Estimation of dissolved oxygen in water:

Reagents: Manganous sulphate solution Alkali-iodine reagent

Starch indicator

0.00025 No Sodium thiosulphate solution

Procedure:

. 20 mL of sample water was taken in a dry breakers.

. Adding 3 drops of Manganous sulphate solution and 2 drops of alkali iodine solution to it.

. Mix well and allow the precipitate to settle down.
. Adding 2 drops of conc. H2 SO4 and mix well till the precipitate

dissolves.
.20 drops of the above solution was taken in a dry test tube.

. Adding 1 drop of starch to the tube. A blue colour of the solution appears.

. Adding Sodium thiosulphate solution drop wise with a calibrated pipette to the tube till the blue colour disappears.

. Counting the number of drops of Sodium thiosulphate solution used.

Calculation:

Volume of water = No. of drops x Drop value of the pipette = p mL Volume of Na2 S2 O3 = No. of drops x Drop value of the pipette = q mL Dissolved oxygen (ppm) = 2 x q / p
Result: The DO of the sample is ___________mg/L or ppm.