1. Chapter 8 Bulk Electrolysis: Electrogravimetry and Coulometry

2.  8A Electrolytical Analysis  8B Electrogravimetric Methods  8C Coulometry  8D Other Coulometric Methods

3. What are electrolytical analysis and coulometry? André Marie Ampère (1775-1863)

4. 8A Electrolytical Analysis Pt + - Cu 2+ H+H+ SO 4 2- OH - R A Figure 8A-1 Apparatus for electrolysis Analysis IUPAC Anode: Oxidation reactions Cathode: Cu 2+ +2e - Cu Anode: 2H 2 O 4H + +O 2 +4e - Cathode: Reduction reaction So, in CuSO 4 solution Electrolytical cell Battery Positive Negative

5. E cell =E 正 -E 负 = 0.31-1.22 = 0.91V 实际分解电压 E applied Overpotential:  理论分解电压 E cell E applied = E + - E - + iR =(E + +  + ) - (E - -  - )+iR = (E + - E-) + (  + +  - )+ iR = E cell +  + iR Ohmic Potential; IR Drop Polarization Effects 8A-2

6. AgCd 0.00mR - 0.734 V + - - [Cd 2+ ] = 0.00500 M [Cl-] = 0.200 M 2.00mR - 0.764V + - - AgCd R=15.0Ω I Fig. 8-3 An electrolytic cell for determination Cd 2+ E cell = E right – E left = - 0.734V E applied = E cell – IR = - 0.764V An example for overpotential

7. The metal is deposited on a weighed platinum or other metal cathode, and the increase in mass is determined. Controlled-current electrolytical analysis Controlled-potential electrolytical analysis

8. 8B Electrogravimetric Methods 8B-1Controlled-Current Electrolytical Analysis Figure 8B-1 Apparatus for electrodeposition of metals without cathode-potential control. A two-electrode eletrolytical cell WE: large-surface-area platinum gauze CE: plane Pt Low selectivity Constant current

10. 自动控制阴极电位电解分析实验装置示意图 three-electrode system 自动调节 E 外 constant negative potential 控制阴极电位 Instrument 8B-2 Controlled-Potential Electrolytical Analysis

11. Figure 8B-2 Curve i-E of separation ion A and ion B. 8B-3 Choice of Negative Potential a: E A c: E C To separate A and B, E = E b

12. For example: Seperation of Cu and Bi, Sb, Pb, Sn, Ni, Cd, Zn Seperation of Pb and Cd, Zn, Ni, Zn, Mn, Al, Fe Good selectivity Low speed Character and application

13. three-electrode system

14. 8C Coulometry Controlled-Potential Coulometry Coulometric Titration Determine the charge Q

15. Q = n A = n A : the number of moles of the analyte n: the number of moles of electrons in the analyte half-reaction F: Faraday constant, 96487 C/mol WA =WA = MrMr Faraday Laws 8C-1 Controlled-Potential Coulometry

16. Current efficiency 100 % Current efficiency 使用纯度比较高的试剂和溶剂，通氮气除氧，设法避免电极副反应的发 生，可以保证电流效率达到或接近 100% 。

17. Determination of charge 氢氧库仑计示意图 库仑计：在电路中串联一 个用于测量电解中所消耗 电量的库仑计。常用的库 仑计有化学库仑计，电子 积分仪等。

18. Advantage: accurate, sensitivity, good selectivity Disadvantage: difficult to ensure 100% current efficiency need long time Application: determine mixtures study the electrode process, and the mechanism of various reactions

19. 8C-2 Coulometric Titration Coulometric titrations are carried out with a constant-current source, which senses decrease in current in a cell and responds by increasing the potential applied to the cell until the current is restored to its original level.

20. Figure 8C-1 Conceptual diagram of a coulometric titration apparatus. Instrument Constant current power Electrolysis reaction system electrolytic cells WE CE Timer Clock

21. Double Pt Electrode – End Point Titration small E 外 reversible system current the indicator circuit Irreversible system no current

22. Na 2 S 2 O 3 titrate I 2 i V(Na 2 S 2 O 3 ) End Point Titration Cathode I 2 = 2I - + 2e Anodic 2I - + 2e= I 2 滴定管 Irreversible system Reversible system 双铂电极指示系统 I2I2

23. Irreversible system titrate reversible system Reversible system titrate reversible systemReversible titrate irreversible system

24. Character and Application High accuracy High sensitivity 10 -5 ~10 -9 g/mL In situ produce unstable regents No standard solutions

25. Application

26. 8D Other Coulometric Methods Microcoulometric analysis Automated coulometric titration Sensitive Fast speed Convenient Determination of COD