Chapter 7 Electrochemistry §7.8 Electrode potential

Porous partition diagram How does electrode potential establish? Zinc metal Copper metal ZnSO4 solution CuSO4 solution Zn2+ Cu2+ Porous partition diagram electromotive forces potential difference Daniell cell

7.8.1. Interfacial charge and electrode potential 1) Metal-metal interface: contact potential +  Cu Zn 2) Liquid-liquid interface: Liquid junction is the interface between two miscible electrolyte solutions. KCl solution HCl solution liquid junction potential, liquid potential, diffusion potential

exchange current, electrode potential 3) Liquid-metal: Cu2+ + 2e Cu +  exchange current, electrode potential

7.8.2. Models of electric double layer Holmholtz double layer (1853) 2) Gouy-Chappman layer (1910, 1913) 3) Stern double layer (1924) +  d E +  d E +  d E Compact double layer Diffuse double layer

Cu(s)Zn(s)ZnSO4(m1)CuSO4(m2)Cu (s) 7.8.3 Electromotive forces and relative electrode potential Cu(s)Zn(s)ZnSO4(m1)CuSO4(m2)Cu (s) anode cathode E = c +  + j + + E = c + (l,1- ) + ( +- l,2) =  +-  + (c+  l,1-  l,2) When emf of a cell was measured, we , in fact, measured the potential difference between the two electrodes.

Absolute potential potentiometer arbitrary reference  Can the absolute potential of electrode be unmeasured? Only the difference between two electrodes, i.e., electromotive of the cell E = +  can be measured.

definition (2) Normal/Standard Hydrogen Electrode (NHE/SHE) In 1953, IUPAC defined normal hydrogen electrode (NHE) as the reference for measurement of electrode potential. IUPAC conventions pure hydrogen gas at standard pressure platinized platinum foil electrode acidic solution with activity of H+ equals to 1. definition  H+/H2 = 0.000000 V.

- NHE || unknown electrode + (3) standard electrode potential The potential of other electrode can be obtained by combination of NHE and any other unknown electrode into an electrochemical cell with NHE serving as negative electrode and the unknown electrode as positive electrode: - NHE || unknown electrode + The sign and the value of the emf of the cell is thus the sign and value of the potential of the unknown electrode. All standard electrode potentials are reduction potentials. Cf. Levine, p. 431-435

Example E = 0.342 V NHE  Cu2+ (a=0.1)Cu the electrode potential of the Cu|Cu2+ (a=0.1) electrode at pressure p and temperature T is thus Because the unknown electrode is always arranged as positive electrode, the electrode reaction is, therefore, written in reduction form. Cu2+ + 2e Cu (reduction)(standard) electrode potential

(4) Nernst equation for electrode SHE||Cu2+ (a=0.1)|Cu Cell reaction: H2(g, p) + Cu2+(a) = 2H+ (a=1) + Cu

7.8.4 Reference electrode Problems with NHE (primary standard): 1)The platinized platinum electrode is easily poisoned by adsorption of impurities from the solution and the gas. 2) An elaborate purification is required to purify the hydrogen before it is passed through the cell. 3) Changes in barometric pressure or in the depth of immersion of the electrode in the solution produce a small variation in the potential of the electrode. 4) The preparation and the maintenance of the unit activity solution are both much complicated.

Hg(l)Hg2Cl2(s)KCl (m) Some electrodes with stable potential usually used as the secondary standard, named as reference electrode Hg Hg2Cl2 past The calomel electrode: Hg(l)Hg2Cl2(s)KCl (m) (T)/V＝ 0.2412 - 6.61 10-4 (T/℃-25) - 1.7  10-6 (T/℃-25)2 - 9 10-10 (T/℃-25)3 saturated calomel electrode (SCE)

Other common reference electrodes 0.799 V 0.2412 V 0.000 V SCE NHE mercury-mercurous sulfate electrode: Hg(l)Hg2SO4(s)SO42(m) ⊖ ＝ +0.640 V Ag+/Ag mercury-mercuric oxide electrode: Hg(l)HgO(s)OH(m) ⊖ ＝ +0.098 V silver-silver chloride electrode: Ag(s)AgCl(s)Cl(m) ⊖ ＝ 0.197 V ⊖(Ag+/Ag) ＝ +0.799 V vs NHE ⊖(Ag+/Ag) ＝ __?___ V vs SCE

7.8.5. liquid junction potential and salt bridge The diffusion of ions is irreversible, which destroys the reversibility of the cell. The value of Ej can reach ca. 30 mV, which is too large for the measurement of emf.

For uni-univalence electrolytes 2) influential factor of Ej Pt(s), H2(g,p)HCl(m)HCl(m′)H2(g, p), Pt(s) Ej On passage of 1 mole of electrons through the cell, t+ mol H+ and t mol Cl pass the boundary For uni-univalence electrolytes

3) Effects of salt bridge Every measurement of emf of a cell whose two electrodes require different electrolyte raises the problem of the liquid junction potential. The problem can be solved either by measuring the junction potential or eliminating it. The salt bridge is often used to connect the two electrode compartments to reduce the junction potential.

4) Salt bridge electrolyte does not react with either solution transference number of cation and anion is close of high concentration. ions K+ NH4+ Cl NO3 102 /S·m2·mol-1 0.7352 0.734 0.7634 0.7144 t+ of some common salt bridge electrolytes c/mol·dm-3 0.01 0.05 0.10 0.20 KCl 0.490 0.489 NH4Cl 0.491 KNO3 0.508 0.509 0.510 0.512

Concentration-dependence of Ej c/mol·dm-3 0.2 1.0 2.5 3.5 Ej 28.2 19.95 8.4 3.4 1.1 + - Why does salt bridge reduce the junction potential.

5) Effects of salt bridge: 6) Elimination of junction potential