be detected from the region of the measuring electrode 3 into the region of the auxiliary electrode 7 as a result of which u201c cross talk u201d is effectively prevented from occurring fig4 shows an exploded view of a detail from a reference electrode according to the present invention which has a sandwich design a layer of the material for the reference electrode 5 u2032 u2033 is arranged in the central area this is surrounded on both sides by a one layer nonwoven material 8 u2032 u2033 each a layer each of the material of the protective electrode 6 u2032 u2033 is located in front of the nonwoven fig5 shows a sectional view of an electrode arrangement according to the present invention with cylinder electrodes which are arranged concentrically an outer cylinder 9 forms the measuring electrode a middle cylinder 10 forms the reference electrode and an inner cylinder 11 forms the auxiliary electrode a cylindrical protective electrode 12 12 u2032 each is arranged between the electrodes the cylindrical protective electrodes 12 12 u2032 are permeable to the electrolyte are approximately at the potential of the measuring electrode and shield the reference electrode against the effect of substances that cause changes in the potential fig6 shows a planar sensor according to the present invention with a.