It is of great help to be familiar to the concept of electrode potential
to understand the behavior of a pH electrode. As already known, a pH
measurement is a potentiometric measurement ( i.e. it measures a potential). However, the output voltage of your pH electrode is the sum of different electrode potentials.
A pH probe delivers a sensitive
electrical potential (voltage) with a number of sources of error. To
achieve an accurate measurement you want to keep all the different
potentials stable, except, of course, the potential of the pH sensitive glass membrane. Most pH sensors in use today are pH combination electrodes. So, let's take a closer look at the different sources of potential for a pH combination electrode.
Six different potentials form the total output voltage (mV) of a pH electrode:
E1 is the potential of the hydrogen sensitive glass - the real pH effect.
E2 is the asymmetry potential. When a pH electrode is in contact with a solution with the same pH as its internal buffer, there should be no potential difference across the glass membrane. However, in reality, there is a potential difference of a few millivolts, and it is known as the asymmetry potential.
Things that limit the ability of the ion exchange mechanism in the glass membrane will cause an asymmetry potential. For example, different gel layer thickness and on the thickness of the glass membrane itself.
E3 is the potential on the inside of the membrane. As the pH of the internal buffer solution does not change in value, this potential should be constant at all times.
E4 is the potential that develops at the surface of the reference (Ag/AgCl) element of the measuring electrode. The potential is kept constant by using an internal buffer with a constant pH value.
E5 is the potential that develops at the surface of the reference (Ag/AgCl) element of the reference electrode. The potential is kept constant by using an electrolyte with constant pH.
If both reference elements are identical, and the reference electrolyte and the internal buffer have the same pH value, then E4 and E5 are equal in magnitude but opposite in sign and cancel one another.
E6 is the diffusion potential of the junction or diaphragm, and the contact point between the measuring solution and the electrolyte.
It ideally would be constant resistance (potential) of the junction during measuring. However, the resistance is always changing, depending on contamination and clogging of the junction or diaphragm, electrolyte dilution, and chemical attacks of the reference element. One of the most common problems with pH sensors is fouling of the junction.
is the electrode potential of primary interest. The potential that really
measures the pH value. Unfortunately, this potential cannot be measured
What the pH meter (voltmeter) measures is a sum of all potentials:
U = E1 + E2 + E3 + E4 + E5 + E6
During ideal conditions, all the potentials E2 - E6 would be constant and E1 would be the only potential that vary during a pH measurement. However, in practice that's not the case, and the way we can compensate for these undesirable electrode potential fluctuations is regular calibration.
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