Dissolved Oxygen
The amount of dissolved oxygen in seawater is measured using the Carpenter modification of the Winkler method. Carpenters modification (1965) was designed to increase the accuracy of the original method devised by Winkler in 1889. Using Carpenters modification, the significant loss of iodine is reduced and air oxidation of iodide is minimized. Rather than using the visible color of the iodine-starch complex as an indicator of the titration end-point, we use an automated titrator that measures the absorption of ultraviolet light by the tri-iodide ion, which is centered at a wavelength of 350 nm.
Complete methods of dissolved oxygen determination can be found here.
Principle
Manganous chloride solution is added to a known quantity of seawater and is immediately followed by the addition of sodium hydroxide iodide solution. Manganous hydroxide is oxidized by the dissolved oxygen in the seawater sample and precipitates forming hydrated tetravalent oxides of manganese.
Mn+2 + 2OH- ----------------------------------------------> Mn(OH)2 (solid)
2Mn(OH)2 + O2 --------------------------------------------> 2MnO(OH)2 (solid)
Upon acidification of the sample, the manganese hydroxides dissolve and the tetravalent manganese in MnO(OH)2 acts as an oxidizing agent, setting free iodine from the iodide ions.
Mn(OH)2 + 2H+ -------------------------------------------> Mn+2 + 2H2O
MnO(OH)2 +4H+ +2I ------------------------------------> Mn+2 +I2 +3H2O
The liberated iodine, equivalent to the dissolved oxygen present in the sample, is then titrated with a standardized sodium thiosulfate solution and the dissolved oxygen present in the sample is calculated. The reaction is as follows:
I2 + 2S2O3 --------------------------------------------------> 2I + S4O6
Sample Drawing
Oxygen samples are always drawn first from the Niskin bottles and should are drawn as soon as possible to avoid contamination from atmospheric oxygen. Approximately 6 inches of Tygon tubing connected to a temperature probe via a y-connector is slipped onto the discharge valve of the Niskin. A calibrated volumetric flask is rinsed three times, then with the seawater still flowing, the end of the tygon tubing is placed into the flask nearing the bottom. The sample is then overflowed with twice the sample volume while making sure that there are no bubbles in the tubing during the overflow process. The tubing is then carefully removed from the sample flask to prevent the influx of bubbles.
Immediately after drawing the sample, 1 ml of manganous chloride solution is added into the flask. This is followed by the addition of 1ml of sodium hydroxide-sodium iodide solution to the sample. Both dispensers should be purged to remove air bubbles prior to the addition of these reagents. The stopper is then carefully placed in the bottle to avoid the trapping of air and the temperature of the seawater at the time of sample draw is recorded. After all samples are drawn, they are shaken vigorously to disperse the precipitate uniformly through the flask. This process is repeated again after the precipitate has settled to the bottom of the flask, or after at least ten minutes.
Sample analysis
Samples are analyzed after all of the precipitate settles to the bottom of the flask, after the second shake. The top of the flask is wiped with a kimwipe to remove moisture containing excess reagent around the stopper and then the stopper is carefully removed. Oneml of 10N sulfuric acid is added to the sample and a stir bar is placed inside the flask. The flask is then secured inside the clean water bath. The tip of the thiosulfate dispenser is placed inside the sample flask and the automated titration can begin with the use of an auto-titrator program.
Calculation and Expression of Results
The auto-titrator uses a UV detector that detects changes in voltage as thiosulfate is added to the sample. The volume of thiosulfate added is recorded at an endpoint once there is no change in voltage. The end point is determined by a least squares fit using a group of data points just prior to the end point, where the slope of the titration curve is steep, and a group of data points just after the endpoint, where the slope of the curve is close to zero. The intersection of the two lines is taken as the endpoint.