How the pH sensor works
The most common pH sensor is the glass electrode. It's used in many
industry applications and in a wide variety of fields. The
glass-electrode method has high reproducibility, and it can measure pH
of various solutions.
Electrochemical sensor
A pH electrode is a potentiometric
or electrochemical sensor that has a voltage output. A potentiometric
sensor consists of two electrochemical cells or electrodes:
- the glass electrode, sometimes called the measuring electrode or active electrode.
- and the reference electrode.
The
electric potential created between the glass electrode, and the
reference electrode is a function of the pH value of the measured
solution.
So once the potential difference has been measured you can
calculate the pH value. How? Nernst equation, see below, will explain
more.
"A battery with a variable voltage"
When
the glass and the reference electrode, are immersed in a solution, a
small galvanic cell is established. If you use a voltmeter and measure
the potential between these two electrodes, you will see a potential
difference.
The pH sensor is essentially a simple single cell
battery. It can be thought of as a battery, with a voltage that varies
with the pH of the measured solution.
Combination electrodes
The majority of pH electrodes available commercially are combination electrodes
that have both the glass electrode and the reference electrode
conveniently placed in one housing. While the reference electrode and
the pH measuring electrode are physically combined into one electrode,
the electrodes still function independently, and characteristic problems
remain.
To learn more about how the pH sensor works, please continue to read:
• Nernst equation - mathematical description of the pH electrode.
• Glass electrode - why the electrical potential of the glass electrode indicate the pH value.
• Reference electrode - makes contact with the sample solution and (should) deliver a stable reference potential.
• pH versus temperature - always take into account the temperature at pH measurements.
• Different potentials of a pH electrode - the measured pH electrode potential is affected by a number of factors.
• Combination electrode - modern electrodes combine the reference and standard electrodes into one unit.
• Nonglass pH electrode - an alternative to the glass electrode where an unbreakable sensor is required.
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Optical pH sensors 
One technique to measure pH is to use a fiber optic sensor. Maybe an uncommon pH technique, but I think it's important to know it exists. The development …
