In the electrometallurgy of aluminium, oxygen gas is evolved at anode. It reacts with graphite or carbon (graphite electrodes ) to form carbon monoxide and carbon dioxide. In case some other metal electrodes act as anode, then oxygen will react with aluminium formed during the process to form aluminium oxide `(Al_(2)O_(3))` which will pass into the reaction mixture. Since graphite is cheaper than aluminium, its wastage or consumption can be tolerated.
