山东大学：《物理化学》课程教学资源（讲义资料）7.8 Electrode potential-for students

87.8 Electrode potential Out-class extensive reading IraN. Levine, pp 426 Section 14.7 standard electrode potentials Section 14.8 concentration cells Section 14.9 liquid-junction potential

§7.8 Electrode potential Out-class extensive reading: Ira N. Levine, pp. 426 Section 14.7 standard electrode potentials Section 14.8 concentration cells Section 14.9 liquid-junction potential

87.8 Electrode potential 7.8.5. liquid junction potential and salt bridge (I)liquid junction potential KCI HCI KCI H The diffusion of ions is irreversible, which The value of E; can reach ca 30 mV, which destroys the reversibility of the cell is too large for the measurement of emf

7.8.5. liquid junction potential and salt bridge (1) liquid junction potential 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. §7.8 Electrode potential

87.8 Electrode potential 7.8.5. liquid junction potential and salt bridge (3)Effects of salt bridge salt bridge Fe electrode Cu electrode Every measurement of emf of a cell whose two electrodes require different electrolyte raises the problem of the liquid junction potential Fe(NO3)2 Cu(NO3)2 solution solution The problem can be solved either by measur ng the junction potential or eliminating it. The salt bridge is often used to connect the two electrode compartments to reduce the junction potential

(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. 7.8.5. liquid junction potential and salt bridge §7.8 Electrode potential

87.8 Electrode potential 7.8.5. liquid junction potential and salt bridge (2) influential factor ofE Pt(s), H2(g p)HCI(m)HCI(mH2(g,P), Pt(s) On passage of I mole of electrons 4G,=t.RTIn-H+t rT in-cr=-nFE through the cell, t, mol H* and t mol pass the boundary RT RT For uni-univalence electrolytes E (+-t)n}(2t4-1)ln F m How can we reduce liquid junction potential?

(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 + - + - H Cl H Cl ' ln ln ' j j a a G t RT t RT nFE a a  = + = − + − For uni-univalence electrolytes ' (2 1) ln ' ( ) ln m m F RT t m m F RT E t t j = + − − = + − 7.8.5. liquid junction potential and salt bridge How can we reduce liquid junction potential? §7.8 Electrode potential

87.8 Electrode potential 7.8.5. liquid junction potential and salt bridge Ions K NH4 Cl- NO 102/Sm2moh10.73520734 0.7634 0.7144 t, of some common salt bridge electrolytes c/mol. dm 0.01 0.05 0.10 0.20 KCI 0.490 0.490 0.490 0.489 NHACI 0.491 0.491 0.491 0.491 KNO3 0.508 0.509 0.510 0.512

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 0.490 0.490 0.489 NH4Cl 0.491 0.491 0.491 0.491 KNO3 0.508 0.509 0.510 0.512 m  7.8.5. liquid junction potential and salt bridge §7.8 Electrode potential

87.8 Electrode potential 7.8.5. liquid junction potential and salt bridge Concentration-dependence of E c/mol dm 0 0.2 1.0 3.5 E / mv 28219.95 8.4 3.4 OOOO Why does salt bridge reduce the junction potential

c/mol·dm-3 0 0.2 1.0 2.5 3.5 Ej /mV 28.2 19.95 8.4 3.4 1.1 Concentration-dependence of Ej Why does salt bridge reduce the junction potential. + + + + + - - - - - 7.8.5. liquid junction potential and salt bridge §7.8 Electrode potential

87.8 Electrode potential (4) Requirements of salt bridge Discussion section electrolyte (1)What are the effects of salt bridge in 1)does not react with either solution constructing reversible cell? 2)transference number of cation and (2) How can we eliminate junction potential anion is close of a cell with two electrolyte? f of high concentration (3)How can we measure the junction potential between two electrolytes (4)How can we measure the membrane potential separating two different electrolyte?

(1) What are the effects of salt bridge in constructing reversible cell? (2) How can we eliminate junction potential of a cell with two electrolyte? Discussion section (3) How can we measure the junction potential between two electrolytes? (4) How can we measure the membrane potential separating two different electrolyte? (4) Requirements of salt bridge electrolyte 1) does not react with either solution 2) transference number of cation and anion is close 3) of high concentration. §7.8 Electrode potential