KMnO ₄ can be prepared from K ₂ MnO ₄ as per reaction (3 MnO ₂²⁻+2 H ₂ O 2 MnO ₄⁻+ MnO ₂+4 OH ⁻ ). The reaction can go to completion by removing OH ⁻ ions by adding

Step-by-step solution

Since, OH ⁻ are generated from weak acid ( H ₂ O ) , a weak acid ( . like . CO ₂ ) should be used to remove it because of strong acid ( HCl ) reverse the reaction. KOH increases the concentration of OH ⁻ , thus again shifts the reaction in backward side. CO ₂ combines with OH ⁻ to give carbonate which is easily removed. SO ₂ reacts with water to give strong acid, so it cannot be used.