In my school there are jars which are turned into Lechlanché cells. Basically the cell has aqueous ammonium chloride solution as the electrolyte which is obviously transparent in the beginning. The electrodes used are zinc and the second electrode is a porous pot which has a carbon rod and crushed manganese(IV) Oxide. The electrolytes in cells which have been used for a long time have turned blue. I want to know where this colour is coming from. Because ammonium salts are colourless, zinc complexes cannot show colour due to d-d transition and I have never seen a blue coloured manganese compound. So what is it that is forming due to which the solution is turning blue?