Chapter 8 Bulk Electrolysis: Electrogravimetry and Coulometry

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

What are electrolytical analysis and coulometry? André Marie Ampère ( )

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

E cell =E 正 -E 负 = = 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

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

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

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

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

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

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

three-electrode system

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

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, C/mol WA =WA = MrMr Faraday Laws 8C-1 Controlled-Potential Coulometry

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% 。

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。

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

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.

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

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

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

Irreversible system titrate reversible system Reversible system titrate reversible systemReversible titrate irreversible system

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

Application

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