Yokogawa PH72 Personal pH/ORP Meter - Manual (Page 77)

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
؉ e Ag ؉ Cl - - , If
␣ represents the activity, from
equation 9.6
(Activity of solid
␣؍1, then ␣ ؍1, ␣ ؍1)
(
␣؍m␥ m: molality, ␥: activity coefficient)
E
؍ E ؉
ln
R T
F
␣
␣ ␣
؇
'
AgCl
AgCl
AgCl Ag
Ag Cl-
؍ E ؊
ln
␣
R T
F
؇
Cl-
؍ E ؊
ln m
␥
R T
F
؇
Cl- Cl-
E'
AgCl
can be obtained using the average ion activity coefficient,
␥
؎KCl
, of the KCl (m)
solution instead of the activity coefficient of Cl – (actual measurement is impossible).
E
؍ E ؊
ln m
␥
R T
F
؇
'
AgCl KCl ؎KCl
(9.7)
The relationship of the molality m, molarity c, activity coefficient at molality
␥
؎
, and
activity coefficient at molarity y
؎
, 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 ]
␥ : activity coefficient at molality
y
: activity coefficient at molarity
m
؍
c
d
؊0.001 c W
(9.8)
y
؍ ␥
m d
c
(9.9)
؎
؎
؎
؎
0
0
From equations 9.7 and 9.9,
E
؍ E ؊
R T
F
؇
'
AgCl
(9.10)
ln
c y
d
؎
0