AbstractThere is a requirement for on-line monitoring of elements in molten aluminium and one possible approach is to use electrochemical sensors which produce a voltage that is related to the concentration of an element in the melt. The principle of electrochemical sensors is outlined and examples given for hydrogen, lithium, and sodium determination in molten aluminium using sensors based upon calcium hydride, lithium silicate–phosphate, and sodiumβ-alumina, respectively: sensors could also be used to control the Hall–Heroult cell and possible examples are the measurement of the NaF/AlF3ratio in the cryolite bath, the evolved CO/CO2ratio from the anode, as well as the concentration of sodium and lithium in the aluminium tapped from the cell.MST/571