Sulfide by Iodometry: Separation First, Titration Second

The iodometric determination of sulfide in water is deceptively simple on paper: sulfide reduces iodine, and the unconsumed iodine is titrated with thiosulfate. In real water matrices, however, the method only works when sulfide is first separated from the substances that share its reducing character.

The core chemistry is a two-step redox sequence. Hydrogen sulfide reacts with a known excess of iodine in acidic solution, producing elemental sulfur and iodide: H₂S + I₂ → S + 2HI. The iodine that remains is then back-titrated with a standard sodium thiosulfate solution, with starch serving as the endpoint indicator. The difference between the iodine added and the iodine found gives the sulfide content.

Because direct titration is easily biased by coexisting reductants, the standard approach interposes an acidification–purge–absorption step. The sample is acidified, converting dissolved sulfide species to volatile H₂S, which is carried by an oxygen-free gas stream—typically nitrogen—into an absorber. Zinc acetate is a common absorbent, precipitating sulfide as ZnS; alkaline solutions such as sodium hydroxide are also used and can give higher recoveries because they neutralize the weakly acidic H₂S more rapidly. This separation is what makes the iodometric finish selective: the volatile sulfide is removed from non-volatile reducing interferences before any iodine is introduced.

Interferences that survive the purge must be handled explicitly. Sulfite is a classic positive interferent, and adding formalin to the absorber masks it by forming a sulfite–formaldehyde adduct. Thiosulfate can be removed by washing the zinc sulfide precipitate with a 10% KOH solution, although this step must be validated for the particular matrix because dissolved organic matter can still exert an iodine demand. Nitrite is usually a negligible interferent under the acidic purge conditions.

The method is best suited to sulfide concentrations above roughly 0.4 mg/L; at lower levels, the iodine consumed becomes too small relative to the blank and the precision deteriorates. Samples with high sulfide loads should be diluted or taken in smaller portions so that the iodine excess remains sufficient to drive the reaction cleanly.