ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
1-9
pH electrode
Description
The pH electrode (E777) is a pH-sensitive glass electrode. The pH-sensitive glass
membrane is located at the tip and seals the inner buffer solution with a constant
and known pH.
The air bubble allows for expansion
of the inner buffer solution when the
electrode is thermostatted to 37
o
C.
The potential difference across the
glass membrane is due to a change
in the charge balance at the
membrane.
The glass membrane is sensitive to
H
+
ions. The metal ions in the glass
are exchanged with protons on either
side of the membrane, from the inner
buffer solution on one side and the
sample on the other.
A difference in the ion exchange on either side of the membrane occurs if the H
+
concentration (and therefore pH) is unequal on both sides. The number of positive
and negative ions is no longer equal, so the potential difference across the
membrane changes. If the H
+
concentrations on either side of the membrane are
equal, the potential difference will theoretically be 0 mV.
Electrode contact
Glass membrane
Inner buffer
solution
Electrode
The theoretical sensitivity of the pH electrode at 37
o
C being equal to −61.5 mV
per pH unit, using pH = −log [H
+
], and converting concentration to activity, the
Nernst equation can be expressed as:
Nernst equation
EE615pH m
sample 0
=− ×.
V
Designation
The following symbols are used:
−61.5 mV/pH
= Theoretical sensitivity of the pH electrode at 37
o
C
E(pH,Cal2) = Potential of the pH electrode chain from a calibration
measurement on Cal 2 solution
E(pH,Cal1) = Potential of the pH electrode chain from a calibration
measurement on Cal 1 solution
E
0
(pH,Cal1) = Standard potential of the pH electrode chain with a nominal
pH = 7.4 (the approximate pH of Cal 1 solution)
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