Q1(d). Colour change of phenolphthalein

Answer in brief

Increasing pH favours the conjugated, quinonoid dianion of phenolphthalein, which absorbs visible light and appears pink.

Key points

• At low pH the predominantly closed lactone form has interrupted conjugation and is colourless in the visible region.
• Deprotonation and ring opening produce a delocalised quinonoid dianion; its extended conjugation shifts absorption into the visible region.
• A simplified net equilibrium is H₂In ⇌ 2H⁺ + In²⁻; removing H⁺ by adding base favours the pink form.
• Below pH 8 the coloured fraction is too small to see; above the transition range the pink form predominates.
• In very strongly alkaline solution, further OH⁻ addition can give a colourless carbinol form and fading.