Question

On addition of catechol to aqueous solution of boric acid, it will :

• A. increase the pH of solution
• B. decrease the pH of the solution
• C. form polymeric metaborate species
• D. leave the solution undisturbed.

Answer 1

The correct option is B decrease the pH of the solution

Boric acid is a weak monobasic acid.
When certain organic polyhydroxy compounds such as glycerol, mannitol, catechol or sugars are added, $B(OH{)}_3$ behaves as a strong monobasic acid.

Here, catechol forms a very stable complex with $\left[B\right(OH{)}_4{]}^{–}$, removing it from the solution.

Thus, the removal of one of the products shifts the equilibrium in the forward direction. This will increase the $H^{+}$ concentration solution. Thus the pH of solution will get decreased.
$B(OH{)}_3+2H_{2}O\rightleftharpoons B(OH{)}_4^{-}+H_3{O}^{+}$