The reaction of potassium cyanide with sodium hydroxide gives the cyanide ion. After that cyanide ion reacts with the thiosulphate ions and cool and acidify in presence of the hydrochloric acid. It gives the thiocyanide ion.

$\mathrm{KCN} + \mathrm{NaOH}\to \mathrm{NaCN} + \mathrm{KOH}$

 ${\mathrm{S}}_2{{\mathrm{O}}_3}^{2-} +{\mathrm{CN}}^{-} \stackrel{\mathrm{HCl}}{\to }{\mathrm{SCN}}^{-} + {{\mathrm{SO}}_3}^{-}$

The thiocyanate ion reacts with the iron $\left(\mathrm{III}\right)$ chloride and formed the $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$ and the complex has the blood-red colour.

${\mathrm{FeCl}}_3 +{\mathrm{SCN}}^{-}\to \mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}(\mathrm{blood} \mathrm{red} \mathrm{colour})$