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.