potentiometry
/ poh-ten-shee-OM-uh-tree /
Picture dipping two probes into a glass of liquid and watching a voltage appear on a meter, the way a thermometer shows a temperature. You are not pushing current through the liquid and forcing anything to happen — you are simply listening to the tiny voltage the solution itself produces, and that voltage tells you about what is dissolved in it. That quiet act of listening to a voltage is potentiometry.
More precisely, potentiometry measures the voltage (potential difference) between two electrodes dipped in a sample while essentially no current flows. One electrode, the indicator electrode, develops a potential that depends on the concentration (more exactly, the activity) of a chosen ion; the other, the reference electrode, holds a fixed, known potential as a stable baseline. The difference between them is read and converted into concentration using the Nernst equation.
It matters because it is the basis of the everyday pH meter and of ion-selective electrodes for ions like sodium, potassium, fluoride, and calcium — cheap, fast, and usable right in a sample without destroying it. The honest caveat is that the signal responds to ion activity rather than directly to concentration, so the answer depends on ionic strength and on a clean, well-maintained reference; a drifting or contaminated reference quietly ruins the reading.
A pH meter is the most familiar potentiometric instrument: a glass indicator electrode plus a reference electrode read a voltage in the sample, and the meter simply displays that voltage already converted into a pH number — no current, no titration needed to get a quick answer.
Potentiometry reads a voltage at near-zero current to find ion concentration.
Do not confuse potentiometry (measure voltage, no current) with voltammetry or coulometry (apply a voltage and measure or count current). Potentiometry watches; the current-based methods actively drive a reaction.