Abstract: An apparatus and method are disclosed for the continuous monitoring of pH in a fluid stream flowing at a low volumetric flow rate. The stream is directed through an enclosed conduit with a glass electrode sealed therein. The conduit wall facing the electrode has contour approximately complementary in shape to the sensing surface of the electrode with a narrow gap between the opposing surfaces through which the fluid passes, forcing rapid and uniform flow of the fluid over the electrode surface. Continuous operation is thereby permitted for extended time periods without substantial fouling of the surfaces.