Bi³⁺ ion readily gets hydrolyzed in water to give bismuthyl ion.
${\mathrm{Bi}}^{3+}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{BiO}}^{+}+2{\mathrm{H}}^{+}$

This equilibrium can be moved in the forward direction by

While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium.

Hence, This equilibrium can be moved in the forward direction by making the solution alkaline.