Aquatic Ecology374
First Term of year 1432-1433
Nuha AL-Abdulhadi
Dr.promy
lab 9
Chemical Properties of Water
3-Dissolved Oxygen Measurements
( Winkler’s Method)
INTRODUCTION
Oxygen is a clear, colorless, odorless, and
tasteless gas that dissolves in water. Small but
important amounts of it are dissolved in water.
It is supplied by diffusion of atmospheric (air)
oxygen into the water and by production of
oxygen from photosynthesis by aquatic plants.
Oxygen: Aquatic life depends on it
Both plants and animals depend on dissolved oxygen for
survival. Lack of dissolved oxygen can cause aquatic
animals (e.g. fish, macroinvertebrates) to quickly leave the
area or face death. Under low oxygen conditions, the
aquatic animal community changes quickly. Under
extreme conditions, lack of oxygen can kill aquatic plants
and animals. Measuring dissolved oxygen is probably the
most significant water quality test to determine the
suitability of a stream for fish and many other aquatic
organisms.
Factors Affecting Oxygen Levels
1)Oxygen is removed from the water by chemical reactions, the decay process
and respiration of living organisms, including fish, bacteria, fungi and
protozoans.
2)Water temperature and atmospheric pressure affect the capacity of water to
hold dissolved oxygen. Cold water at high atmospheric pressure holds more
dissolved oxygen than warm water at low atmospheric pressure.
3)Oxygen levels also are affected by the degree of light penetration (turbidity,
color and water depth) and the degree of water turbulence or wave action.
4)Dissolved oxygen (D.O.) is reported as milligrams of oxygen per liter of
water (mg/L) which can be called parts by weight per million (ppm). Different
aquatic organisms have different oxygen needs.
Aquatic plants produce oxygen by photosynthesis during daylight hours but
they also use oxygen for respiration. During the night or on heavily overcast
days, respiration removes oxygen while photosynthesis stops or drastically
slows down.
Principles of Winkler’s Method:
Oxygen combines with Mangnous
hydroxide to form higher hydroxides,
which on acidification liberate iodine
equivalent to that of oxygen fixed.
This iodine is titrated by standard
Sodium thiosulfate solution using
starch as an indicator.
Materials
Water Samples, Titration assembly, Sodium thiosulfate titrant
(0.025), Narrow mouth 250 ml BOD bottles, Sulfuric acid
(concentrated), Starch indicator, Manganous sulfate solution,
alkaline- potassium iodide ,Pipettes, Flasks
Procedure:
A. Collecting the Sample
1. Collect your sample in roughly one-foot deep, normally
moving water.
4. Facing upstream, slowly lower the bottle so opening of the
bottle faces away from you and water current is entering
bottle.
5. Allow the bottle to fill with water gradually, turning it to
allow air bubbles to float out.
6. Cap bottle while still submerged and leave extra water in
the neck of the bottle.
Materials
Water Samples, Titration assembly, Sodium
thiosulfate titrant (0.025), Narrow mouth 250 ml
BOD bottles, Sulfuric acid (concentrated), Starch
indicator, Manganous sulfate solution, alkalinepotassium iodide ,Pipettes, Flasks
Procedure:
A. Collecting the Sample
1. Collect your sample in roughly one-foot deep,
normally moving water.
4. Facing upstream, slowly lower the bottle so
opening of the bottle faces away from you and water
current is entering bottle.
5. Allow the bottle to fill with water gradually,
turning it to allow air bubbles to float out.
6. Cap bottle while still submerged and leave extra
water in the neck of the bottle.
B. Testing for Dissolved Oxygen
Put on protective gloves. If your skin comes in contact with any solution or titrant,
rinse the area liberally with water.
Remove the stopper and add 1 ml Manganous sulfate.
Add 3 ml alkaline potassium iodide into the sample and insert the stopper, and shake
vigorously, holding on to the top. If oxygen is present, a brownish-orange precipitate
will form.
Allow the sample to stand until the precipitate settles halfway.
Remove the stopper and slowly add 2 ml sulfuric acid.
Insert the stopper and shake vigorously to dissolve the precipitate. Shake and wait
until all the precipitate is dissolved. The yellow color is from iodine.
Take a suitable aliquot 200 ml in a flask and titrate with Sodium thiosulfate solution
till the colour changes to very light yellow.
Add 3ml of starch solution.
The prepared sample will turn blue from the added starch solution.
Continue adding drops and mixing until the prepared sample turns from blue to
colorless (the end point).
Observations:
S.N
1234567-
ml of Sample
ml of titrant
used
D.O. mg l -1
Calculation:
Science 1 ml of 0.025 N sodium thiosulfate is equivalent to
0.2 mg oxygen.
D.O. mg l -1 = (8X1000XN) X v
Where:
V
V= volume of sample
v= volume of titrant used (ml)
N= normality of the titrant
Result. Express D.O. in mg l 1
NUHA