Principle:
The experiment is based on the principle of iodometric titration. When a strong
alkaline solution of MnSO4 (divalent) is added to a water sample
containing dissolved oxygen, it picks up oxygen to form basic MnO(OH)2
(tetravalent) which is needed to oxidize KI to I2 in presence of H2SO4.
The liberated I2 is titrated against standard sodium thiosulphate
using starch as indicator. By knowing the volume of sodium thiosulphate
consumed DO content can be determined. The reactions are shown below:
MnSO4 + 2KOH
-------àMn(OH)2 + K2SO4
2 Mn(OH)2 +O2
----à 2 MnO(OH)2 (Basic manganic
oxide)
MnO(OH)2 + H2SO4
----àMnSO4 +
2H2O + [O]
2KI + H2SO4 +
[O] -----à K2SO4
+ H2O + I2
I2 + 2Na2S2O3
------à 2NaI + Na2S4O6
Procedure:
·
Pipette out 300ml of water sample into a
clean glass stoppered bottle. Add 3ml of manganous sulphate solution and 3ml of
alkaline potassium iodide solution.
·
The bottle is stoppered and shaken well for
10- 15min. The precipitate is allowed to settle down.
·
Then add 1ml of conc. H2SO4
and bottle is stoppered and mix well until the precipitate dissoveles
completely. The iodine is liberated.
·
100 ml of this solution is pipette out into
a clean conical flask. Add 2ml of freshly prepared starch indicator.
·
The solution is titrated against standard
solution of sodium thiosulphate till the blue color disappears. Record the
volume of sodium thiosulphate consumed.
Calculation:
Volume of water sample taken= 100ml
Volume of sodium thiosulphate consumed = V1
ml
Normality of sodium thiosulphate used = N1
Normality of oxygen content in water = V1 x N1
100
= N2
Hence oxygen content in water = N2
x Equivalent weight of oxygen
= N2
x 8 x 1000 mg/dm3
No comments:
Post a Comment