RADIOMETER ABL800 FLEX, Reference Manual

Page: 8 / 206

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
General information,
Continued
Nernst equation
(continued)
n = charge on the ion
F =
Faraday constant (96487 coulomb
×
mol
−
1
)
x
a
= activity of x
The Nernst equation is rearranged to express the activity as a function of the
potential
E
sample
. Having measured E
sample
the activity can be calculated since all
other quantities are already known. Finally the analyzer converts activity to
concentration.
Strictly speaking, the potential of an electrode chain or the magnitude of current
flowing through an electrical chain is related to the activity of a substance, and not
its concentration.
Activity expresses the ‘effective concentration’ of a species, taking non-ideality of
the medium into account.
Activity and concentration are related by the following equation:
a
x
=
γ
c
x
where:
a
x
= the activity of the species x
γ
= the activity coefficient of species x under the measurement conditions
(for ideal systems
γ
= 1)
c
x
= the concentration of species (mmol/L)
NOTE: To be exact, activity is related to the molality of species x, i.e., the number
of mmoles per kg of solvent. However molality is converted to concentration
(molarity).
The analyzer automatically converts activities into concentrations
[
1
]
. The term
concentration is therefore used in explanations of the measuring principles for each
of the electrodes further on in this chapter.
The potentiometric measuring principle is applied in the pH, p CO
2
, and electrolyte
electrodes. It is slightly different for the
p
CO
2
electrode, however, since the Nernst
equation is not directly applied.
Calibration is an analytical process defining the functional relationship between the
obtained readings or analytical responses and the concentration or other quantities
present in the calibration material (liquid or gas). Thus, a calibrating solution or a
gas mixture (for
p CO
2
calibrations) is drawn into the measuring chamber and the
analyzer adjusts itself to measure the known value of the liquid or gas.
Calibration
Continued on next page
1-3