The Diffusion Potential of a pH Sensor
Another challenge of pH sensor design is the diffusion potential (E6) of the liquid junction. This potential always develops at the phase boundary between the internal KCl electrolytes and the process liquid. The diffusion potential is attributed to the different migration velocities of ions, which again depends on the polarity and size of the ion type.
The illustration below explains the diffusion potential between two HCl solutions of different concentration:
The H+ ions diffuse nearly five times faster to the right than the Cl- ions. This creates a voltage potential across the boundary of the two solutions. In order to keep the diffusion potential at the liquid junction of a reference electrode as small as possible, the different ions in the reference electrolyte should have identical ionic mobility. With a 3M KCl solution this ideal condition is nearly reached.
In general it can be said:
- The higher the KCl concentration of the reference electrolyte, the lower the diffusion potential.
- The larger the flowrate of the reference electrolyte through the diaphragm, the smaller the diffusion potential.
The more the pH value of the measured solution differs from pH 7, the larger the diffusion potential.
Diffusion Potentials of Various Solutions and Saturated KCl Electrolyte
| 1.0 | mole HCl | = | 14.1 mV |
| 0.1 | mole HCl | = | 4.6 mV |
| 0.01 | mole HCl | = | 3.0 mV |
| 0.1 | mole HCl | = | 1.8 mV |
| Buffer pH | 1.68 | = | 3.3 mV |
| Buffer pH | 4.01 | = | 2.6 mV |
| Buffer pH | 4.65 | = | 3.1 mV |
| Buffer pH | 4.01 | = | 2.6 mV |
| Buffer pH | 4.65 | = | 3.1 mV |
| Buffer pH | 7.00 | = | 1.9 mV |
| Buffer pH | 10.1 | = | 1.8 mV |
| 0.01 | mole NaOH | = | 2.3 mV |
| 0.1 | mole NaOH | = | -0.4 mV |
| 1.0 | mole NaOH | = | -8.6 mV |
From the above example it can be seen that different measured solutions will create different diffusion potentials at the liquid junction of a reference electrode.
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