and sweep out the apparatus. Wash the contents of both receivers into a 250-ml glass-stoppered conical flask containing 5 ml of a 25 percent w/v solution of sodium acetate, adjust the volume of the liquid to approximately 125 ml and add 0.3 ml of formic acid. Rotate the flask until the colour due to the bromine is discharged, add 0.6 ml of formic acid, stopper the flask and mix the contents thoroughly so as to remove any excess of bromine from the vapour above the liquid in the flask. After allowing to stand for 1 to 2 minutes, add 1 g of potassium iodide and a few ml of 1 M sulphuric acid and titrate the liberated iodine with 0.1 M sodium thiosulphate.
Perform a blank titration and make any necessary correction.1 ml of 0.1 M sodium thiosulphate is equivalent to 0.0005172 g of methoxyl (CH3O).

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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