Solution of a salt in dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$ or acetic acid produces deep blue colour with starch iodide solution. The salt contains :

$I_2$ is released from potassium iodide by nitrite ion, making starch blue.

$\begin{array}{l}2{\mathrm{NO}}_2^{-}+3l^{-}+4{\mathrm{CH}}_3\mathrm{COOH}\to l_3^{-}+2\mathrm{NO}\uparrow +4{\mathrm{CH}}_3{\mathrm{COO}}^{-}+2{\mathrm{H}}_2\mathrm{O}\\ l_2+\text{ starch }\to \text{ blue colour }\end{array}$

The process has the reaction,

$2{\mathrm{NO}}_2^{-}+3{\mathrm{I}}^{-}+4{\mathrm{CH}}_3\mathrm{COOH}$

                             $\to {\mathrm{I}}_3^{-}+2\mathrm{NO}\uparrow +4{\mathrm{CH}}_3{\mathrm{COO}}^{-}+2{\mathrm{H}}_2\mathrm{O}$

                                          $\Downarrow$

With starch gives blue colouration.