Title: Determination of Sulphur Dioxide
- Objective: To determine the sulphur Dioxide content in the sample.
2. Procedure:
Use Method A unless otherwise directed.
- Preparation of Reagents:
Note: All solutions should be prepared in Distilled/Purified Water.
- Bromophenol Blue Solution: Dissolve 0.1 g of Bromophenol Blue with gentle heating in 1.5 ml of 0.1 M Sodium Hydroxide and 20 ml of ethanol (95%) and add sufficient water to produce 100 ml.
- 1 M Sodium Hydroxide Volumetric Solution: Dissolve 4.2 g of Sodium Hydroxide in sufficient water to produce 1000 ml. Standardise the solution before use.
- 2 M Hydrochloric acid: Dilute 17 ml of Hydrochloric acid to 100 ml with water.
- 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.) neutralised 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 hydrochloride 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 being examined, heat gently and boil for 45 minutes. Turn off the current of nitrogen or carbon dioxide, disconnect the absorption tubes and titrates the contents with 0.1 M sodium hydroxide. Each ml of 0.1 M sodium hydroxide is equivalent to 0.003203 g of SO2.
Repeat the operation without the substance being examined; 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 neutralised 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 being examined 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 15 minutes and allow to cool. Titrate with 0.1 M sodium hydroxide using bromophenol blue solution as indicator until the colour changes from yellow to violet-blue.
1 ml of 0.1 M sodium hydroxide is equivalent to 0.003203 g of SO2.