What is the standard output characteristic of a pH sensing device in millivolts?

• A. A current signal in amperes.
• B. A potential difference in millivolts.
• C. A frequency signal in hertz.
• D. A digital code representing pH units.

Answer: (B)

The key idea is that a pH sensor outputs a voltage signal, not a current, a frequency, or a digital code. A glass pH electrode and a reference electrode form a electrochemical cell that generates a potential difference whose value changes with hydrogen ion activity. The measurement instrument reads this voltage, which is naturally expressed in millivolts, making the standard output characteristic a potential difference in millivolts. At room temperature (about 25°C), the sensitivity is roughly 59 millivolts per pH unit, meaning each unit change in pH shifts the voltage by about 59 mV in magnitude. Temperature affects this slope a bit, but the signal remains a voltage. So the sensor’s raw output is a millivolt-level voltage, while the final pH value is obtained by the instrument’s electronics translating that voltage into a pH reading.