Q1(a). Carbonate titration endpoint

Answer in brief

It can indicate the first endpoint, carbonate to bicarbonate, but not complete two-proton neutralisation.

Key points

• CO₃²⁻ + H⁺ → HCO₃⁻ has an equivalence region near pH 8.3, within the phenolphthalein transition.
• Further acid gives HCO₃⁻ + H⁺ → CO₂ + H₂O, with an acidic final equivalence region.
• Phenolphthalein is already colourless before this second endpoint; methyl orange or an appropriate potentiometric endpoint is used for complete neutralisation.
• Thus the printed statement is too broad. A valid carbonate assay can use phenolphthalein if the one-proton stoichiometry is deliberately used.