4. Determination of dissolved oxygen content of water sample by winkler's method

4. DETERMINATION OF DISSOLVED OXYGEN CONTENT OF WATER SAMPLE BY WINKLER'S METHOD

Expt. No.

Date:

AIM

To determine the amount of dissolved oxygen in the given water sample by Winkler's method, a standard solution of K₂Cr₂O₇ of strength .......... N is given.

PRINCIPLE

Oxygen is dissolved in water to the extent of 7-9 mgs/lit at a temperature range of 25° – 35°C. The estimation of dissolved oxygen in water is useful in studying corrosion effects of boiler feed water and in studying water pollution. The amount of dissolved oxygen in water is estimated using Winkler's reagent (Potassium bromide + Potassium bromate). Water sample is collected carefully avoiding aeration/deaeration in ground stoppered flask. Initially manganous sulphate and alkali-iodide reagents are added and the reactions occur as follows

Mn²⁺ + 2OH^- → Mn (OH)₂ ↓ (White)

Mn(OH)₂ + ½ O₂ → MnO(OH)₂ + ↓ (Yellow brown)

The precipitate dissolves in concentrated sulphuric acid liberating iodine and the liberated iodine is titrated against Na₂S₂O₃.

MnO(OH) ₂  + 2H₂SO₄ → Mn(SO₄) ₂ + 3H₂O

Mn(SO₄) ₂ + 2KI → MnSO₄ + K₂SO₄ + I₂

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2 Nal

MATERIALS REQUIRED

1. Sodium thiosulphate solution 2. Std. Potassium dichromate solution 3. Dil H SO. 4. 5% KI 6. Starch indicator 7. Manganese sulphate 8. Alkali-iodide mixture 9. Con. H2SO4 10. Burette, pipette, iodine flask.

Step II : DETERMINATION OF DISSOLVED OXYGEN

TITRATION – II

Water sample Vs. Sodium thiosulphate

Calculation

Volume of sodium thiosulphate, V₁ = 20 ml

Strength of sodium thiosulphate, N₁ = 22.0 N

Volume of water sample, V₂ = 20 ml

PROCEDURE

TITRATION - I

Standardisation of Sodium Thiosulphate

The burette is washed and rinsed with sodium thiosulphate solution. Then the burette is filled with the given sodium thiosulphate solution upto zero mark. 20 ml of .......... N potassium dichromate solution is pipetted out into a clean conical flask. To this, 5 ml of sulphuric acid and 15 ml of 5% potassium iodide solution are added. This is titrated against sodium thiosulphate solution. When the solution becomes straw yellow colour, starch indicator is added and then titration is continued. The end point is disappearance of blue colour and appearance of light green colour. The titration is repeated to get concordant value.

TITRATION – II

Determination of dissolved oxygen

100-150 ml of the water sample is taken in the iodine flask, 2ml of manganese sulphate and 2ml of alkali-iodide mixture are added. The stopper is replaced and the flask is inverted and shaked several times for the rough mixing of reagents. The flask is left aside for some time. When half of the precipitate settles down, the stopper is removed and 2 ml of concentrated sulphuric acid is added.

The stopper is replaced and the flask is inverted several times for complete dissolution of the precipitate to get a clear yellow solution. 20 ml of this solution is pipetted out in a clean conical flask and titrated against standardised sodium thiosulphate solution. When the solution becomes light yellow, starch indicator is added. The titration is continued until the blue colour disappears. From the titre value the strength of dissolved oxygen is calculated and hence the amount of dissolved oxygen in the water sample is calculated.

RESULT

Amount of dissolved oxygen in sample water = ………. mgs/lit.

Step I: STANDARDISATION OF SILVER NITRATE

TITRATION – I

Standard NaCl Vs AgNO₃

Calculation of the normality of Silver Nitrate

Volume of Sodium Chloride V₁ = 20 ml

Strength of Sodium Chloride N₁ = ……… N

Volume of Silver Nitrate V₂ = .............. ml

Strength of Silver Nitrate N₂ = ?

According to the law of volumetric analysis