Winkler's method overestimates dissolved oxygen in natural waters: Hydrogen peroxide interference and its implications

Abstract

In the Winkler's reaction scheme used for the determination of dissolved oxygen in natural waters, hydrogen peroxide reacts with iodide to form tri-iodide ion and leads to an over-estimation in the concentration of oxygen. In pure solutions, the reaction is slow but it is accelerated in the presence of Mn + 2 and reaches completion when Mn(OH) 2 is first formed under basic condition before the acidification of the solution, as in this reaction scheme. Each mole of hydrogen peroxide appears as 0.5 mol of apparent oxygen. This interference further exacerbates the recently realized over-estimation by the presence of naturally occurring iodate. Thus, the use of solubility equations of oxygen that are based on manometrically determined equilibrium concentrations, which are free from these chemical interferences, is highly recommended. The presence of hydrogen peroxide may have led to an over-estimation in the global efflux of oxygen from the ocean to the atmosphere by 1% and an over-estimation in the biological contribution to the % saturation anomaly of oxygen by 0.03%. At the present level of sophistication at which oxygen data are being used in data manipulation, these over-estimations should be taken into account and corrected for. Assuming that there is negligible change in their concentrations after sample collection, the distributions of hydrogen peroxide and iodate in the oceans are sufficiently well known that schemes for such corrections may be devised. However, they will degrade significantly the presently claimed precision in the determination of oxygen. Since the concentration of hydrogen peroxide is known to change upon storage, these variations will affect the estimation of community respiration rates and primary production by measuring the changes in the concentration of oxygen in incubated samples. This error may be especially prominent in the estimation of the former.