DISSOLVED OXYGEN
AIM:- To
determine the concentration of dissolved oxygen in the sample water by Azide
modification method.
APPARATUS:-
Glassware along with BOD bottle.
THEORY:-
Dissolved oxygen is one of the most important constituents of natural water
system. A certain amount of dissolved oxygen is essential for aerobic decomposition
of waste to avoid nuisance conditions in rivers and to maintain the growth of
fish and other aquatic animals. Oxygen is generally absorbed by water from
atmosphere and unpolluted natural surface waters are usually saturated with it.
Except oxygen the presence of any other gas dissolved in water is not desirable
and steps should be taken to remove the same. This test is the basis of BOD
test, which is an important parameter to evaluate pollution potential of the
wastes. This test determines the quality of raw water and keeps proper check on
stream pollution. Dissolved oxygen in the water is determined by the modified
winkler’s method, also called Azide modification method, whose principle is
discussed below.
Dissolved
oxygen present in the sample water oxidizes manganous sulphate to produce
manganese hydroxide (a brown colour precipitate) after addition of NAOH and KI.
Upon acidification, manganese reverts to its manganous form and liberates
iodine from KI, equivalent to dissolved oxygen content in the sample water. The
liberated iodine is titrated against sodium thisosulphate using starch as
indicator.
RELAVENCE:-
The presence of dissolved oxygen in water near its saturation level is an
indication of its purity. A stream must have a minimum of 2 ppm of dissolved
oxygen to support fish and other higher life forms. On the other hand, high
amount of dissolved oxygen corrodes the distribution system. The permissible
limit of dissolved oxygen in the domestic water supplies is 5-6 ppm.
REAGENTS:-
- Manganese sulphate solution:- Dissolve 480 grams mnso4.4H2o in distilled water and make it to 1 lit.
- Alkali – Iodide – Azide reagent:- Dissolve 500 gram Noah and 150 gram KI in distilled water and dilute to 1 lit. Add 10 gram sodium Azide (NAN3) dissolved in 40ml distilled.
- Concentrated H2SO4
- Starch:- Dissolve 5grams starch in 1 lit boiling distilled water. Preserve by adding a few drops of Toluene.
- Sodium Thiosulphate solution: Dissolved 6.205 grams Na2s2o3.5H2o in distilled water and dilute it to 1 lit. Standardize it using potassium dichromate solution.
- Standard potassium dichromate solution:- Dissolve 1.226 grams K2CR2O7 in distilled water and dilute it to 1 lit.
- Potassium Iodide Solution:- Dissolve 20 grams KI in 100ml distilled water.
PROCEDURE:-
- Standardization of sodium Thiosiphate Titrant:-
a) Take
100 ml distilled water in a flask and add 10ml KI solution. Add 10ml 1+9 H2SO4.
b) Add
20ml K2CR2O7 solution and store it in a dark
place for five minutes.
c) Titrate
with Na2s2o3 titrant, adding starch towards the
end when a pale straw colour develops. Continue titration to colourless end
point. Note down the volume of titrant used.
d) Determine
the normality of titrant using the relation N1V1 = N2V2
N1 = normality of K2CR2O7
= 0.025
V1 = volume of K2CR2O7
– 20ml
V2 = volume of Titrant consumed
N2 = normality Titrant (unknown)
Adjust the normality of Titrant to 0.025N using the relation
N1V1 = N2V2.
- Estimation of D.O. in the sample water:-
a) To
the sample in the BOD bottle, add 2ml manganese sulphate solution, 2ml of
alkali azide reagent, well below the surface.
b) Stopper
and mix by inverting bottle at least 15 times. Let the brown ppt settle for 5
minutes.
c) Add
2ml conc. H2So4 and stopper. Gently invert to dissolve
the ppt totally.
d) Take
203ml of the bottle water in the flask and titrate with sodium thiosulphate
titrant.
e) Add
1-2 ml starch and continue titration till the first disappearance of pale blue
colour.
OBSERVATIONS:-
|
S.No
|
Burette
|
Vol. of Titrant used
|
|
|
Initial
|
Final
|
||
|
|
|
|
|
CALCULATIONS:-
D.O. in mg/l = ml
of 0.025N sodium thiosulphate used.
RESULTS:-
INTERPRETATION OF
RESULTS:-
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