Orthoboric acid in aqueous medium is

${\mathrm{H}}_3{\mathrm{BO}}_3 + {\mathrm{H}}_2\mathrm{O} \rightleftharpoons {\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}^{-} + {\mathrm{H}}^{+}$

${\mathrm{H}}_3{\mathrm{BO}}_3$ is a weak monobasic acid. It self does not donate proton but by accepting lone pair from water it forms complex and proton is produced in solution.