DETERMINATION OF SULFUR-DIOXIDE IN AIR ON THE BASIS OF THE CATALYZED AUTOXIDATION OF CO(II) IN AN AZIDE MEDIUM

A method used for the determination of S(IV) in precipitation was adap ted for measurements of SO2 in air. A solution containing tris(hydroxy melhyl)-amino-methane (TRIS) and sodium azide is used to trap SO2 duri ng 30 min at an air flow of 15-40 L/h(-1). The addition of a solution containing perchloric acid, Co2+ and Mn2+ leads to a formation of a mi xture of azido-cobaltate(III) complexes (pH 5.2-5.4 for the final HN3/ N-3(-)-buffer), proportional to the total [S(IV)] in the absorption so lution. Spectrophotometric measurement is performed at 365 nm with a s lope of (3.07 +/- 0.03) x 10(4) L . mol(-1). cm(-1) of the calibration curve, Detection limit is 8 x 10(-8) M in the working solution with 5 cm light path cuvettes. This results in a relative detection limit of 2.8 mu g SO2/m(3) under the sampling conditions chosen. Interference of oxidants and H2S is avoided by inserting in the sampling train a ca rtridge containing FeSO4 . 7H(2)O and silver wool upstream of the abso rber. Oxides of nitrogen do not interfere. The method provides results comparable with those obtained with a coulometric monitor based on th e I-2/I- system.