4.10  Determination of sulphur dioxide as sulphate ions on impregnated filters

The extraction of sulphate from the alkaline impregnated filters is described in Section 3.6. For the Thorin method the extract has a too high pH to permit a direct analysis and the solution has to be treated with a cation resin before analysis.

4.10.1  Determination of sulphur dioxide as sulphate by ion chromatography

The procedure after the extraction from the filter (Section 3.6) for Dionex systems and Waters systems are described in Section 4.1.

4.10.2  Determination of sulphur dioxide as sulphate spectrophotometric by the barium perchlorate – Thorin method

4.10.2.1  Field of application

This method can be used for the determination of sulphur dioxide in ambient air after the sulphur dioxide has been absorbed on a potassium hydroxide impregnated filter (see Section 3.6). The concentration range is 0.1 to 8 mg SO2 per litre leaching solution.

4.10.2.2  Principle

Sulphur dioxide is absorbed on a potassium hydroxide impregnated filter as sulphite. During the sampling period and storage the sulphite will be partly oxidized to sulphate. The filters are extracted with water, and hydrogen peroxide is added to oxidize the remaining sulphite to sulphate. Before analysis by the Thorin method (see Section 3.6.3), the extracted solution has to be treated with a cation exchange resin to remove the potassium and to neutralize the solution.

4.10.2.3  Instrumentation

Same as listed in Section 4.2.1 In addition 100-150 ml polyethylene beakers or 30 ml centrifuge tubes are needed for the extraction.

4.10.2.4  Chemicals

Same as listed in Section 4.2.1. In addition: Hydrogen peroxide (H2O2), 30%. Traces of sulphate can be removed from the ion exchange resin by washing with 0.1 M sodium hydroxide, 0.1 M hydrochloric acid and distilled water before use.

4.10.2.5  Reagents

Same as listed in Section 4.2.1.

4.10.2.6  Calibration

Proceed as described in Section 4.2.1.

4.10.2.7  Analytical procedure

  1. Leaching of the filters; proceed as described in Section 3.6.1.2.
  2. Analysis; proceed as described in Section 4.2.1.

4.10.2.8  Expression of results

The concentration of sulphur dioxide in the air expressed as microgram of sulphur per cubic metre, is given by:

         

a  = concentration of sulphur in milligrams per litre in the leaching solution, read from the calibration curve.
v1  = the volume, in litres, of the leaching solution.
v2  = the volume, in cubic metres, of the air sample.

4.10.2.9  References

Healy, C. and Atkins, D.H.F. (1975) The determination of atmospheric sulphur dioxide after collection on impregnated filter paper. Harwell, U.K. Atomic Energy Authority (AERE-R 7956).


Last revision: November 2001