Alkalinity in Water

Alkalinity means the find the amount of acid  neutralizing in water. Alkalinity is the total of all bases that occupying in the water and basically find in carbonate, bicarbonate and hydroxide. More alkalinity level can make scale formation. Total Alkalinity maximum level is 500 mg/l.

The alkalinity of water is attributed to the presence of the following substances in it

Caustic alkalinity      

1. Sodium or potassium hydroxides (NaOH or KOH)
2. Sodium or potassium carbonates   (Na₂C0₃ or K₂CO3)
3. Sodium or potassium bicarbonates   (NaHCO3 or KHCO3)

Temporary Hardness    

1.   Calcium bicarbonate [Ca(HCO3)2]
2.   Magnesium bicarbonate [Mg(HCO3)2]

These can be estimated separately by titration against standard acid using phenolphthalein and methyl orange indicators. The determination is based on the following reaction.

(ⁱ)   [OH^–]             +           [H⁺]  ——->   H2O

(ii)   [CO₃^2-]        +            [H⁺]   —–>    [HCO3^–]

(iii)  [HCO₃ ^–]      +           [H⁺]   ——>   H₂O  + CO2

The titration of the water sample against a standard acid upto phenolphthalein end-point marks that completion of reactions (i) and (ii) only. This amount of acid used thus corresponds to hydroxide plus one-half of the normal carbonate present. On the other hand, titration of the water sample against a standard acid to methyl orange end•point marks the completion of reactions (i), (ii) and (iii).

Hence. the amount of acid used alter the phenolphthalein end•point corresponds to one-half of normal carbonate plus all the bicarbonates ; while the total amount of acid represents the total alkalinity (due to hydroxide bicarbonate and carbonates).

The possible combinations of ions causing alkalinity in water are

(i)   OH-  only or
(ii)  CO3 2- only or
(iii) HOCO3 –  only or
(iv)  OH – and CO# 2- together or
(v)   CO3 2- and HCO3 2- together

The possibility of OH^–  and HCO₃^–  ions together it ruled out because They combine instantaneously to force CO₃^2- ions.
OH-  + HCO3-  ———> CO3 2-  +   H2O

 e.g  NaOH + NaHCO3  ——-> Na2CO₃ + H2O

Thus, OH^– and HCO^3- ions cannot exist together in water. On the basis of same reasoning all the three OH^{–  ,}  CO₃^2-  and HCO₃^2-  cannot exist together.

Alkalinity  Procedure.

Pipette out 100 ml of the water sample in a clean titration flask. Add to a it 2 to 3 drops of phenolphthalein indicator. Run in N/50 H2So4  (from a burette) till the pink colour is use discharged. Then to the same solution add 2 and a drops of methyl Orange. Continue the titration till the pink colour reappears.