Question

Why should we boil water and shake the solution in the volumetric determination of alkalinity of soda ash?

Answer 1

Crude sodium carbonate, Na₂CO₃, is commonly called soda ash. It is frequently used as a commercial neutralizing agent. Besides the carbonate small amounts of sodium hydroxide, NaOH, and sodium hydrogen carbonate, NaHCO₃, may also be present. Titrating with standard acid, usually HCl, makes it possible to determine the total alkalinity of the soda ash. It is common practice to report the total alkalinity as percent sodium carbonate or sodium oxide, Na₂O.

The solution is boiled to remove the dissolved CO₂. This will change the pH back to somewhere between 8 and 9 and the indicator color back to blue. The solution is then cooled in an ice bath and the titration is continued until the solution becomes yellow-green. Removal of the CO₂ changes the aspects of this last part of the titration to one of a strong base titrated with a strong acid, which explains the accuracy of better than 0.1% that can be obtained by this method.

Hence we will boil & shake the solution in the volumetric determination of alkalinity of soda ash to remove any dissolved CO₂ gas.