Chemical Catalyst Interference in the Winkler Titration Determination of Dissolved Oxygen—A Method for Correction

Author:

M.B. Lakin

Mixing Equipment Co., Inc.

This paper investigates the chemical interference caused by cobalt catalysts in the Winkler titration method for determining dissolved oxygen in water. It introduces a corrective procedure using a chemical blank to account for the additional iodine liberated by cobalt precipitates, improving the accuracy of oxygen measurements and calculated mass transfer coefficients. The study highlights the effects of catalyst concentration, pH, and repeated sulfite additions, providing a reliable method to correct errors and ensure reproducible results in unsteady-state reaeration tests.

Key Learnings

  • Cobalt (II) ions used as catalysts in unsteady-state reaeration tests can interfere with the Winkler titration, causing overestimation of dissolved oxygen levels.
  • The interference arises from the formation of cobalt (III) hydroxide precipitates, which release additional iodine during acidification, skewing results.
  • Using a chemical blank allows for correction of these errors, yielding more accurate dissolved oxygen measurements and mass transfer coefficients.
  • The magnitude of interference depends on cobalt concentration, pH, number of sodium sulfite additions, and aeration duration.
  • Dissolved oxygen probes provide more consistent readings and can be used in combination with the chemical blank method for validation.
  • The chemical blank method is simple, reproducible, and aligns with standard chemical analysis practices, allowing greater flexibility in catalyst concentrations.
  • Modified Winkler methods, such as azide and Rideal-Stewart variations, can correct for additional interferences like nitrites or reducing compounds.
  • Failure to correct for chemical interference can lead to significant errors (up to 28%) in calculated oxygen mass transfer coefficients.
  • Proper correction improves reliability and reproducibility in evaluating aeration efficiency for water treatment systems.
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