Electrochemical Study on Reaction Direction in the Winkler Method for Dissolved Oxygen Determination

The Winkler method for dissolved oxygen（DO）determination was expressed by four half-cell reactions whose electrode potentials defined by Nernst equation were calculated from the standard Gibbs free energy of formation. The virtual electron transfer from the lower electrode to the higher one visualized the direction of the redox reaction. pH change reversed the high-low relationship between DO and Mn electrodes; hence, the oxidation of Mn by DO occurs only in basic conditions. Some important points in analytical procedures and stability of reagents were discussed from the viewpoints of electrochemistry.