Reference Electrodes
1-Standard Hydrogen Electrode SHE
The schematic construction of hydrogen reference electrode is in figure (3) .A
platinum foil specimen is suspended in a solution contain unit activity H+ which is
bubbled with purified hydrogen to removed dissolved oxygen and establish the
standard state for H2 at 1 atm pressure.
Figure (3) standard hydrogen electrode
The hydrogen electrode is connected to another half-cell through a solution bridge
which contains a porous glass barrier to permit charge transfer and potential
measurement but not mass transfer of the acid solution in the electrode.
2-Standard Calomel Electrode
The calomel reference electrode has long been a standard reference electrode used in lab. it
consists of mercury in equilibrium with Hg 22 the half cell reaction is
Hg 2 Cl2  2e   2Hg  2Cl 
E   0.268v
Figure (4) show the calomel electrode apparatus
A combination of Hg2Cl2 , mercury and a solution of chloride ions provides very
stable and reproducible potential and for convenience of use in laboratories , the
chloride solution is a saturated solution of potassium chloride KCl , in this case the
standard calomel electrode is called saturated calomel electrode ( SCE). Electrode
potential = 0.24 volt at 298 ○K
The mercurous chloride and mercury are at unit activity . Therefore , the electrode
potential can be written as:
E calomel  E  
0.0592
log[ Cl  ]
2
The electrode potentials of calomel electrode
Electrode
Hg/Hg2Cl2/KCl(1.0 N)
Hg/Hg2Cl2/KCl(0.1 N)
Potential (volt)
0.289
0.3356
3- Silver-Silver Chloride electrode
This electrode is composed of a silver wire coated with silver Chloride and immersed
in a solution of chloride ions, the overall electrode l reaction is :
AgCl  e   Ag  Cl 
The Nernst equation gives the electrode potential
E Ag / AgCl  0.222  0.0592 log[ Cl  ]
It is clear that the potential is dependent on Cl- same as for
calomel electrode , When at low concentration – log (Cl-)
in above equation can be replaced by pH:
E Ag / AgCl  0.222  0.0592 pH
Other two reaction involve a dynamic equilibrium of
deposition and dissolution of silver together with
solubility equilibrium between silver chloride and its ions
The following are the values of EAg/AgCl for different HCl
concentration
Concentration (M)
0.1
0.01
0.34
0.001
0.40
4-copper-copper sulfate electrode
Electrode potential (volt)
0.28
This is a reference electrode which is easy robust and stable , it is used mainly in
cathodic protection measurements such as the measurement of pipe-to-soil potential.
It has lower accuracy than other electrodes .It consists of copper metal placed in a
solution contain copper sulfate placed in non
conducting holder with a porous plug .the
copper sulfate crystals maintain the solution at
a fixed ion concentration and it is easily to
recharged when it becomes contaminated.

ECu
/ CuSO4  0.316v
The reaction of Cu-CuSO4 half cell is :
Cu 2  2e   Cu
The electrode potential :
ECu / CuSO4  0.34 
0.0592
log[ Cu  2 ]
2