Alkalinity of Water
Alkalinity can be defined as the ability of water to neutralize acids. Alkalinity of
water is due to the presence of carbonates, bicarbonates and hydroxides of sodium,
potassium, magnesium and calcium present alone or in combination. However,
hydroxide and bicarbonates cannot exist together as they react to form carbonate ions.
Naturally occurring alkalinity in water functions as the Earth's natural buffering
system. Even when small amounts of strong acids, in the form of acid rain, get added
to natural water, the water resists the change in pH. The acid converts carbonates to
bicarbonates and converts bicarbonates to carbon dioxide, with both reactions
resulting in very little change in pH of water.
Theoretically, a pH of 7.0 represents a neutral system, with a higher pH being alkaline
and lower pH being acidic. However, in water chemistry, the neutral pH is not
achieved as some alkalinity is present, even at a pH of 5.
Types of Alkalinity:
1. Carbon dioxide alkalinity: At a pH of 4.2-4.5 or lower, all alkalinity is in the form of
carbon
dioxide.
2. Carbon dioxide and bicarbonate alkalinity: Carbon dioxide and bicarbonate are in a
balance between the pH range of 4.2-4.5 at the low end and 8.2-8.4 at the high end. At a pH
of 8.2-8.4, there is no carbon dioxide and all alkalinity is in form of bicarbonate.
3. Bicarbonate and carbonate alkalinity: Bicarbonate and carbonate are in a balance
between the pH range of 8.2-8.4 at the low end and 9.6 at the high end. At a pH of 9.6, there
is no carbon
dioxide or bicarbonate, and all alkalinity is in form of carbonate.
4. Hydroxyl alkalinity: As the pH increases above 9.6, hydroxyl alkalinity starts to occur.
Most naturally occurring water sources have pH between 6 and 8.4, so the presence of
hydroxides is the result of manmade activity.
Estimation of Alkalinity:
The estimation of alkalinity due to the presence of different ions can be carried out by
titration of water with a standard acid solution in the presence of appropriate indicators, that
is, phenolphthalein and methyl orange. Volume of acid used up to phenolphthalein end point
corresponds to the complete neutralization of hydroxide ions and half-neutralization of
carbonate ions (up to the formation of bicarbonate ions) and is known as P-alkalinity. The
volume of acid used up to methyl orange point corresponds to the complete neutralization of
hydroxide, carbonate and bicarbonate ions and is known as M-alkalinity
Principle
The volume of acid used can be used to determine the strength of various ions can be
determined using volume of acid.
+ ¿→H 2 O ¿
OH −¿+H ¿
+¿→ HCO3−¿ ¿ ¿
2−¿+ H ¿
CO 2
+¿→ H 2 CO3 ¿
−¿+ H ¿
HCO 3
The M-alkalinity measurement is based on a sulphuric acid titration using methyl
orange as indicator that changes from yellow at a pH of 4.5 to orange at a pH of 4.4 at
the end point.
The P-alkalinity measurement is based on a sulphuric acid titration using
phenolphthalein indicator that changes from pink at a pH of 8.3 to colourless at a pH
of 8.2 at the end point.
Procedure
About 100 ml of water sample is taken in a conical flask and 2-3 drops of
phenolphthalein indicator are added to it.
Titration is carried out against standard sulphuric acid solution till the disappearance
of pink colour. The volume of titre (V1) is noted.
Then 2-3 drops of methyl orange indicator are added to the same solution and titrated
further with acid solution till the pink colour develops again. The titre value is noted
(V2).
Calculations
Volume of titre = 100 ml.
Strength of acid = N
Volume of acid used for P-alkalinity = V1
Volume of acid for M-alkalinity = V1 + V2
P-alkalinity in terms of CaCO3 equivalents = V1 × N × 50/100 (g/L) or V1 × N × 50 ×
106/105 ppm
M-alkalinity in terms of CaCO3 equivalents = (V1 + V2) × N × 50/100 (g/L) or (V 1 + V2) ×
N × 50 ×106/105 ppm
The interpretation of the results is summarized in Table :
Interpretation of alkalinity values
−¿ ¿
Alkalinity OH −¿¿ 2−¿ ¿
CO 3 HCO 3
P=0 0 0 M
P = (½)M 0 2P 0
P ¿(½)M 0 2P M-2P
P ¿(½)M 2P-M 2(M-P) 0
P=M P 0 0
Numerical: A 100 ml water sample required 25 ml of 0.05 N sulphuric acid
for phenolphthalein end point and another 10 ml for methyl orange end point.
Determine the nature and amount of Alkalinity present in water?
Solution:
Given that:
Volume of water sample = 100 ml
Volume of sulphuric acid for phenolphthalein end point = 25 ml
Normality of sulphuric acid = 0.05 N
Using N1V1 = N2V2, normality of phenolphthalein alkalinity is:
N1 = 0.05 × 25/100
Phenolphthalein alkalinity in terms of CaCO3 equivalent is:
P-alkalinity = 0.05 ×25 ×50/100 = 0.625 g/L = 625 mg/L = 625 ppm
Volume of sulphuric acid consumed for methyl orange end point = 25 + 10 = 35 ml
Using N1V1 = N2V2, normality of methyl orange alkalinity is:
N1 = 0.05 × 35/100
Methyl orange alkalinity in terms of CaCO3 equivalents is:
M-alkalinity = 0.05 ×35 ×50/100 = 0.875 g/L = 875 mg/L = 875 ppm
Since P-alkalinity > M-alkalinity/2, alkalinity is due to both OH −¿¿ and CO 2−¿
3
¿
ions.
Alkalinity due to carbonate ions ¿= 2(M-alkalinity – P-alkalinity) = 2(875 - 625) = 500
ppm.
Alkalinity due to hydroxyl ions (OH −¿¿ = 2(P-alkalinity – M-alkalinity) = 375 ppm.
