Determination of Sulphur Dioxide

Use Method A unless otherwise directed.

Method A

Apparatus

A round-bottomed flask of 1000 to 1500 ml capacity is connected with a water-cooled reflux condenser whose upper end is connected with two absorption tubes in series. The flask is provided with a gas inlet tube which reaches nearly to the bottom of the flask. Each absorption tube contains 10 ml of hydrogen peroxide solution (20 vol) neutralized with 0.1 M sodium hydroxide using bromophenol blue solution as indicator.

Procedure

Place in the flask 500 ml of water and 20 ml of hydrochloric acid. Connect the flask with the condenser and absorption tubes, pass through it a steady current of nitrogen or carbon dioxide which has been bubbled through sodium carbonate solution and gradually heat the liquid until it boils. Maintain the current of nitrogen or carbon dioxide, allow the solution to boil for about 10 minutes and then cool the flask by gradual immersion in water. Introduce, by momentarily removing the stopper of the flask, 50 to 100 g of the substance under examination, heat gently and boil for 45 minutes. Turn off the current of nitrogen or carbon dioxide, disconnect the absorption tubes and titrate the contents with 0.l M sodium hydroxide. 1 ml of 0.1 M sodium hydroxide is equivalent to 0.003203 g of SO2.
Repeat the operation without the substance under examination; the solution in the absorption tubes remains neutral.

Method B

Apparatus

A 500-ml three-necked round-bottomed flask is fitted with a water-cooled reflux condenser, 200 mm long, the upper end of which is connected to an absorption tube. The flask is fitted with a 100-ml dropping funnel and a gas inlet tube which reaches nearly to the bottom of the flask. The absorption tube contains 10 ml of hydrogen peroxide solution (10 vol) previously neutralized to bromophenol blue solution.

Procedure

Place 150 ml of water in the flask and pass a stream of carbon dioxide at a rate of 100 ml per minute for 15 minutes. Connect the absorption tube and without interrupting the flow of carbon dioxide introduce through the funnel the prescribed quantity of the substance under examination and 80 ml of 2 M hydrochloric acid. Boil for 1 hour, disconnect the absorption tube and stop the flow of carbon dioxide. Wash the contents of the absorption tube into a 250-ml conical flask, heat on a water-bath for IS minutes and allow to cool. Titrate with 0.1 M sodium hydroxide using bromophenol blue solution as indicator until the color changes from yellow to violet-blue. 1 ml of 0.1M sodium hydroxide is equivalent to 0.003203 g of SO2.

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