Abstract: A device and method are provided for monitoring the metal content of aqueous systems. The device involves the use of a porous membrane permeable to the metal ions to be detected in an aqueous phase and an organic liquid medium containing a trapping agent for the ions and capable of forming therewith a complex soluble in the organic liquid medium. The organic medium with its trapping agent is contained in a housing with an opening covered by the membrane which can be presented to the aqueous phase.The device and method allow the aqueous and organic phases to wet opposite faces of the membrane so that ion transfer can take place across the membrane. The trapped ions are analyzed at intervals by conventional means.