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JEE Main levelSingle correct optionDifficulty 2 of 3pb017

Boric acid cannot be titrated sharply with NaOH using phenolphthalein, but on adding glycerol or mannitol it can be. This is because the polyol

  1. Aforms a stable cyclic complex with the borate, which removes and makes the acid effectively stronger
  2. Bconverts boric acid into a strong Bronsted acid by donating a proton to it
  3. Cacts as a buffer and fixes the pH at 7
  4. Dreduces boric acid to diborane

Answer: A

The cis-diol groups of glycerol or mannitol chelate forming a stable five-membered ring complex. This shifts the equilibrium to the right, so the solution behaves as a stronger acid and gives a sharp end point.
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