Modification of iodometric determination of total and reactive sulfide in environmental samples.

In iodometric determination of sulfide two reactions are taking place when alkaline solution is added to HCl acid-iodine. The main oxidation reaction (1), H(2)S+I(2)=2HI+S; and side reaction of sulfide (2), S(-2)+4I(2)+8OH(-)=SO(4)(2-)+8I(-)+4H(2)O. Preference of reaction (2) over (1) is dependent on pH increasing to >7. When sulfide solution of pH 9 was mixed with HCl acid-iodine, the recovery exceeded 120%, but the recovery of a solution with a pH of 13 exceeded 200%. To eliminate the side reaction in iodometric titration, the sulfide solution must be acidic when it is mixed with HCl-iodine. To avoid the side reaction (2), the pH of sulfide solutions were adjusted with acetic acid to pH 5.5, mixed with HCl-iodine solution and then titrated with standard thiosulfate with precision and accuracy <+/-3%.