Roche 9180 - Physical Principle

Roche Diagnostics
54 Instructions for Use · Version 10.0
4 Theoretical foundation 9180 Electrolyte Analyzer
Principles of operation
Figure 4-1 Measurement principle
Physical principle
An ion-selective electrode is connected with a reference electrode to form a
measuring system (see Figure 4-2 Physical principle (p. 55)). When immersed in a
solution that contains the relative ion, the Nernst equation applies:
Equation 4-1
or
Equation 4-2
(+) for cations
(–) for anions
The equation can also be written:
Equation 4-3
A Glass capillary D Inner solution G Measured solution
B Reference electrode E Ion-selective electrode
C Voltmeter F Ion-selective membrane
B
C
D
F
A
E
G
E
measured electrical potential
E’
electromotive force of the system in standard solution
a
i
activity of ion measured
R
general gas constant (8.31 J/Kmol)
T
temperature
n
valence of measured ion
F
Faraday constant (96.496 A·s/g)
f
i
activity coefficient
c
i
concentration of measured ion
S
slope of the electrode
EE
RT
nF
----------- a
i
ln=
EE
RT
nF
----------- f
i
c
i
ln=
E
E Sf
i
c
i
log=