Blog entry by Lina Middendorf
My core task during the expedition is to determine the oxygen content of water samples collected at various depths with the CTD rosette. Although the CTD is equipped with oxygen sensors, these sensors require calibration – and that’s where the Winkler method comes in. This classical titration principle is very straightforward: a chemical reaction is carried out in a precise stoichiometric ratio, and from the volume of a standard solution required to complete the reaction, the oxygen concentration can be calculated.
Water samples are taken directly from the CTD rosette into so-called Winkler bottles – glass bottles with a ground-glass stopper and an exact internal volume. They are filled to overflowing to ensure no air bubbles remain, as even tiny amounts of atmospheric oxygen could distort the result. Immediately after sampling, the oxygen in the water is “fixed” chemically to prevent changes before analysis. Oxygen is a dissolved gas subject to a dynamic equilibrium with its surroundings; temperature fluctuations or agitation could alter its concentration. To avoid this, manganese (II) chloride solution and an alkaline potassium iodide solution are added. These reagents react with the dissolved oxygen to form a beige-brown precipitate. The bottles are sealed without air bubbles, shaken, and stored in the dark to prevent photochemical side reactions. Once the precipitate has settled completely, sulfuric acid is added. This dissolves the precipitate and releases iodine into the solution, turning it a characteristic deep orange.
The released iodine is quantified by titration with a sodium thiosulfate solution of known concentration. Before that, the exact concentration of the sodium thiosulfate is determined, providing a correction factor (titer) for precise results. During the titration, the sodium thiosulfate is added while stirring, gradually reducing the iodine and changing the solution from orange to pale yellow. At this stage, starch solution is added, which forms a blue-violet complex with any remaining iodine – a highly visible indicator for the endpoint. The titration continues until the blue color disappears completely, signalling that all iodine has been reduced. The volume of sodium thiosulfate used is then used to calculate the original oxygen content.
Switching colour during titration. Images by L. Middendorf.
The Winkler method is a redox titration – a reaction in which one species is oxidized while another is reduced. In simplified terms:
1. Fixation: Manganese (II) ions precipitate as manganese (II) hydroxide, which is oxidized by the dissolved oxygen to manganese (III/IV) hydroxide.
2. Acidification: In acidic conditions, manganese is reduced back to Mn (II), and iodide ions are oxidized to iodine.
3. Titration: Iodine is reduced back to iodide by thiosulfate, which is oxidized to tetrathionate.
The amount of iodine released is directly proportional to the oxygen content of the original water sample.
Dissolved oxygen is a key parameter in understanding ocean processes. It reflects biological activity, mixing of water masses, and can serve as an indicator for long-term environmental changes. Accurate oxygen measurements are therefore essential for interpreting the data collected during the expedition.