1 Measuring electrode potentials

Measuring the electrode potential of a cell • It is not possible to measure the absolute potential of a half electrode on its own. It is only possible to measure the potential difference between two electrodes. • To measure it, it has to be connected to another half-cell of known potential, and the potential difference between the two half-cells measured. • by convention we can assign a relative potential to each electrode by linking it to a reference electrode (hydrogen electrode), which is given a potential of zero Volts. The Standard Hydrogen Electrode The potential of all electrodes are measured by comparing their potential to that of the standard hydrogen electrode. The standard hydrogen electrode (SHE) is assigned the potential of 0 volts. The hydrogen electrode equilibrium is: H2 (g) 2H+ (aq) + 2eBecause the equilibrium does not include a conducting metal surface a platinum wire is used which is coated in finely divided platinum. (The platinum black is used because it is porous and can absorb the hydrogen gas.) In a cell diagram the hydrogen electrode is represented by: Pt |H2 (g) | H+ (aq) Components of a standard hydrogen electrode. To make the electrode a standard reference electrode some conditions apply: 1. Hydrogen gas at pressure of 100kPa 2. Solution containing the hydrogen ion at 1 M (solution is usually 1M HCl) 3. Temperature at 298K 4. Platinum Electrode Secondary standards The Standard Hydrogen Electrode is difficult to use, so often a different standard is used which is easier to use. These other standards are themselves calibrated against the SHE. This is known as using a secondary standard – i.e. a standard electrode that has been calibrated against the primary standard. The common ones are: silver / silver chloride E = +0.22 V calomel electrode E = +0.27 V Standard conditions are needed because the position of the redox equilibrium will change with conditions. For example, in the equilibrium: Mn+ (aq) + n e- M(s) An increase in the concentration of Mn+ would move the equilibrium to the right, so making the potential more positive. Standard Electrode Potentials The standard conditions are : •All ion solutions at 1M •temperature 298K •gases at 100kPa pressure •No current flowing When an electrode system is connected to the hydrogen electrode system, and standard conditions apply the potential difference measured is called the standard electrode potential, E Standard electrode potentials are found in data books and are quoted as Li+ (aq) | Li (s) E= -3.03V more oxidised form on left They may also be quoted as half equations Li+ (aq) + e- Li (s) E= -3.03V but again the more oxidised form is on the left Pt electrode Pt electrode 1M HCl 1M FeSO4 and 0.5 M Fe2 (SO4 )3 Salt bridge KNO3 (aq) Pt|H2 |H+ ||Fe3+,Fe2+|Pt H2 gas at 100kPa Solution containing metal ions (e.g. Fe2+) at 1 mol dm-3 concentration Metal electrode e.g. Fe H2 gas at 100kPa 1M HCl Pt electrode Salt bridge KNO3 (aq) N Goalby chemrevise.org Note: in the electrode system containing two solutions it is necessary to use a platinum electrode and both ion solutions must be of a 1M concentration so [Fe2+] = 1M and [Fe3+ ] = 1M .