Chemistry – Experimental techniques and chemical analysis - Acid-base titrations | e-Consult

Phenolphthalein is a suitable indicator for the titration of hydrochloric acid with sodium carbonate because it changes colour over a specific pH range that coincides with the pH change during the reaction. Phenolphthalein is colourless in acidic solutions (pH 8.3).

In the reaction between hydrochloric acid and sodium carbonate, a neutralization reaction occurs, producing water and carbon dioxide. The carbon dioxide produced reacts with the water to form carbonic acid (H₂CO₃), which is a weak acid. This leads to an increase in the pH of the solution. The end-point is reached when the pH rises above 8.3, causing the phenolphthalein to change from colourless to pink. This pink colour indicates that the hydrochloric acid has been completely neutralised by the sodium carbonate. The amount of pink colour is proportional to the amount of acid present.