Oxygen represents about 21% of the air and it naturally dissolves into the ocean through gas exchanges between the water and the atmosphere. The concentration of dissolved oxygen in seawater is an important parameter used by physicist, chemist and biologist in oceanography. It enables to study the water masses, the redox potential of a water column or the biological processes that consume/release oxygen for example.
The Winkler method is a rapid and accurate way to measure dissolved oxygen in water samples. It owes its name to its inventor Lajos Winkler, a Hungarian chemist, who made it up in 1888 while he was still a doctorate student. The method was then improved in 1965 by Carpenter.
- BOD Bottles: glass bottles specially designed to carry out the Winkler test. Our enemies are air bubbles and the special shape of these bottles makes easier the exclusion of these bubbles that will affect the accuracy of the oxygen measurements. All the bottles used have been numbered and calibrated so the exact volume of each BOD is known;
- Reagent 1: MnCl2 (manganese (II) chloride solution);
- Reagent 2: NaI, NaOH (sodium iodine basique solution);
- Reagent 3: H2SO4 (sulphuric acid);
- Sodium Thiosulfate: NaS2O3;
- Metrohm titrator;
- Computer & Tiamo software;
- Teflon plunger
The objective is to estimate the production and the respiration of the planktonic community inside the mesocosms. As we study two oxygen-related biological processes, we are going to estimate them measuring oxygen differences after light or dark incubations.
The sampling of the mesocosms has to be done before the sunrise (5 am ready to leave the harbour)! Back on land, the BOD are filled with the seawater collected and a part of the bottles is directly fixed (means that the dissolved oxygen is trapped) adding reagents 1 & 2. The amount of oxygen measured is the initial concentration (T0), the initial state of the system. The rest of the BOD are incubated in the dark or in the light for 24H. After that time period, these bottles are also fixed and the concentration of oxygen is measured.
The oxygen difference between T0 and T24 light incubations represents the production while the difference between T0 and T24 dark incubations represents the respiration of the community.
Just a bit of chemistry…
The addition of reagents 1 & 2 in the sample (the fixation) traps the oxygen: in the basic environment created by the addition of NaOH, manganic hydroxides are formed resulting in a brown precipitate. The dissolved oxygen oxidize the manganese and convert the Mn(II) hydroxides into Mn(III) hydroxides, so the amount of Mn(III) actually corresponds to the amount of oxygen. This first reaction needs at least 5h to take place. After that, the sample is acidified just before the analysis. In an acid environment, the manganic hydroxides dissolve. The Mn(III) released oxidise the iodide ions (I-) previously added (reagent 2) to iodine (I2). Finally, the iodine is titrated by the thiosulfate (iodine is reduced and thiosulfate oxidised), the orange colour of the sample disappear slowly during the titration. At the end, the equivalent volume of thiosulfate is determined by the titrator and as we know that four moles of thiosulfate react with one mole of oxygen, the amount of oxygen in the sample can be determined. Considering the exact volume of the bottle (the BOD have all been previously calibrated) the oxygen concentration can be computed in µmol/L.