Method for Nitrite Titration

The following method is suitable for the determination of most of the pharmacopoeial sulphonamide drugs and their preparations. It may also be used for other pharmacopoeial drugs for which nitrite titration is recommended.

Apparatus

A suitable open vessel of about 200 ml capacity is fitted with two similar clean platinum electrodes and a stirrer. The electrodes may be of platinum foil 0.5 cm square and should be placed 1.5 cm apart. They may be cleaned by immersing for a few seconds in boiling nitric acid containing a small amount of ferric chloride, followed by washing with water.

The polarizing voltage may be obtained from a 1.5-volt dry cell and potentiometer or another convenient device which enables a small but definite voltage to be applied across the electrodes. The current flowing in the system is indicated by a series galvanometer which should have adequate sensitivity.

Method

Weigh accurately about 0.5 g in the case of a sulphonamide or otherwise the quantity specified in the individual monograph and transfer to the titration vessel. Add 20 ml of hydrochloric acid and 50 ml of water stir until dissolved, cool to about 150. Immerse the platinum electrodes in the solution and apply a voltage of about 50 mV across the electrodes when the polarization of the electrodes takes place. Place the burette tip just above the surface of the solution and stir the solution gently, maintaining the temperature at about 150. The titration may be carried out manually or by means of an automatic titrator.

In the manual titration, add 0.1 M sodium nitrite slowly and when the titration is within 1 ml of the endpoint, add the titrant in 0.1 ml portions, allowing not less than 1 minute between additions. (The galvanometer needle deflects and then returns to approximately its original position until the end point is reached). At the end-point, when a slight excess of sodium nitrite is present, the electrodes are depolarised, current flows and a permanent deflection of the needle is obtained.

NOTE-It will be necessary to adjust the sensitivity of the galvanometer or the applied voltage before the titration is begun in order to obtain an adequate deflection at the endpoint.

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