Alkalinity in water is measured by titrating a water sample with sulfuric acid and monitoring the pH. Alkalinity is caused by hydroxides, carbonates, and bicarbonates and is expressed in units of mg/L of calcium carbonate. The amount of acid needed to reduce the pH to 8.3 measures phenolphthalein alkalinity, while the amount to reduce to 4.5 measures total alkalinity. Different combinations of hydroxides, carbonates, and bicarbonates can be present depending on the initial pH. Alkalinity data provides information useful for water treatment processes like coagulation, softening, and corrosion control.

1. Hardness of Water

2. INTRODUCTION
• What is alkalinity?
Alkalinity of a water is the measure of its capacity to neutralize acids i.e. to
absorb hydrogen ions without significant change in pH.
• Parameters which causes the alkalinity
Alkalinity is caused by the presence of hydroxide ions, carbonates and
bicarbonates.
At pH <8.3 , only bicarbonates are found to occur , because at lower pH all
carbonates are converted into bicarbonates by the action of CO2 and H2O.
CO2 + H2O + CaCO3 Ca(HCO3)2

3. If its pH falls below 4.5 since the all are converted into carbonic acid.
As far as natural water concern, algae in natural water usually found to be rich
in carbonates.
By the photosynthesis the algae provides CO2 and the carbonates are converted
into the bicarbonates and hence the algae rises the pH of water to above 8.3.
At pH 8.3 or more carbonates as well as bicarbonates both may exist
Hydroxide are however found to exist at much higher values of pH say about 10
to 11.

4. Total alkalinity in water may consist of,
I. Only hydroxide
II. Only carbonates
III. Hydroxide and carbonates
IV. Carbonates and bicarbonates
V. Only bicarbonates
First four possibilities will exist when the pH of water is more than 8.3 and
fifth possibility will exist only when the pH is in between 4.5 to 8.3.
It is assumed that hydroxide and bicarbonate alkalinity cannot be present
together in the same sample , although this may not be 100% true always.

5. Environmental Significance
Alkalinity in water has a little public health significance
Highly alkaline waters are usually not pleasant to taste.
Sometimes chemically treated waters are alkaline so standards are
sometimes established on that chemically treated waters
The principal objection of alkalinity in water is the reactions that occur
between alkalinity and certain cations in waters.The resultant precipitates
can corrode the pipes and other accessories of water distribution system.

6. Measurement of Alkalinity
Alkalinity of a water sample can be measured by titrating the given water
sample with a standard solution of sulphuric acid.
Further adding of sulphuric acid will results in increasing the H+ ions and
reducing its pH value.
The amount of H2SO4 added till the pH of the mixture is reduced to 8.3, as
detected by the use of an indicator dye- phenolphthalein.
This will help in the determining the alkalinity existing in water above pH of
8.3.
Further addition of sulphuric acid , till the pH is reduced to about 4.5 will help in
determining the total alkalinity in the water.
NOTE- No alkalinity can exist below pH 4.5.

7. Alkalinity is measured as CaCO3 (having equivalent weight = 50),its one mole
will weigh 50 gm/l, or 50 mg/ml.
That means 1 ml of alkalinity consumed by the use of 1 ml of N.H2SO4 will
weigh 50 mg and hence,
Alkalinity (as CaCO3) in water sample in mg/l
= [ml of H2SO4 used * N*50]*1000
ml of water sample

8. For highly alkaline waters, the first step is to titrate it by adding the acid to
reduce its pH up to 8.3.
The second step is to titrate it to reduce its pH to an indicated value of about
4.5.
If the initial value of pH is less than 8.3, the second step itself will become the
first step in the method of titration.
The end point of titration is change in the color of water with indicator dyes at
specific pH values.

9. Indicator
Phenolphthalein
If pH is more than 8.3 it turns water to pink.
At pH = 8.3 , it changes the water from pink to colorless.
Phenolphthalein gives us the amount of acid required to reduce the pH of water
upto 8.3.
Methyl-orange
Is used as an indicator , since it changes from red/orange to yellow in orange at
pH 4.6
The alkalinity determined upto pH = 8.3 , by phenolphthalein ; i.e 10x is the
usually called the phenolphthalein alkalinity and represented by P.
The alkalinity determined upto pH = 4.5 ; i.e 10y is called the total alkalinity and
is represented by T.

10. Hydroxide only:
Sample containing only hydroxide when the pH is more than 10.
Titration is completed at the phenolphthalein end point.
In this case the hydroxide alkalinity is equal to P.
Carbonates only:
Samples containing only carbonate alkalinity have a pH of 8.3 or more.
The titration to phenolphthalein end point is equal to the one half of the total
titration.
In this case total alkalinity is twice the phenolphthalein alkalinity i.e T = 2P.

11. Hydroxide and carbonate alkalinities both:
samples containing hydroxide as well as carbonate alkalinity have a high pH
usually above 10.
The titration from the phenolphthalein end point to methyl orange end point
represents one half of the carbonate alkalinity.
Carbonate – bicarbonate alkalinities both:
Samples containing carbonate and bicarbonate alkalinities have a pH > 8.3 and
less than 11.
The titration to the phenolphthalein end point represents one half pf the
carbonate alkalinity.
So the carbonate alkalinity is calculated as equal to 2P and bicarbonate
alkalinity is equal toT – 2P.

12. Bicarbonates only:
• Samples having a pH less than 8.3 will have only bicarbonate alkalinity which is
equal to total alkalinity .
• In this case P = 0 and phenolphthalein will not produce any pink color when
added to the water.

13. APPLICATION OF ALKALINITY DATA
1. Chemical coagulation
2. Water softening
3. Corrosion control
4. Buffer capacity
5. Industrial wastes