Dissolved oxygen (DO) refers to the level of oxygen present in water or other liquids. It is important for assessing water quality and supporting aquatic life. Chemical oxygen demand (COD) measures the amount of oxygen required to chemically break down pollutants in water. DO enters water through diffusion from air and as a byproduct of photosynthesis. It can be measured using electrochemical, optical, or colorimetric methods. COD is determined by using potassium dichromate as an oxidizing agent under acidic conditions, then measuring the amount of chromium formed.

1. D.O & C.O.D
( DISSOLVED OXYGEN DEMAND)
AND
CHEMICAL OXYGEN DEMAND )
CHETAN SINGH
PUNDEER
(12 CE 036 )Chetan Singh Pundeer (12 CE 036) 1

2. WHAT IS D.O ?
 Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other
liquids. It is an important parameter in assessing water quality because of its influence on the
organisms living within a body of water. In limnology (the study of lakes), dissolved oxygen is an
essential factor second only to water itself . A dissolved oxygen level that is too high or too low can
harm aquatic life and affect water quality.
 Non-compound oxygen, or free oxygen (O2), is oxygen that is not bonded to any other element.
Dissolved oxygen is the presence of these free O2 molecules within water.The bonded oxygen
molecule in water (H2O) is in a compound and does not count toward dissolved oxygen levels. One
can imagine that free oxygen molecules dissolve in water much the way salt or sugar does when it is
stirred .
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3. WHAT IS C.O.D ?
 COD (Chemical Oxygen Demand) is the amount of oxygen required to
degenerate all poution in a chemical way (by adding oxidising agents and heating).
In general with chemical destruction you can remove more polution than with the
biological way.
 . It is expressed in milligrams per liter (mg/L) also referred to as ppm (parts per
million), which indicates the mass of oxygen consumed per liter of solution.
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4. WHERE DOES D.O COMES FROM ?
 Dissolved oxygen enters water through the air or as a plant byproduct. From the air,
oxygen can slowly diffuse across the water’s surface from the surrounding atmosphere,
or be mixed in quickly through aeration, whether natural or man-made 7. The aeration of
water can be caused by wind (creating waves), rapids, waterfalls, ground water discharge
or other forms of running water. Man-made causes of aeration vary from an aquarium air
pump to a hand-turned waterwheel to a large dam.
 Dissolved oxygen is also produced as a waste product of photosynthesis from
phytoplankton, algae, seaweed and other aquatic plants.
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6. METHOD FOR D.O
 There are three methods available for measuring dissolved oxygen concentrations. Modern
techniques involve either an electrochemical or optical sensor. The dissolved oxygen sensor is
attached to a meter for spot sampling and laboratory applications or to a data logger, process monitor
or transmitter for deployed measurements and process control.
 The colorimetric method offers a basic approximation of dissolved oxygen concentrations in a
sample. There are two methods designed for high-range and low-range dissolved oxygen
concentrations. These methods are quick and inexpensive for basic projects, but limited in scope and
subject to error due to other redoxing agents that may be present in the water .
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7. METHOD FOR COD
 Potassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is
usually achieved by the addition of sulfuric acid.)
 In the process of oxidizing the organic substances found in the water sample,
potassium dichromate is reduced (since in all redox reactions, one reagent is oxidized
and the other is reduced), forming Cr3+. The amount of Cr3+ is determined after
oxidization is complete, and is used as an indirect measure of the organic contents of the
water sample.
Chetan Singh Pundeer (12 CE 036) 7

8. THANK YOU
Submitted To:- Ms. Himadri Rajput
Submitted By:- Chetan Singh Pundeer
(12 CE 036)
Chetan Singh Pundeer (12 CE 036) 8