One of our interests is to measure profiles of water column oxygen concentration. Oxygen is an indicator of biological activity and can also be used as a water mass tracer as different depths and ocean currents have distinct oxygen characteristics.

On TEAL-SHIPS cruises, we calibrate our full water column measurements of oxygen with “Winkler titrations” run on board the vessel. In these pictures, Melanie Cohn and Riley Morse perform a series of steps: First, water is collected from Niskin bottles at discrete depths (rinsed thoroughly and no air bubbles, please). Second, reagents manganese chloride and sodium iodide are used to spike the samples, preserving the oxygen concentration and inhibiting any microbial activity. The sample solutions are mixed and then stored in a dark place. Third, a titration is performed where a series of solutions are added to samples while they are mixed on a stirring plate. Once the reaction is at equilibrium, the titrated sample returns to a clear color. The initial and final volume of the thiosulfate titrant used for each sample is recorded at this equilibrium point and used to calculate the original oxygen concentration in the sample, generally 2-8 mg O₂ per liter of seawater.