What does a pH probe do? What is its output?

• A. It generates a current, with output in amperes.
• B. It generates a voltage corresponding to a fixed millivolt value.
• C. It generates a temperature reading in Celsius.
• D. It generates an electrical potential, with output equal to the difference between millivolt signals.

Answer: (D)

The pH probe is an electrochemical sensor that converts hydrogen ion activity in a solution into an electrical potential. It consists of a sensing glass electrode and a reference electrode; the chemical potential difference between them when immersed in the sample produces a small voltage in the millivolt range. The measurement device reads this differential voltage and, after calibration (and temperature compensation), displays the pH value. The relationship is roughly linear with pH and follows the Nernst equation, giving about -59 mV per pH unit at room temperature (the slope shifts with temperature). It is not a current source and it does not directly output a temperature reading—the signal is a differential electrical potential, i.e., the difference between millivolt signals from the two electrodes.