What is a pH meter?

In order to measure pH, a measuring instrument sensitive to the hydronium ions that determine the pH value is required. This is a voltmeter.
The measuring principle consists of taking a sensor with a glass membrane sensitive to hydronium ions (pH electrode) and observing the reaction between the membrane and the sample: measuring potential.
This potential is compared to a reference potential delivered by a non-pH-sensitive electrode, by diffusion of an electrolyte towards the sample through a diaphragm: reference electrode (Ag/AgCl).

The pH of a solution is therefore the potential difference between the two electrodes according to Nernst’s equation :

  E = E0 + 2.3RT / nF * log [H3O+]

  E = measured potential
  E0 = constant
  R = gas constant
  T = temperature in degrees Kelvin
  n = ionic charge
  F = Faraday’s constant

For simpler use, both electrodes are combined in one single electrode.

Illustration of an electrode in cross-sectional view. The view shows a cylinder with a cable running through it. At the end is the membrane, inside which is the internal buffer. In the body are the internal and reference electrodes, the reference electrolyte and the reference diaphragm.
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