dissolved oxgen its definition and how it is available has been mentioned.also its methods of determination has shortly explained.

1. A presentation on the topic of

2.  Dissolved Oxygen is the amount of gaseous oxygen (O2) present
in the water in its dissolved state.
 DO is the most important indicator of the health of a water body
and its capacity to support a balanced aquatic ecosystem of plants
and animals.
 A higher dissolved oxygen level indicates a better water quality.
 If dissolved oxygen levels are too low, some fish and other
organisms may not be able to survive.

3.  Mostly the dissolved oxygen in water comes from oxygen in the air.
 Also it comes from the photosynthesis process done by aquatic plants.
 Water temperature and the volume of moving water affects dissolved oxygen
levels. Oxygen dissolves easier in cooler water than warmer water.
 Salinity is also an important factor in determining the amount of oxygen a body of
water can hold. As the amount of dissolved salts in the water increases, the amount
of oxygen the water can hold decreases. Hence ,for lower saline water, it increases.

4. Dissolved Oxygen Measurement
Methods
There are three methods available for measuring
dissolved oxygen concentrations in water:-
 Titration Method
 Colorimetric Method
 Optical or electrochemical Sensor.

5. TITRATION METHOD
The method for dissolved oxygen analysis is known as “Winkler method”. Also known
as the Iodometric method.
 Chemicals Required:
1. Manganous Sulphate Solution (Mnso4.4H2O)
2. Alkali-iodine Oxide
3. Conc. Sulphuric Acid(H2SO4)
4. Starch Solution
5. Std. Sodium Thiosulphate Solution(0.025 N)
6. Std. Potassium Dichromate Solution (0.025 N)
WINKLER METHOD:-

6.  Procedure:-
1. Take 300 ml of water sample in the bottle. Add 2 ml of manganous sulfate solution
and 2 ml of alkali oxide solution to the sample.
2. Mix the solution obtained by inverting the bottle for at least 15 times.
3. Precipitate begin to form and allow it to settle ,leaving a clear supernatant liquid
above ,shake again.
4. After two minute of settling ,remove the stopper and add 3 ml of conc. H2SO4 by
allowing acid to run from the neck of the bottle.
5. Apply the stopper and mix again until the dissolution is complete.
6. take about 203 ml of the solution from the bottle to flask.

7. 7. Titrate with 0.025 N sodium thiosulfate solution to pale straw
colour.
8. add 1.2 ml starch solution and continue the titration until blue color
disappears and note down the volume of sodium thiosulfate added
which DO value in mg/lit.
 Calculation:-
The formula to calculate amount of DO will be given as :-
= * * *
Amount of
do present
in water
sample
(mg/li.)
Total volume
of sodium
thiosulphate
req.
Normality of
sodium
thiosulphate
Eq. Wt.
of
oxygen
Volume of sample
1000

8. Chemical reactions :-
1. White precipitate formation-
2.Brown precipitate formation-
3.After adding acid-
4. Titration-
Mn2+ + 2OH
- Mn(OH)2
4 Mn(OH)2(s) + O2(aq) + 2 H2O 4 Mn(OH)3(s)
2Mn(OH)3 + 2I
- +6H+ I2 + 6H2O + 2Mn2+
2S2O3
2- + I2 S4O6
2- + 2I-

9. COLORIMETRIC METHOD
 There are two variations of dissolved oxygen analysis by the colorimetric
method.
 They are known as the indigo Carmine method and the Rhodazine D
method. Both variations use colorimetric reagents that react and change
color when reacted with oxygen in the water .
 These interactions are based on the oxidation of the reagent, and the extent
of the color change is proportional to the dissolved oxygen concentration .
 a spectrophotometer, colorimeter or a simple comparator is used for
measuring DO.

10. Indigo carmine method
 The indigo carmine method is used for measuring dissolved
oxygen concentrations between 0.2 and 15 ppm (mg/L).
 This method produces a blue color, the intensity of which is
proportional to the dissolved oxygen concentration
 this method is not affected by temperature, salinity or dissolved
gases

11. Rhodazine D method
 The Rhodazine D method is used to determine very low
dissolved oxygen concentrations i.e. upto parts per billion (ppb)
 Rhodazine D reagents react with dissolved oxygen to form A
deep rose-colored solution .
 this colorimetric method is not affected by salinity or dissolved
gases present in the water sample .
 In addition, sample color and turbidity can affect the accuracy of
the readings . this method is time-dependent, as the analysis must
be made within 30 seconds of mixing the reagent

12. Optical sensor
 Optical dissolved oxygen sensors measure the interaction between oxygen and
certain luminescent dyes.
 When exposed to blue light, these dyes become excited (electrons gaining
energy) and emit light as the electrons return to their normal energy state .
 When dissolved oxygen is present, the returned wavelengths are limited or
altered due to oxygen molecules interacting with the dye. The measured effect is
inversely proportional to the partial pressure of oxygen .
 As oxygen crosses the membrane, it interacts with the dye, limiting the intensity
of the luminescence . The intensity of the returned luminescence is measured by a
photo detector, and is used to calculate the dissolved oxygen concentration.

13. SIGNIFICANCE OF DO IN WATER
1. The level of oxygen is a much more important measure of water
quality .
2. Dissolved oxygen is absolutely essential for the survival of all
aquatic organisms (not only fish but also invertebrates such as
crabs, clams, zooplankton, etc.).
3. Oxygen affects a vast number of other water indicators, not
only biochemical but also like the odor, clarity and taste.

14. 4. As dissolved oxygen levels in water drop below 5.0 mg/l, aquatic life is put under
stress. The lower the concentration, the greater the stress. Oxygen levels that remain
below 1-2 mg/l for a few hours can result in large fish kills.
5. Total dissolved gas concentrations in water should not exceed 110 percent.
Concentrations above this level can be harmful to aquatic life. Fish in waters
containing excessive dissolved gases may suffer from "gas bubble disease"; however,
this is a very rare occurrence. External bubbles (emphysema) can also occur and be
seen on fins, on skin and on other tissue.
6. In a community water supply a high do level is good because it makes drinking water
taste better. However, high DO levels speed up CORROSION in water pipes. For this
reason, industries use water with the least possible amount of dissolved oxygen.