This International Standard specifies a spectrophotometric method, known as the tetrachloromercurate (TCM)/pararosaniline method, for the determination of the mass concentration of sulfur dioxide in ambient air within the range of 20 μg/m^(3) to about 500 μg/m^(3). The sampling period is 30 min to 60 min. If a longer sampling period than 60 min is used, or higher concentrations of sulfur dioxide (up to about 2 000 μg/m^(3)) are expected, care is necessary to ensure that the concentrations of sulfur dioxide in the absorption solution given in clause 6, paragraph 2 are not exceeded. This can be achieved by a reduction of the volume flow rate during sampling. Sample solutions obtained by this procedure may be stored for up to 24 h before making measurements, provided that they are kept in a refrigerator at about 5 °C. Substances which are known to interfere and which might be present in the air being sampled are listed in 7.5 Indications of the precision and accuracy of the method, and of the lower detection limit are given in 8.2. Detection limit, standard deviations and interferences qualify the TCM-method for orientating field measurements in the higher concentration range. When more accurate measurements are necessary, instruments which are specially tested and calibrated should be used.