Iodometric Determination of Iron in Water

Iodometry determines iron in water indirectly, through the oxidation of iodide by ferric iron. Ferrous iron does not oxidize iodide under the conditions of the method, so the procedure measures ferric iron unless a preliminary oxidation step is included. For total iron, dissolved ferrous iron must first be converted to the ferric form; for ferric iron alone, that step is omitted.

The principle is straightforward. In an acidic medium, ferric iron oxidizes iodide to iodine:

2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

The liberated iodine is then titrated with a standard sodium thiosulfate solution:

I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

Starch is added near the end point, and the disappearance of the blue starch–iodine complex marks the end of the titration. The amount of thiosulfate consumed is stoichiometrically related to the ferric iron originally present.

Sample preparation begins with acidification. Hydrochloric or sulfuric acid is added to keep ferric iron in solution and to provide the acidity needed for the iodide oxidation. Nitric acid should be avoided because it can oxidize iodide itself. The acid concentration must be controlled: too little acid allows ferric iron to hydrolyze, while too much acid promotes air oxidation of iodide and decomposition of thiosulfate. A moderate acidity, with the solution near pH 3–4 before the final titration stage, is generally suitable.

If total iron is to be determined, ferrous iron is oxidized before the iodide is added. Hydrogen peroxide in acidic solution is commonly used; excess peroxide must then be removed, for example by boiling, because it also oxidizes iodide and would produce a positive error. Other oxidizing agents, nitrite, and copper can interfere in the same way and must be removed or masked before the determination.

After acidification and any necessary oxidation, excess potassium iodide is added. The flask is stoppered, mixed, and kept in the dark for a period sufficient for the ferric–iodide reaction to approach completion. The dark condition is not optional: light accelerates the air oxidation of iodide, raising the blank and biasing results upward. After standing, the liberated iodine is titrated with sodium thiosulfate. The titration may proceed relatively quickly at first, but the flask should not be shaken violently, since iodine is volatile. When the yellow color becomes faint, starch indicator is added and the titration continues dropwise until the blue color disappears and does not return within about half a minute.

A blank determination is essential. It follows the same procedure with the same reagents but without the water sample, and its thiosulfate volume is subtracted from the sample volume. The blank corrects for iodide oxidized by air and by trace oxidants in the reagents. Sodium thiosulfate solutions are unstable and should be standardized before use and stored away from light.