Determination of Total Alkalinity of Water Sample

What is Alkalinity of Water ?

The alkalinity of water is due to the presence of hydroxyl ions (OH^-) from NaOH or KOH, carbonate ions (CO₃^2-) from sodium carbonate or potassium carbonate and bicarbonates(HCO₃^-)from sodium bicarbonate or potassium bicarbonate or temporary hardness (calcium bicarbonate and magnesium bicarbonate).

Estimation:

            The alkalinity of water can be estimated by titrating water with a standard acid using phenolphthalein and methyl orange as indicators.

      When titrated with acid, the reactions taking place are as follows:

                  OH^-  + H+   →  H₂O                          

              CO₃^2- + H+    →  HCO₃^-

                 HCO₃^-  + H+   → H₂O + CO₂

The titration of water sample with a standard acid up to the  phenolphthalein   endpoint marks the completion of reactions (i)and (ii) only. The amount of acid consumed corresponds to hydroxide plus one half of the carbonate present. Titration of the water sample with a standard acid to methyl orange endpoint marks the completion of all the three reactions above. Hence, the amount of the total acid consumed corresponds to the total alkalinity and the amount of acid used after the phenolphthalein   endpoint corresponds to one half of the carbonate plus all the bicarbonates.

               The alkalinity of water can be due to OH^-  only or carbonate only or bicarbonate only or due to their combinations. But, OH^-  and bicarbonate can not exist together because they combine instantaneously to form carbonate ions.

                      OH^- + HCO₃^-  → CO₃^2- +  H₂O

On the basis of same reasoning, all the  three, OH^-,carbonate and bicarbonate cannot exist together.

Procedure:

         

             Pipette out 100ml of the water sample into a conical flask. Add 2-3 drops of Phenolphthalein indicator. Titrate with standard sulphuric acid solution till the pink color just disappears. Note down the titre value. Then add 2-3 drops of methyl orange indicator to the same solution and titrate further with the standard acid solution till pink color reappears. Again note down the titre value.