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