Estimation of Dissolved oxygen (DO) in a water sample

Dissolved oxygen content in a water sample is indicative of its quality. Amount of dissolved oxygen in the water of river, pond, etc., determines whether biological flora and fauna, other aquatic living organisms viz. fishes will survive in it or not. Dissolved oxygen is also determined in waste water/ industrial effluent since it is an index of its pollution. Although nowadays, more sensitive methods like electrodes are available, still titrimetric method is in common use.  It is generally defined as number of milli-litres of oxygen gas per litre of water (ml. L^-1).

Principle:

The estimation of dissolved oxygen is done using titrimetric method. In the alkaline condition, manganese sulfate gets converted into manganous hydroxide. The oxygen present in the liquid combines with manganous hydroxide and oxidizes manganese to tetravalent state, MnO(OH)₂, and this MnO(OH)₂ on acidification liberates iodine equivalent to that of oxygen fixed. The liberated iodine is titrated using standard sodium thiosulfate solution and starch as indicator.

Sodium thiosulfate upon reaction with iodine forms sodium tetrathionate and sodium iodide. Here, progress of the reaction is monitored by using starch as indicator since starch after binding iodine gives blue colored complex.

2Na₂S₂O₃ + I₂ ==== Na₂S₄O₆ + 2NaI 

Reagents:

Sodium thiosulfate (N/40): Weigh 6.20 gm of sodium thiosulfate (Na2S2O3.5H2O), dissolve in water and make the volume 1 litre.  

Manganous sulfate (48%): Weigh 48 gm of manganous sulfate and dissolve in 100 ml of water.

Potassium iodide (15%) in potassium hydroxide (KOH) (70%): Prepare 70% KOH by dissolving 70 gm of KOH in 100 ml of water. Thereafter, weigh 15 gm of potassium iodide and dissolve in 70% KOH and make the volume 100 ml using 70% KOH.

Starch indicator (1%): Weigh 1 gm of soluble starch, add water and dissolve by gentle heating. Make the volume 100 ml using water.

Sulfuric acid (36N): Use commercial analytical grade sulfuric acid of 36 normality.

Protocol:

1. Pipette known volume of the liquid sample in a 250 ml glass bottle avoiding bubbles.

2. Add 2 ml each of manganous sulfate and alkaline potassium iodide solution in succession right at the bottom of the bottle using separate pipets and thereafter replace the stopper.

3. Shake the bottle well and allow the brown precipitate formed to settle. This brown precipitate is of MnO(OH)₂.

4. Add 2 ml of concentrated sulfuric acid and shake well to dissolve the brown precipitate.

5. Titrate the liberated iodine against standard sodium thiosulfate using starch as an indicator. At the end point, blue color will disappear.

If we check the stoichiometry of the reactions, one mole of oxygen will be equivalent to 2 moles of iodine.

After calculating the number of moles of iodine produced, calculate the number of moles of oxygen present in the sample. The dissolved oxygen content is generally expressed as mg/100 ml sample.

Source:

‘Biochemical Tests: Principles and Protocols’, Viva Books Pvt Ltd, New Delhi, India, 2012.