Determination of Dissolved Oxygen in Seawater by Winkler Titration Using The Amperometric Technique

Abstract

This paper describes procedures to be used for the determination of dissolved oxygen in discrete samples of seawater. These procedures are based on the modified Winkler titration method (Carpenter 1965). These modifications reduced the loss of I2 during the titration due to volatilization by optimizing the concentrations of the “pickling” reagents to encourage the formation of the more stable triiodide complex I3- and by adopting the whole-bottle titration method which eliminates the loss of I2 during transfer of sample aliquots. This procedure is suitable for the measurement of the full range of oceanic oxygen concentrations (0-400 μmol kg-1) in uncontaminated seawater. The typical precision that can be achieved using automated amperometric endpoint detection systems is ±0.15 μmol kg-1 Carpenter (1965) established the accuracy of the method as <0.1% or ±0.3 μmol kg-1. This procedure is unsuitable for seawater containing hydrogen sulfide (H2S). In oxygen-deficient regions (<5 μmol kg-1) a high concentration of nitrite (NO2-) may cause a positive oxygen bias. Polluted waters may contain reducing substances that will react with the liberated I2 resulting in a negative oxygen bias