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 22 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