Abstract: A metal sulphide ionic membrane for use in monitoring heavy metal ions in solution comprises a mixture of at least four metal sulphides in relative proportions such that the membrane is capable of responding electrochemically to all of the corresponding metal ions when said ions are contained in solution, e.g. at concentrations in the range from 10.sup.-5 up to 1M. A preferred embodiment of the ionic membrane consists of a mixture of silver 15 to 65% by weight, cadmium 15 to 65% by weight, lead 30 to 65% by weight and copper 3 to 9% by weight sulphides. The ionic membranes are preferably prepared by co-precipitation of sulphides followed by hot pressing and may be detachably mounted in suitable electrodes, internal electrical connections made either by way of filling solutions or by direct connection, preferably biassed connection, of an internal conductor to the internal surface of the electrode pellet.