Acceleration of reaction by glass magnet stirring shortens the time for the determination of dissolved oxygen in seawater by the Winkler method

Abstract

Dissolved oxygen (DO) is one of the core parameters in ocean investigation among various disciplines. The Winkler method, a classical approach, is extensively employed for DO determination. This method uses the reaction of Mn²⁺ to I₃^‐− as a proxy to quantitatively convert DO to iodine, followed by titration with sodium thiosulfate solution. However, this method is time-consuming and laborious due to the shaking and/or re-shaking of the DO bottle after adding the pickling reagents and waiting for the precipitate to settle. In this study, we implemented a stirring operation using a glass-coated magnetic stir bar at 1000 rpm for 1 min to replace the traditional static settling. The precision of DO measurements obtained via manual titration (coefficient of variation, CV < 0.2%) was comparable to that of the traditional method. Conversely, a polytetrafluoroethylene (PTFE) coated magnetic stir bar was unsuitable due to the release of pre-adsorbed oxygen. Comparative experiments conducted during an Indian Ocean cruise demonstrated that the DO measurements obtained using the improved method were in good agreement with those obtained using the traditional method. The coefficient of determination (R²) from the linear regression of the two methods was 0.999, and the ratio of the results averaged 1.00 ± 0.02. Our study also revealed that the combined effect of Mg²⁺, Ca²⁺, OH⁻, and HCO₃⁻ increases the settling time after the addition of pickling reagents. Overall, this modification represents a useful and labor-saving advancement in the determination of a classic oceanographic parameter, with potential for widespread adoption by scientists and technicians.