Hydrogen-Reference Electrode
Hydroflex®
Dr. Hans-Joachim Kohnke
Gaskatel GmbH
Holländische Str. 195
Kassel / Germany
Busan ISE anual meeting 2005
H2 Reference Electrode
HydroFlex Alkaline Electrolyser
EloFlux
Gas Diffusion Electrode
BiPlex
Products
Content
• History
• GHE, RHE and SHE
• Construction of Hydroflex (GHE)
• Activation
• Pressure influence - Open questions
• Multifunctional use in laboratories
• Resumé
History
3)log(*
*303.2
2ln*
1
221
2
)(
221
formularppHF
TRE
formularpH
a
F
TRE
formulareHH
H
H
Nernst recommands in 1912 the hydrogen potential at a not affected metal
like Platnium as in international standard:
Reasons:
• Short reaction time
• Reproducable
• long time stability
SHE & RHE
RHE (reversable hydrogen electrode):
• no electrolyte bridge
• no drift of the potential
• no contamination by foreign ions
SHE (standard hydrogen electrode):
• inner electrolyte with a = 1
• electrolyte bridge to test cell
• temperature control
• pressure control
• no diffusion polarisation at salt bridge
V
WE CE RE
H2
Electrodes
Type Inner electrolyte T [°C] advantages Potential vs SHE [V]
Ag/AgCl KCl < 135 Non toxic 0.19 – 0.28
Hg/HgCl KCl < 60 Stable 0.24 – 0.33
Hg/HgSO4 K2SO4 n.k No Cl ions 0.61 – 0.65
Hg/HgO NaOH High pH 0.09 – 0.17
H2/H+
No particular
electrolyte
-40 to 300°C ......... 0 - 0.059*pH
Construction I
Hydrogen source
Contact plug 4mm
Contact wire
Platinum gas diffusion electrode
Length
120 m
m
Tube 8mm filled with hydrogen
Construction II
Hydrogen source:
• switchable (off, on ...)
• storable for > 3 years
• operation time 1 – 12 month
• replacable in minutes
• domestic waste
Reference electrode:
• temperatur range –30 to 300°C
• pressure atmospheric ( 0.1 bar)
• any Mounting position
Animation
00:00:00 00:00:26 00:00:53 00:01:20 00:01:55 00:02:30 00:02:57 00:03:20
Activation time
start up with 0.07ml/h in KCl
0
0.1
0.2
0.3
0.4
0.5
0 10 20 30 40 50 60 70
time [h]
Po
ten
tia
l [V
]
-0.12
-0.07
-0.02
0.03
0.08
0.13
pre
ss
ure
[b
ar]
Oxygen
consumption
Nitrogen
removing
Strong alkaline
Hydroflex in 1m KOH, hydrogen production rate
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
15 17 19 21 23 25 27 29
time [h]
Po
ten
tial [m
V]
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
pre
ssu
re [
bar]
0.034 ml/h 0.068 ml/h0.41 ml/h
Weak alkaline
Hydroflex in 0.1 m KOH
0
2
4
6
8
10
12
14
16
18
80 82 84 86 88 90 92 94
Time [h]
Po
ten
tia
l [m
V]
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
pre
ss
ure
[b
ar]
bubble 0.1 mm
no bubble
bubble 2 mm
Pressure
Pressure observations:
• High pressure corresponds to bubble point
• Low pressure corresponds to filling pressure
• Pressure increase depends on hydrogen production rate
Potential
Potential observations: According to the Nernst law the pressure variation will cause a potential
variation. Without changing the activity, pressure variation of about 50 mbar
corresponds to:
0.37 mV
These are the variation of the potential in 1m KOH.
At the thermodynamic equilibrium the chemical potential of hydrogen in gas
and in the chemosorbed phase are identical. Due to formular 2:
The activity changes due to 50 mbar pressure change corresponds to 0,02. In
solutions with a low activity – like 0.1 m KOH – this would change to a
potential change of:
7.2 mV.
2lnln 2
1Hpa
Exposition to air
Hydroflex, exposition to air
-0.050
0.000
0.050
0.100
0.150
0.200
0.250
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
time [h]
Po
ten
tia
l [V
]
hydrophilic hydrophobic
Electrolyte change
Electrolyte change, from HCl to KOH
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.0 5.0 10.0 15.0 20.0
Time [h]
Po
ten
tia
l [V
]
hydrophilic hydrophobic
Resumé
Today
• Small RHE available
• High temperature range
• No inner electrolyte, no diaphragma
• Exchangable hydrogen source
• Usable in alkaline and acid electrolytes
Future
• Hydrogen level indicator
• Temperature measurement
• Whole pH-range
• pH electrode based on Hydroflex