Potassium cyanide is made alkaline with NaOH and boiled with thiosulphate ions. The solution is cooled and acidified with HCl and this solution with iron ( III ) chloride produces :-

Step-by-step solution

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. KCN   +   NaOH → NaCN   +   KOH S 2 O 3 2 -   + CN -   → HCl SCN -   +   SO 3 - The thiocyanate ion reacts with the iron ( III ) chloride and formed the Fe ( SCN ) 2 + and the complex has the blood-red colour. FeCl 3     + SCN - &#85