How a pH electrode works

What does the pH value actually tell you?

The pH value describes how acidic or alkaline a liquid is. The scale runs from 0 to 14: values below 7 are acidic, 7 is neutral, and above that it becomes alkaline. Lemon juice sits at around pH 2, pure water at 7, soapy water at about 10.

Where other everyday substances sit on the scale is shown in the table of pH values of common liquids - from battery acid to caustic soda.

And to feel the logarithm behind it at a slider: the interactive pH scale converts every value into ion concentrations live.

And to place the second big water parameter next to pH: pH and ORP compared explains why the millivolt display on the pool controller measures something entirely different.

One component deserves a closer look of its own: the junction is the inconspicuous transition point where most measurement problems arise - and its design determines which electrode suits which sample.

Behind it lies the concentration of hydrogen ions (H⁺) in the solution. And here it gets interesting: the scale is logarithmic. A solution at pH 5 contains ten times as many hydrogen ions as one at pH 6 - and a hundred times as many as one at pH 7. Even small shifts on the scale mean large chemical differences. That is exactly why precise measurement pays off.

How does an electrode measure pH?

The heart of every classic pH electrode is a wafer-thin membrane of special glass, usually shaped as a small bulb at the lower end. When it is immersed in a liquid, hydrogen ions attach to the glass surface - on the outside from the sample, on the inside from a buffer solution with a known pH that is sealed inside the electrode.

If the ion concentrations inside and outside differ, a tiny electrical voltage builds up across the membrane. At 25 °C this is about 59 millivolts per pH unit - a relationship described by the Nernst equation. The meter measures this voltage and converts it into the pH value. The electrode itself displays nothing; it only delivers the signal.

The construction at a glance

Every measurement also needs a second building block: the reference electrode. It supplies a constant comparison potential against which the membrane voltage is measured. It stays in contact with the sample through a small porous spot in the shaft - the junction (also called the diaphragm).

In the past, the measuring and reference electrodes were two separate probes. Today both are almost always combined in one shaft: the combination electrode. When people say "pH electrode", this is usually what they mean.

Why calibrate?

A pH electrode is a living piece of chemistry. The glass membrane changes with every measurement, the reference system ages, and no two electrodes behave exactly alike. That is why the electrode is regularly checked against buffer solutions whose pH is precisely known - typically pH 7 and pH 4. The meter learns how "its" electrode is currently behaving and corrects the deviations.

Calibration produces two key figures: the zero point (the voltage at pH 7, ideally close to 0 mV) and the slope (the voltage change per pH unit). If the slope drops well below the ideal value of 59 mV, the electrode is approaching the end of its life.

The role of temperature

The 59 millivolts per pH unit apply exactly only at 25 °C. At other temperatures the relationship shifts - at 60 °C it is already about 66 mV. Good meters compensate automatically (keyword ATC, automatic temperature compensation), either via a temperature sensor built into the electrode or a separate probe. If you measure without compensation, at least keep the sample and the buffer solutions at a similar temperature.

What next?

Now you know the principle. The next logical step: finding out which design suits your application - because there is a world of difference between a rugged gel electrode for the aquarium and a refillable laboratory electrode.

Continue to electrode types →