Marine Biogeochemistry

Method / Description Oxygen

APPARATUS:
Eppendorfpipettes (1-5 cm3); 10 cm3 glass pipette
Various volumetric flasks und beakers
Various cylinders
Various bottles to store the reagents
Stoppered glass bottles of predetermined volume
Syringes with long needles (should reach the bottom of the stoppered glass bottles)

REAGENTS:
Manganese-II-chloride: 40 g of MnCl2 * 5 H2O are dissolved in 100 cm3 pure water. Stable.
Alkaline iodide: 60 g of KI and 30 g KOH are seperately dissolved in about 30 cm3 of pure water. The solutions are mixed and made up to 100 cm3 with pure water. Stable
H2SO4 (9 M): Mix 50 cm3 conc. H2SO4 and 50 cm3 pure water. Let cool and store in PE-bottle.
Sodium thiosulfate titrant: (0.02M): Merck-Titrisol™
Zinc-iodid-sol.: Merck.™
Iodat-standard: Dry KIO3 at 105°C and dissolve 356.7 mg in pure water and make up to exactly 1 dm-3. The oxidation capacity of this standard solution is 0.01 N (10 meq. electrons dm-1) of electrons. 

THIOSULFAT-CALIBRATION:
Prior to use, the [S2O3]- titrant has to be standardized as follows: 1 cm3 of H2SO4 (9 M), 0.5 cm3 of alkaline iodide and 0.5 cm3 of manganese-II-chloride are seperately added to 50 cm3 of pure water and the beaker is shaken  to prevent precipitation of Mn-hydroxides. Exactly  10.00 cm3 of the iodate-standard are added and the iodine is titrated with the thiosulphate solution until a light yellowish colour remains. At this point 1 cm3 zinc iodid solution is added and the titration is continued until the blue colour disappears. From the consumption of thiosulphate solution (V), the f-factor of the titrant is calculated as:
 

PROCEDURE:
Immediately after recovery the water is subsampled into stoppered glas-bottles of predetermined volume. The bottles should be rinsed with at least 100 cm3 of sample and the final sample has to be totally free of air bubbles. After adding 0.5 cm3 manganese-II-chloride and 0.5 cm3 alkaline iodide for the fixation of the oxygen, the bottles are carefully stoppered so that no air bubble is trapped. The bottles are well shaken and the precipitate is allowed to settle for about 30 minutes. Store cool.

Prior to titration 1 cm3 H2SO4 (9 M) are carefully pipetted and the re-stoppered bottles are shaken to dissolve the Mn-hydroxides. Pour the whole content of the stoppered bottle into an Erlenmeyer flask and titrate with the sodium thiosulfate titrant (0.02 M) until a light yellowish colour remains. At this point 1 cm3 zinc iodid solution is added and the titration is continued until the blue colour disappears.

CALCULATION:

a = consumption of thiosulphate solution  (in cm3)
b = volume of the sample bottle (in cm3)
f =  factor of the thiosulphate solution

INTERFERENCES:
During the handling of the samples and reagents it might happen that air oxygen is entrained into the samples. The oxygen blank is about 10 µM.

Contact

Head:

Prof. Dr. Klaus Wallmann
Tel.: 0431-600 2287
Fax: 0431-600 2928

Technique /Operator

Bettina Domeyer
Tel.: 0431-600 2230
Fax: 0431-600 2928

Anke Bleyer
Tel.: 0431-600 2230
Fax: 0431-600 2928

Regina Surberg
Tel.: 0431-600 2211
Fax: 0431-600 2928