RADIOMETER ABL800 FLEX, Справочное руководство

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1. Potentiometric measuring principles ABL800 FLEX Reference Manual
pCO
2
electrode
The p CO
2
electrode (E788) is a combined pH and Ag/AgCl reference electrode
mounted in a plastic jacket, which is filled with a bicarbonate electrolyte.
Description
The jacket is covered by a 20
µ
m silicone membrane
moulded on a 50
µ
m nylon net.
The net both reinforces the
silicone membrane and serves
as a spacer in order to trap a
layer of the electrolyte between
the membrane and the glass tip
of the electrode. The electrolyte
also contains glycerol to
prevent collection of air
bubbles in the electrode jacket
thus improving electrode
stability.
Electrolyte
Ag/AgCl reference band
Membrane
Electrode contact
Electrode jacket
The membrane allows any uncharged molecules of CO
2
, O
2
, N
2
to pass through it.
Charged ions such as H
+
will not pass. Consequently, dissolved CO
2
from the
sample will diffuse into the thin layer of bicarbonate electrolyte until the
equilibrium is reached.
This produces carbonic acid:
H
2
O + CO
2
⇔
H
2
CO
3
Carbonic acid dissociates according to the following equilibrium reaction:
H CO H H CO
2 3 2 3
⇔ + + −
The release of H
+
ions changes the H
+
concentration, and therefore the pH of the
solution on one side of the pH-sensitive glass membrane.
The concentration gradient of H
+
ions on the other side of the membrane affects
the potential difference across the glass membrane. This change in potential across
the glass membrane is measured by the voltmeter.
Nernst equation The Nernst equation is used to convert the potential reading into a pH value:
mV) ( pH 5 . 61
0 glass
× − =
E E
where:
E
glass
= potential difference across the glass membrane
E
0
= standard electrode potential
61.5 mV/pH = theoretical sensitivity of the pH electrode at 37
o
C
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