Experiment No

Experiment No.3

D.O. DISSOLVED OXYGEN (By Winkler method)

Objective:

To determine the amount of dissolved oxygen in a given sample water by titration process.

Apparatus Required:

· Beaker

· Burette

· Pipette of 100ml capacity

· Conical flask

· Stand & clamps

· Titration flask

Chemical Required:

Sample water
Manganese sulphate
Alkali-iodide-acid
50% Sulphuric acid
N/80 Sodium thisulphate (Na₂S₂O₃)
Starch indicator

Theory:

The presence of oxygen in dissolved form in water is known as dissolved oxygen (D.O.). It is very necessary to keep water fresh and sparkling. But excess amount of oxygen causes corrosion to the pipe materials. So it is necessary to calculate the amount of oxygen. In lab it is calculated by titration process which as follows.

If N be the normality of the D.O. in the sample of water of 100ml, then amount of D.O. is equal to N times equivalent weight (eq wt) of oxygen.

i.e. strength of D.O. = N * eq wt of oxygen (gm/l)

= N * 8000 ppm

N is calculated from the titration equation,

N*V=N₁*V₁

Or, N = N₁*V₁ / V ---------------------------------------- (I)

Where, N₁ = normality of the Na₂S₂O₃ solution

V₁ = volume of Na₂S₂O₃ required to neutralize

V = volume of water sample (100ml)

During the titration process, following reaction takes place

Step 1:

Mn⁺⁺ + ½O₂ + 2OH ------------- MnO₂ ↓+ H₂O (brown ppt.)

Mn⁺⁺ + 2OH ------------------ Mn (OH)₂↓ (white ppt.)

MnO₂ + 2I -------------------- Mn +I₂ + 2H₂O

Step 2:

2MnSO₄ --------------------------- 2Mn + 2SO₄

2NaI + 2 NaN --------------------- 4Na + 2I + 2N

4Na + 2Ho + O ------------------- 2Na + 4OH

2Mn + 4OH ------------------------ 2Mn(OH)₂

4Na + 2So -------------------------- 2NaSO

2MnSO + (2NaI + NaN) + 2HO + O ---------- 2NaSO + 2Mn(OH)₂ + 2I +2N

Procedure:

The water sample of 300ml was poured into 300ml bottle and then glass stopper was inserted carefully so that the air bubbles are not tapped in the bottle.
2ml of manganese sulphate was added well below the surface of the liquid and immediately 2ml alkali iodide Acid was added to water surface.
The stopper was replaced so that air bubbles was tapped and the bottle was inverted for several times to mix the contents well.
The bottle was placed on the bench to allow the precipitate to settle down to the bottom one third of the bottle.
4ml of 50% Sulphuric acid was added and the bottle was stoppered.
The burette was filled with N/80 standard sodium thiosulphate solution and set up the reading as zero.
The contents of bottle was again mixed by the process of inversion.
The water sample of 100ml was taken in the conical flask and titrated it against the sodium thisulphate till pale straw colour was observed.
Then 2ml of starch indicator was added which results the sample was turned blue and the titration was continued slowly until the blue colour disappears and becomes colourless.
The record on the burette was noted.
The process was repeated till the two concurrent reading were obtained.

Observation:

S.N.	Initial burette reading (ml)	Final burette reading (ml)	Difference (ml)	Vol of Na₂S₂O₃ (ml)
1.	0	4.7	4.7
2	7.1	9.4	4.7	4.7

Calculation:

Volume of sample water, V = 100ml

Volume of Na₂S₂O₃ V₁=4.7ml

Normality of Na₂S₂O₃ N₁= N/80

From titration equation (I),

Normality of sample water, N = N₁*V₁/V = (N/80) * 4.7/100= 5.875E-4N

Strength of D.O. = N * 8000 mg/lit

= 5.875E-4 * 8000 = 4.7 mg/lit

= 4.7 ppm

Conclusion:

In this way the amount of dissolved oxygen is determined in the lab, which is 4.7 ppm. For domestic purpose the water having 5-6 ppm of dissolved oxygen is used. So the value obtained in lab is less than desirable level. So the water used in the lab is not suitable for domestic purpose.