for a potentiostatically operable , polarographic - amperometric sensor , in particular for biomedical use , wherein a working electrode and a reference electrode are covered by an electrolyte layer which is separated from the medium to be measured by a semipermeable membrane , the invention proposes to provide the counter electrode in the form of an external electrode which -- for example -- can be an outer metal coating surrounding the sensor chip in a three electrode system fabricated as micro - sensor with the integrated technique , and which can be made -- for example -- of a stainless steel material or of a conductive plastics material .