Iodometric Determination of Chlorine Dioxide in Water

Iodometric titration remains one of the most widely used analytical methods for determining chlorine dioxide (ClO₂) concentration in water. Though increasingly complemented by instrumental techniques, this classical wet-chemical approach is valued for its reliability, simplicity, and minimal equipment requirements. Here is a concise guide to understanding and performing the method.

Principle of the Method

The determination rests on chlorine dioxide's strong oxidizing property. When a water sample is acidified and treated with excess potassium iodide (KI), ClO₂ oxidizes iodide ions (I⁻) to elemental iodine (I₂) in a quantitative stoichiometric reaction. The liberated iodine is then titrated with a standard sodium thiosulfate (Na₂S₂O₃) solution, using starch as an indicator. 

The endpoint is reached when the blue starch-iodine complex turns colorless. The volume of thiosulfate consumed directly relates to the original chlorine dioxide concentration through simple stoichiometric calculation.

Basic Procedure

Sample collection is the first critical step. Water samples should be protected from direct sunlight and excessive agitation to prevent ClO₂ volatilization or photodecomposition. A measured volume of sample is transferred into an iodine flask, followed by the addition of potassium iodide and acidification with sulfuric or hydrochloric acid to create the acidic environment necessary for the reaction. The flask is stoppered, shaken, and placed in the dark for a sufficient reaction period.

After the reaction is complete, the liberated iodine is titrated with standard sodium thiosulfate solution. When the titrated solution turns pale yellow, a few drops of starch indicator are added, turning the solution deep blue. Titration continues dropwise until the blue color just disappears—this is the endpoint. The ClO₂ concentration is calculated from the thiosulfate volume and its known molarity.

Interferences and the Continuous Titration Approach

A significant limitation of the simple iodometric method is its lack of selectivity. Any other oxidizing species present in the sample—such as free chlorine, ozone, chlorite, or chlorate—will also oxidize iodide and contribute to the titration, causing positively biased results. To address this, the continuous iodometric titration (also known as the five-step iodometric method) has been developed. 

This refined procedure exploits the fact that different chlorine species—ClO₂, free chlorine (Cl₂), chlorite (ClO₂⁻), and chlorate (ClO₃⁻)—react with iodide optimally at different pH values. By performing sequential titrations under controlled pH conditions (neutral vs. strongly acidic) and with selective chemical masking, each species can be distinguished and quantified individually. This approach is specified in standard methods such as HJ 551-2016 for wastewater analysis.