Abstract: A modular potentiometric sensor includes a housing having measuring and reference half-cells, a temperature sensor, and solution ground combination assembly. An electrical conductor of the combination assembly extends through the housing, while remaining electrically isolated from the housing and half-cells, terminating at an electrically and thermally conductive end cap. Seals at opposite ends of the housing permit portions of the half-cells and the combination assembly to extend therethrough. The seals, measuring half-cell, and the combination assembly define an electrolyte compartment for the reference half-cell. The end cap provides close thermal coupling to the test fluid while also serving as a test fluid ground that is electrically isolated from the electrolyte compartment.