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pH Measurement

Measuring pH involves comparing the potential of solutions with unknown [H+] to a known reference potential. pH meters convert the voltage ratio between a reference half-cell and a sensing half-cell to pH values. Today, most electrodes are combination electrodes with both the reference and sensing half-cells in the same body.

When the pH electrode is immersed in the solution to be measured, a potential is established on the surface of the sensing glass membrane. If the unknown solution is neutral, the sum of fixed voltages on the inner surface of the glass membrane and on the sensing conductor approximately balances the voltage on the outer surface of the glass membrane and the reference half-cell. This results in a total potential difference of 0 mV and a pH value of 7.

In acidic or alkaline solutions, the voltage on the outer membrane surface changes proportionally to changes in [H+]. The pH meter detects the change in potential and determines [H+] of the unknown by the Nernst equation:

E = E° + 2.3RT
 nF
log unknown [H+]
 internal [H+]
>
E = E° + 2.3RT  log unknown [H±]


nF internal [H+]

where:
  E = total potential difference (measured in mV)
  E° = reference potential
  T = temperature in Kelvin
 n = number of electrons
  R = gas constant
  F = Faraday’s constant
  [H+] = hydrogen ion concentration

 

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