Abstract: An ion selective combination is formed by creating a measuring electrode from an elongated tube whose wall is made with a depression along its length. That wall depression is covered with a material of a kind that can be pierced by the needle of a syringe or otherwise treated so that an electrolyte, or a liquid to form an electrolyte, can be injected into the cavity formed by the depression and that covering material. The electrolyte in the cavity then serves as the electrolyte of a reference cell which is completed by some non-metallic means for conducting ions from the interior of the cavity to the exterior where contact is made with a material to be tested. That ion path can be completed by a salt bridge or by ionic conduction through the pliant or piercable cover over the cavity in which the electrolyte is housed. An electrode having those features is combined with a coaxial cable so that the electrode structure is formed on the end of the cable.