A multisyringe flow injection Winkler-based spectrophotometric analyzer for in-line monitoring of dissolved oxygen in seawater

An integrated analyzer based on the multisyringe flow injection analysis approach is proposed for the automated determination of dissolved oxygen in seawater. The entire Winkler method including precipitation of manganese(II) hydroxide, fixation of dissolved oxygen, dissolution of the oxidized manganese hydroxide precipitate, and generation of iodine and tri-iodide ion are in-line effected within the flow network. Spectrophotometric quantification of iodine and tri-iodide at the isosbestic wavelength of 466 nm renders enhanced method reliability. The calibration function is linear up to 19 mg L−1 dissolved oxygen and an injection frequency of 17 per hour is achieved. The multisyringe system features a highly satisfying signal stability with repeatabilities of 2.2% RSD that make it suitable for continuous determination of dissolved oxygen in seawater. Compared to the manual starch-end-point titrimetric Winkler method and early reported automated systems, concentrations and consumption of reagents and sample are reduced up to hundredfold. The versatility of the multisyringe assembly was exploited in the implementation of an ancillary automatic batch-wise Winkler titrator using a single syringe of the module for accurate titration of the released iodine/tri-iodide with thiosulfate.