2.3.2. Determination of sulfur dioxide
It would be essential to undertake a standard test whenever samples are to be analysed, under identical conditions for semi-quantitative measurements.
To establish a standard reference color card, after 0.5 g zinc powder was placed in the bottom of the tube, 10 mL standard solutions and 2 mL of 7.0 mol L−1 hydrochloric acid were added. The tube was sealed with a cap on which a wet lead acetate paper was suspended. The mixture was allowed to react for 3 min. A standard reference color card was prepared in correspondence to the concentration levels (6.0, 15.0, 30.0, 60.0, 100.0 μg mL−1) (Fig. 1b). The result formed a light-brown to brown color in the concentration range of 6.0–100.0 μg mL−1. Color intensity of concentrations exceeding 100.0 μg mL−1 did not facilitate visual comparison of sulfur dioxide concentrations.
To detect the sulfur dioxide concentration in samples, 10 mL of sample filtrate and 2 mL of hydrochloric acid were added to the tube in which 0.5 g zinc powder was added previously. The samples were mixed well and sealed with the cap quickly. For semi-quantitative detection of sulfur dioxide levels, the color of the final solution was compared to the standard reference color card after 3 min.