Assay of Metronidazole


To determine the percentage purity of a given sample of Metronidazole tablet.


A substance with very weak acid or basic property does not give a sharp endpoint in an aqueous solution can be titrated with non-aqueous solvents. Non-aqueous titration is based on Bronsted-Lowry theory According to this theory an acid is a proton donor, i.e. a substance which tends to dissociate to yield a proton and a base is a proton acceptor, i.e. a substance which tends to combine with a proton. When an acid HB dissociates it yields a proton together with the conjugate base B of the acid.
Metronidazole is a weak base and it is assayed by non-aqueous titration. When a weakly basic drug is dissolved in an acidic solvent with titration of
perchloric acid, the basic property of a mineral is enhanced. If a strong acid like perchloric acid is added to acetic acid, the acetic acid behaves as a base and combine with a proton donated by perchloric acid to form an onium ion which acts as a strong acid, the onium ion has readily donated the proton to the base. The indicator used in this titration is crystal violet. The use of acetic anhydride in this solution combines with the water molecule to form acetic acid.


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Preparation of Perchloric acid 0.1M

Mixed 8.5 ml of Perchloric acid in 900 ml of glacial acetic acid shake vigorously and continuous stirring. Then added 30 ml acetic anhydride and
make up the volume to 1 litre with glacial acetic acid and allowed to stand for 24 hours before use. The acetic anhydride reacts with the water in perchloric acid and some traces in glacial acetic acid thereby making the resulting mixture practically anhydrous.

Standardisation of 0.1 N Perchloric acid

About 0.5g of Potassium hydrogen phthalate were weighed accurately and transfer in a 100 ml conical flask, previously dried at 120oC for 2 hours.
50ml of glacial acetic acid was added, warmed if necessary, to dissolve the salt completely. Cooled and titrated with 0.1M perchloric acid using crystal violet 0.1ml (2 drops) as an indicator. The endpoint is colour changes from violet to emerald green.
Each ml of 0.1M Perchloric acid = 0.02042g of Potassium Hydrogen Phthalate

Assay of metronidazole

About 3.0g of metronidazole sample was weighed accurately and transfer into a 250ml conical flask, 50ml of Glacial acetic acid was added, warmed
gently. Cooled and titrated with 0.1M perchloric acid using α-Naphthol benzene as an indicator. The endpoint is colour changes from Blue to Blue-green.

Each ml of 0.1M perchloric acid = 0.02042g of Potassium hydrogen Phthalate.


The Molarity of 0.1M Perchloric acid = _____________
The Percentage purity of given metronidazole was found to be = ________________

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