A student carried out an acid-base titration by adding NaOH solution from a buret to an Erlenmeyer flask containing HCl solution and using phenolphthalein as indicator. At the equivalence point, she observed a faint reddish-pink color. However, after a few minutes, the solution gradually turned colorless. What do you suppose happened?

Shift in equilibrium position

Explanation:

The reddish-pink colour signalled the equivalence point. That is, pH=PKind. This was the point at which we have commonly the called the end point of the titration which has been carried out. As the solution was left to stand after this equivalence point was attained, the equilibrium position shifted again towards the acid side and the solution turned colourless once more. The equilibrium position indicated by the colour change depends on the H+ concentration of the solution.