YOKOGAWA PH72-21-E-AA, Руководство пользователя

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IM 12B03D02-01E
9-10
9. Technical Information
9.9 Reference Electrode
The oxidation-reduction potential in measurement is a value relative to the reference
electrode. If different types of reference electrodes are used, the oxidation-reduction
potentials of identical solutions are apparently different. In the electrochemical field, the
hydrogen electrode is generally used as a reference electrode. However, it has
complicated construction and is impractical. Therefore, in Yokogawa's ORP sensors, an
Ag/AgCl electrode filled with a 3.3 mol/l KCl solution is used as a reference electrode.
The relationship between the Ag/AgCl electrode and the standard hydrogen electrode can
be derived from the Nernst equation based on the following cell scheme.
Electrode (Pt) H
2
Electrolyte solution Liquid junction or salt bridge KCl(m) AgCl Ag
The reaction formula is
AgCl
1
e Ag
1
Cl
-
-
, If
a
represents the activity, from
equation 9.6
(Activity of solid
a5
1, then
a 5
1,
a 5
1)
(
a5
m
g
m: molality,
g
: activity coefficient)
E
5
E
1
ln
R T
F
a
a a
8
'
AgCl
AgCl
AgCl Ag
Ag Cl-
5
E
2
ln
a
R T
F
8
Cl-
5
E
2
ln m
g
R T
F
8
Cl- Cl-
E'
AgCl
can be obtained using the average ion activity coefficient,
g
6
KCl
, of the KCl (m)
solution instead of the activity coefficient of Cl
–
(actual measurement is impossible).
E
5
E
2
ln m
g
R T
F
8
'
AgCl KCl
6
KCl
(9.7)
The relationship of the molality m, molarity c, activity coefficient at molality
g
6
, and
activity coefficient at molarity y
6
, is as follows.
m : molality [mol/kg]
c
: molarity [mol/l] or [M]
d : density of solution [g/cm
3
]
W : molecular weight of solute
d
: density of solvent [g/cm
3
]
g
: activity coefficient at molality
y : activity coefficient at molarity
m
5
c
d
2
0.001 c W
(9.8)
y
5 g
m d
c
(9.9)
6
6
6
6
0
0
From equations 9.7 and 9.9,
E
5
E
2
R T
F
8
'
AgCl
(9.10)
ln
c y
d
6
0