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Presented By:
Dr. Alka Sharma
Assistant Professor
Deptt. of Chemistry
IPS Academy, IES, Indore


Alkalinity of water is due to the presence of those
types of substances in water which have tendency
to increase the concentration of OH- ions either by
hydrolysis or by dissociation of water.
Factors:1. The presence of salts of weak organic acids
which undergo hydrolysis and consume H+ ions of
water. As a result concentration of OH- ions
increases in water and water becomes alkaline.
2. The presence of HCO3-, HSiO3- and SiO3-2 ions
in water which makes the water alkaline because
they have tendency to take up H+ ions from water.
Hence the concentration of OH- ions increases in
water and water becomes alkaline
Alkalinity is a measure of ability of water to
neutralize the acids .It is classified as
1. Hydroxide Alkalinity (Due to OH- )
2. Carbonate Alkalinity (Due to CO3-2)
3. Bicarbonate Alkalinity or Temporary hardness
(Due to HCO3-)
Alkalinity are also expressed in terms of CaCO3
equivalent like hardness.


A Known volume of hard water sample is
titrated against standard acid using
phenolphthalein & methyl orange as
indicator. Color Change at end point
Phenolphthalein: Pink to colourless
Methyl
orange: Pale yellow to organish red.
Reaction
P
M
P & M represent the volume of cosumed acid by using
Phenolphthalein & Methyl orange indicator respectively
Conclusions :
S. NO.
1
2
3
4
5
Result up to
P & M end
point
P=0
P=M
P = 1/2M
P ˃ ½M
P˂½M
OH-
CO 32-
HCO 3-
0
P or M
0
2P - M
0
0
0
2P or M
2(M-P)
0
M
0
0
0
M-2P
Determination of Phenolphthalein Alkalinity:
N1V1 = N2V2
V1 = volume of water Sample
N1= Normality of water Sample
V2= volume of acid consumed with P (A ml)
N2= Normality of acid (N/50)

N1 = N2V2 /V1
Strength (mg/l) = Nor X E.weight of CaCO3 X 1000
Determination of Methyl Orange Alkalinity or Total
Alkalinity:
N1V1 = N2V2
V1 = volume of water Sample
N1= Normality of water Sample
V2= Total volume of acid consumed with Methyl
orange (B ml) or (A+B ml)
N2= Normality of acid (N/50)
N1 = N2V2 /V1
Strength (mg/l) = Nor X E.weight of CaCO3 X 1000
( in terms of CaCO3 equivalent)
The type & amount of alkalinity is determined by
comparing the volume of P & M using the above table.