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 22+Hg the half cell reaction is

-- +®+ ClHgeClHg 22222 vE 268.0=o

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:

]log[2

0592.0 --= ClEEcalomelo

The electrode potentials of calomel electrode

Electrode Potential (volt) Hg/Hg2Cl2/KCl(1.0 N) 0.289 Hg/Hg2Cl2/KCl(0.1 N) 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 :

-- +®+ ClAgeAgCl

The Nernst equation gives the electrode potential

]log[0592.0222.0/--= ClE AgClAg

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:

pHE AgClAg 0592.0222.0/ +=

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) Electrode potential (volt) 0.1 0.28

0.01 0.34 0.001 0.40

4-copper-copper sulfate electrode

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.

vE CuSOCu 316.04/ =o

The reaction of Cu-CuSO4 half cell is :

CueCu =+ -+ 22

The electrode potential :

]log[2

0592.034.0 2/ 4

++-= CuE CuSOCu