Question

When a strong acid is titrated with a strong base using phenolphthalein, the color changes suddenly at the end point. The color can be switched back and forth by addition of only a single drop of acid or base. The reason for the abruptness of this color change is that?

Answer 1

Answer:

Keto - enol tautomerism

Explanation:

Phenolphthalein is an organic molecule (formula: C20H14O4), which is used in to determine the final volume in an acid-base titration, this is, as the molecule changes from one to another color, depending on the Ph of the solution (in acids solution, the molecule remains colorless, while in basics solutions it remains pink)  :

The molecule of phenolphthalein is a week acid, which losses a proton (H+) when it´s in solution: the undissociated molecule is colorless, while the correspondent anion (without a H+) is pink. This may be simplified with the following reaction:

H₃In⁺ ⇄ H₂In (colorless) ⇄ In⁻² (pink) ⇄ In(OH) ⁻³

- When adding a base (for example, NaOH) to phenolphthalein (initially, a colorless solution), molecule losses one H⁺ and we obtain In⁻², which is pink

- When adding an acid, the original structured is recovered, so color turns from pink to colorless

This change color is not only explained with adding an acid or a base, but also with phenolphthalein structure, that leads to a keto-enol tautomerism: as molecule has 2 hydroxyl groups (- OH) and 1 carbonyl group (C = O), compounds with this structure have an equilibrium between both groups:

R1 = C H– OH ⇄  R1 – C = O

With this change, phenolphthalein structure changes and hence, color solution changes as well