In Winkler iodometric analysis, which substances interfere?

• A. Nitrates and total chlorine
• B. NO2-N (nitrites), Ferrous Iron, suspended solids
• C. Dissolved oxygen and pH
• D. Alkalinity and hardness

Answer: (B)

In Winkler iodometric analysis, the amount of iodine generated (and thus the dissolved oxygen detected) depends on redox reactions that convert DO into iodine. Substances that either produce iodine independent of DO or consume iodine will skew the result.

Nitrite (NO2−) can oxidize iodide to iodine without any DO present, giving a falsely high iodine signal and overestimating dissolved oxygen. Ferrous iron (Fe2+) readily reduces iodine back to iodide, consuming the iodine that should be titrated and causing an underestimation of dissolved oxygen. Suspended solids can physically interfere by adsorbing iodine or the MnO2 precipitate, or by causing turbidity that complicates complete reaction and accurate endpoint determination, leading to erroneous results. These factors make that combination of interferences the most impactful for this method.