Chemical catalyst interference in the winkler titration determination of dissolved oxygen—a method for correction

A new procedure involving the measurement of a chemical blank is introduced to correct the cobalt interference effect associated with the Winkler titration analysis for dissolved oxygen. Unsteadystate reaeration studies employed for mechanical aerator evaluation tests show that when sodium sulfite is used in the presence of a cobalt ion catalyst to deoxygenate the test basin water, a chemical interference is detected in the Winkler determination of dissolved oxygen content. The source to which this influence is attributed is a chemical precipitate that forms in the reaeration process. Evidence suggests that the cobalt (II) ion is oxidized to the (III) oxidation state forming a cobalt (III) hydroxide in the alkaline test environment. Upon acidification of the test samples during the Winkler procedure, the cobalt (III), a powerful oxidizing agent, is released and in turn effects an increase in the amount of free iodine released and measured by titration. The additional iodine liberated in this manner yields erroneous oxygen concentration values which are then used to calculate the oxygen mass transfer coefficient. Considerable deviation from the true value of the coefficient can result.