∵ When ferric nitrate is mixed with aqueous solution of potassium thiocyanate, i.e. (KSCN). The intensity of red colour becomes constant on attaining equilibrium. When, (I) oxalic acid, i.e. (C2H2O4) or (II) mercuric chloride, i.e. (HgCl2) is added at equilibrium. Both (I) and (II) will decrease the intensity of red colour. Due to formation of complex.