Do,cod,bod

DISSOLVED OXYGEN (DO):
• Amount of oxygen dissolved (and hence available to sustain
marine life) in a body of water such as lake, river or stream .
• Dissolved oxygen is the indicator of water pollution.
• DO levels in natural and waste water depend on physical,
chemical and biological activities in water body.
• Amounts of oxygen that can be dissolved in water by
temperature.
• It is usually determined by Winkler’s method known as
iodometric method.
• Method was developed by L.W.Winkler , a hungarian
chemist in 1888 .

The Winkler method with azide modification:
 Alsterberg azide modification.
 Rideal stewart modification .
 Alum flocculation modification .
 Copper sulfate sulfanic acid flocculation
modification .
 Alkaline hypochlorite modification .

Principle :
 The basic principle of this method is that DO reacts
with manganese ions to form manganesedioxide
which further reacts with I¯ (Iodine ion) to liberate I2
in an amount chemically equivalent to the original
dissolved oxygen .
Mn⁺² + ½ O2 +2OH¯ → MnO2 ↓ +H2O
MnO2 +2I¯ + 4H⁺ → Mn⁺² + I2 + 2H2O
This liberated iodine is titrated with sodium thiosulphate
(Na2S2O3) .
I2 + 2S2O3²¯ → 2I¯ + S206²¯

 Manganese sulphate
 Alkali iodide – azide reagent
 Sulphuric acid
 Starch indicator
REAGENT :
 Standard sodium thiosulphate 0.025N

CALCULATION :
If the whole content is used for titration,
DO mg/l = V₁ × N × 1000/V2 – V3
When only a part of the contents have been used ,
DO mg/l = V1 × N × 8 × 1000 / V4 (V2 – V3 )/V2
Where, V1= volume of titrant (sodium thiosulphate)
V2= Volume of sampling bottle
V3= Volume of MnSO4 and KI solutions added
V4= Volume of the part of the contents titrated
N= Normality of sodium thiosulphate (0.025N)

DO LEVEL :
Water quality DO LEVEL
Good quality(20°C sea level 8-9 ppm O2
Moderate pollution 4-5 ppm O2
Highly polluted <4.5 ppm O2

CHEMICAL OXYGEN DEMAND (COD ) :
• COD is an empiric laboratory assay which measured the
amount of organic matter contained in a water sample
with the help of chemical oxidizing agents such as
potassium dichromate and potassium permanganate .
 COD is a means of measuring all organic matter
(biodegradable and non- biodegradable ) .
 This test is useful in studying pollution strength of
domestic and wastewater .
 COD values are greater than BOD values .
 The open reflux method is suitable for a wide range
of wastes is discussed here .

PRINCIPLE :
 The organic matter gets oxidized
completely by K2CR2O7 and silver sulphate
catalyst in the presence of H2SO4 to
produce CO2 +H2O . The excess K2CR2O7
remaining after the reaction is titrated
with Fe(NH4)2 (SO4)2. The dichromate
consumed gives the O2 required for
oxidation of the organic matter .
3(CH2O) +16H⁺ + Cr2O7²¯ → 4Cr³⁺ + 3CO2 + 11H2O

REAGENTS :
Standard potassium dichromate 0.250N
Sulphuric acid reagent
Standard ferrous ammonium sulphate 0.1 N
Ferroin indicator
Mercuric sulphate
Potassium hydrogen phthalte (KHP) standard

Calculation :
COD mg/l = ( B – A ) × N × 1000 / ml sample
Where ,
A = volume of FAS used for sample ( in ml )
B = volume of FAS used to blank ( in ml )
N = normality of FAS

COD LEVEL :
0 – 5 mg / l indicates very clean stream
Chemical oxygen demand , ( 5 days at -20°C ) max :
Parameter standards
COD
Inland surface water : 250 mg/l
Marine coastal area : 250 mg/l

BIOLOGICAL OXYGEN DEMAND (BOD) :
 BOD is the amount of dissolved oxygen required to
breakdown the organic material of the sample by
aerobic biological microorganisms at certain
temperature over a specific period of time .
 This parameter is determined with the consumption
of oxygen by microorganisms during 5 days of
incubation at 20°C .
{CH2O} + O2 → CO2 + H2O

 BOD indicates a amount of pollution of water bodies.
 Lower BOD indicates the water is a good quality for
aquatic life or little aerobic acitivity whereas the
higher BOD indicates the water is highly polluted.
 When BOD is higher , then DO becomes lower .
 so, aquatic organisms are unable to survive at
normal rate , it becomes stressed , asphyxiate and
may die .

Determination of BOD :
 The most common and popular method for
determination of biological oxygen demand is
standard method which is recognized by U.S . EPA .
 It is not accurate quantitative test , although it
is widely used as an indicates the amount of
pollution .

Procedure :
• Take 2 samples of water .
• Record the DO level (ppm) of one immediately using
the method described in the dissolved oxygen test .
• Place the second water sample in an incubator in
complete darkness at 20°C for 5 days . If you don’t have
an incubator, wrap the water sample bottle in aluminium
foil or black electrical tape and store in a dark place at
room temperature (20°C or 68°F) .
• After 5 days, take another dissolved oxygen reading
(ppm) using the dissolved oxygen test kit .
• Subtract the day 5 reading from the day 1 reading to
determine the BOD result in ppm.

CALCULATION :
 The difference between the two DO levels
represents the amount of oxygen required for the
decomposition of any organic material in the
sample .
BOD mg/l = D₁ - D₂ / V
Where, D₁ = initial DO
D₂ = final DO
V = volume of sample

BOD LEVEL :
BOD LEVEL (mg/l) Remarks
1mg/l very clean
2mg/l clean
3mg/l fairly
5mg/l doubtful
10mg/l contaminated
BOD of some collected samples from different sources:
• Normal water = 0-3
• River water = 5-20
• Sewerage water = 50-100
• Industrial water = 150-1000
• Land for irrigation = 100
• Marine coastal area = 100
• Inland surface water = 30
• Public sewers = 350

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