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), H2S + I-2 = 2HI + S; and side reaction of sulfide (2), S-2 + 4I(2) + 80H(-) = SO42- + 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%. (C) 1999 Elsevier Science B.V. All rights reserved.