On determination of low oxygen concentrations with Winkler technique

The accuracy and detection limit of the oxygen technique is an actual problem in studying the processes that occur in the redox zone. The formal accuracy of the Winkler technique is 1 μM (0.02 mL L⁻¹) and its detection limit is about 3.0 μM (0.06 mL L⁻¹). These values are significantly higher than the similar characteristics (in molar concentrations) for such parameters as the hydrogen sulphide, nitrates, manganese, and others. In this work, we describe some recommendations for increasing the accuracy of the Winkler technique. The results of the application of these recommendations for the suboxic zone of the Black Sea are presented. During the 100th cruise of the R/V Professor Shtokman, argon-filled balloons were attached to the upper valves of Niskin bottles during the sampling, which allowed protecting the samples from contamination with atmospheric oxygen. The titration was performed with an automatic Metrohm Titrino burette with potentiometric end point detection. That allowed us to significantly increase the accuracy and decrease the detection limit compared with the visual techniques. The oxidized forms of metals present in the sea water (Mn(IV), Mn(III); Fe(III)) were measured, which allowed us to correct the Winkler technique calculations for the oxidizers. The studies performed during the 100th cruise of the R/V Professor Shtokman confirmed the absence of a layer of the coexistence of oxygen and hydrogen sulphide.