Alkalinity is the water quality parameter that determines the presence of dissolved hydroxide, carbonates and bicarbonates in the water. These may either be present alone or in the mixture form. Alkalinity also causes the hardness of water and increases turbidity of water. Such water can corrode water pipes, boilers, and can damage lever, kidney etc hoever it maintains PH value of water.

Determination of alkalinity

The extent of alkalinity present in the water is determined by volumetric titration of supplied water sample with standard acid using double indicator, first with phenolphthalein indicator and then second with methyl orange indicator.

In first titration, sample of water is taken in a conical flask and 2-3 drops of phenolphthalein is added as indicator. Then, it is titrated against standard acids solution taken in burette till pink color gets discharged. It is termed as phenolphthalein end point and volume consumed in the burette is noted. At this stage, hydroxides are completely neutralized and carbonate changes into bicarbonates as,

                    OH   +   H+   ——> H2O

                     CO3-2   +    H+ —— >HCO3

In second titration, methyl orange indicator is added and titration is further continued till the color of the solution changes from yellow to pink color. It is termed as methyl orange end point and volume is noted from beginning. At this stage, bicarbonate is completely neutralized to carbonic acid as,

                   HCO3   +   H+ ——–> H2CO3

Let,    x ml= volume of acid required up to phenolphthalein end point

       Y ml= vol. of acid required from beginning to methyl orange end point.

Conditions(vol. in ml) CO3-2 HCO3 OH
If      X=0 ml Nil present nil
If      y=x  ml Nil nil present
If      X =1/2y Present nil nil
If      X>1/2y Present nil Present
If     X<1/2y Present present nil

Sample test

S.N. Vol.of water (ml)           Burette reading( HCl)
Initial reading(ml) phenolphthaleinendpoint(x) Methylorange end point (y)
1 50 0.0 5.2   5.0 19.8   20.0
2 50 0.0 5.0 20.0
3 50 0.0 5.0 20.0


Volume of HCl for phenolphthalein end point(x) =  5.0ml

Volume of HCl for methylorange end point(y) =   20.0 ml

condition   x>1/2y

since x>1/2y, therefore, water contains CO3-2 and HCO3 but not OH

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