An aqueous solution of boric acid undergoes titration with NaOH using phenolphthalein as an indicator. Reaction is:
In cis-1,2-diol, both -OH groups present on the same side and forms chelate complex with B(OH)3. This chelate complex enhances the acidic strength of B(OH)3, which consequently increases the rate of reaction in the forward direction. In trans-1,2-diol, two -OH groups present on the opposite direction, as a result, chelate formation do not take place with B(OH)3.