NAVI F-52 - Page 237

Chapter 16 Reference
16.3 Ion measurement
224 HORIBA
16.3 Ion measurement
16.3.1 Ion electrode
The ion electrode responds (has an ion selectivity) to the activity (concentration) of specific
ions which exist in a solution and then generates electric potential corresponding to the con
-
centration of those ions.
The responsive membrane of the ion electrode is made of a compound or jelly organic sub-
stance that is difficult to dissolve in water. From the viewpoint of responsive membrane
types, the ion electrode is classified into ones for the ion species shown below:
Glass (membrane) electrode
i.g., H
+
, Na
+
, K
+
, NH
4
+
, Ag
+
, Li
+
Solid membrane electrode (with membrane which was formed by pressurizing
single-crystal metallic base that is difficult to dissolve)
i.e., F
-
, Cl
-
, Br
-
, I
-
, S
2
-, CN
-
, SCN
-
,
Ag
+
, Cu
2+
, Cd
2+
, Pb
2+
, Hg
2+
Liquid membrane electrode or plastic membrane electrode
A responsive substance such as an ion exchanging substance and antibiotics is dissolved in a
hydrophobic solution and then retained on polyvinyl chloride to make it oily for use as mem
-
brane.
K
+
, Ca
2+
, NO
3
-
, ClO
4
-
, Cl
-
, NH
4
+
Dissolved gas sensitive electrode (Perforated gas permeable membrane is
combined with a pH electrode or an ion electrode.)
i.g., NH
3
, CO
2
, HCN, H
2
S, HF, NO
2
16.3.2 Ion concentration measurement
When certain ions exist within the solution that is to be measured, the responsive ion elec-
trode membrane generates an electric potential corresponding to the concentration of the ions.
The potential that is generated is measured by the ion meter as potential, using the reference
electrode as the standard. With ion electrodes, the measured potential and the logarithm of the
ion activity within the solution being measured are generally proportional to each other and
are expressed in the following way.
E=+(2.303RT/nF) logγC
E:Measured electric potential (V)
:Standard potential (V), determined according to the system. This includes the standard
potential of the reference electrode and the liquid junction potential.
F: Faraday constant (96,480
R: General gas constant (8.314
T: Absolute temperature (K)
n: Ion charge
γ:Activity coefficient
C: Ion concentration (mol/L)