Sculpture dioxide is an important pollutant of the atmosphere. Its estimation in industrially polluted localities is often desired. An idea of the concentration of sulphur dioxide can be obtained by following methods:
1. Conductimetric method:
This method of analysis of sulphur dioxide has been the basis of commercial analysers for the gas since last fifty years. Sulphur dioxide is collected over a solution of hydrogen peroxide with which it reacts to form sulphuric acid.
SO2 + H2O2 H2SO4
The formation of H2SO4 raises the electrical conductivity of the solution which is proportional to the concentration of the gas in the sample of air. Measurement of conductance of the solution provides an estimate of SO2 concentration in the sample.
3. Amperometric method: Some of the commercial S02 monitors are based on this method in which electrical current is measured. The air sampled is allowed to bubble through a solution of Br2 which causes change in electrical potential as H2SO4 and HBr are produced.
SO2 + Br2 + H2O H2SO4 + HBr
These changes in electrical potential are obviously proportional to the amount of SO2 present in the sample which is measured. The potential may be restored to its original value by regeneratin Br2 electrochemically measuring the current required. It is from these measurements that the concentration of SO2 present in the sample is determined.
3. West-Gaek’s spectrophotometric method:
This method is a better and more accurate method for estimation of SO2 in air samples, and can be applied to concentrations as low as 0.0005 ppm. SO2 is collected over a solution of HgCl4 2– (HgCl2 + KCl) and is allowed to react with HCHO and then with para-rosaniline hydrochloride.
SO2 + H2O + HCHO2 SO2H
The reaction results in the formation of a red-violet dye, the intensity of colour of which is proportional to the concentration of SO2 present in the sample. The intensity of colour is measured spectrophotometrically at 548 nm. The presence of NO2 interferes with the reaction and can be removed by addition of sulphamic acid (H2N – SO3H) which reduces NO2 to nitrogen.