The use ofpHpotentiometric measurements in titrimetric analysis is commonplace in chemical and biological systems. The methodology usually works well, but problems arise in chemical systems involving slow equilibrium processes. This article describes a microprocessor controlled system that measurespHduring the course of titration and will only allow the addition of titrant when there is no detectible drift inpHwith time. The apparatus described has been used in these laboratories for several years and is extremely stable and reliable. ©1998 American Institute of Physics.