Iodometry in COD Determination: Principles and Applications

Chemical Oxygen Demand (COD) is a fundamental parameter in water quality assessment, reflecting the capacity of organic and reducing substances to consume oxygen under controlled oxidative conditions. While the dichromate reflux method remains the standard approach for COD determination, iodometry plays a crucial—though often overlooked—role in this analytical domain, particularly when dealing with challenging sample matrices.

The foundation of iodometry rests on the redox chemistry of iodine. In its indirect form—the variant relevant to COD analysis—the method involves two sequential steps. First, the analyte or its derivative reacts with an excess oxidant to liberate a stoichiometric equivalent of elemental iodine (I₂). 

Second, the released iodine is titrated against a standard sodium thiosulfate (Na₂S₂O₃) solution, with starch serving as the endpoint indicator. The volume of thiosulfate consumed directly corresponds to the amount of iodine released, which in turn reflects the concentration of the original substance of interest.

In the context of COD measurement, iodometry finds its most significant application in correcting for chloride interference—a persistent challenge in wastewater analysis. High concentrations of chloride ions (Cl⁻) are particularly problematic because they are co-oxidized by dichromate during the digestion step, generating chlorine gas (Cl₂) and artificially inflating the apparent COD value. This interference can render standard COD measurements unreliable, especially for industrial effluents and saline waters.

The chlorine-correction procedure elegantly harnesses iodometric principles. Following the conventional dichromate digestion—during which both organic matter and chloride are oxidized—the chlorine gas evolved is swept out of the reaction system and captured in a sodium hydroxide absorption solution. 

An excess of potassium iodide (KI) is then added to this absorbate, and the pH is adjusted to approximately 2–3. Under these acidic conditions, the absorbed chlorine oxidizes iodide ions to free iodine. The liberated iodine is subsequently titrated with standard sodium thiosulfate using starch as the indicator. 

From the thiosulfate consumption, the mass of oxygen equivalent to the chloride-derived interference is calculated—this is the chloride correction value. The true COD of the sample is obtained by subtracting this correction from the apparent COD measured by the standard dichromate method.