The present invention relates to planar electrochemical sensors with membrane coatings used to perform chemical analyses. The object of this invention is to provide unit-use disposable sensors of very simple and economical construction, preferably with only a single membrane coating on an electrode. The invented devices are potentiometric salt-bridge reference electrodes constructed with a heterogeneous membrane coating of a conductor. The heterogeneous membrane is a formulation that concurrently supports non-volatile species transport through an electrolyte compartment and gas transport through a hydrophobic compartment.