In a real time, on-line pH measurement of an aqueous solution, a system provides continuous data on the physical condition of the pH sensor. The system includes a pH sensor having a membrane disposed in the process solution with a first external electrode attached to outer surface of the membrane and a second external electrode adjacent the sensor. The two electrodes in the solution are periodically driven by a short time duration dc signal producing a direct current between the two electrodes. The direction of the current through the solution is periodically reversed. This action results in electrolysis of the H.sub.2 O molecules of the solution between the two electrodes producing controlled changes in hydrogen ion concentration at the sensor. One current direction produces an increase in pH and the other direction produces a decrease in pH. The sensor response to the controlled changes in pH in the solution is a direct indicator of the complete sensor and system performance and is monitored and analyzed by a computer. If a change in the response to the controlled pH changes is detected, the computer attempts to recalibrate the sensor. Visual readouts and threshold alarm circuits are provided which indicate changes in the sensor response to the pH changes and to a failure of the recalibration attempt.