Appendix IX B. Determination of Sulfur Dioxide

Apparatus A round-bottomed flask of 1000- to 1500-mL capacity is fitted with a water-cooled reflux condenser the upper end of which is connected to two absorption tubes in series. The flask is fitted 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) previously neutralised with 0.1m sodium hydroxide VS using bromophenol blue solution as indicator.

Method Place in the flask 500 mL of water and 20 mL of hydrochloric acid. Pass through the flask a steady current of nitrogen or carbon dioxide that has been bubbled through dilute 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 cool the flask by gradual immersion in water. Introduce, while momentarily removing the stopper of the flask, a weighed quantity of 50 to 100 g of the substance being examined, heat gently and boil for 45 minutes. Disconnect the absorption tubes before turning off the current of nitrogen or carbon dioxide and titrate the combined contents with 0.1m sodium hydroxide VS. Each mL of 0.1m sodium hydroxide VS is equivalent to 3.203 mg of sulfur dioxide.

Repeat the operation without the substance being examined. The solution in the absorption tubes remains neutral.

Introduce 150 mL of water R into the flask (A) (see Figure 2.5.29.-1) and pass carbon dioxide R through the whole system for 15 min at a rate of 100 mL/min. To 10 mL of dilute hydrogen peroxide solution R add 0.15 mL of a 1 g/L solution of bromophenol blue R in alcohol (20 per cent V/V) R. Add 0.1 M sodium hydroxide until a violet-blue colour is obtained, without exceeding the end-point. Place the solution in the test-tube (D). Without interrupting the stream of carbon dioxide, remove the funnel (B) and introduce through the opening into the flask (A) 25.0 g of the substance to be examined (m g) with the aid of 100 mL of water R. Add through the funnel 80 mL of dilute hydrochloric acid R and boil for 1 h. Open the tap of the funnel and stop the flow of carbon dioxide and also the heating and the cooling water. Transfer the contents of the test-tube with the aid of a little water R to a 200 mL wide-necked, conical flask. Heat on a water-bath for 15 min and allow to cool. Add 0.1 mL of a 1 g/L solution of bromophenol blue R in alcohol (20 per cent V/V) R and titrate with 0.1 M sodium hydroxide until the colour changes from yellow to violet-blue (V₁ mL). Carry out a blank titration (V₂ mL).

Calculate the content of sulfur dioxide in parts per million from the expression