1 / 26

Chapter 8

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?. Andr é Marie Ampère (1775-1863). R. -. +. A. Pt.

elda
Download Presentation

Chapter 8

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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. R - + A Pt Anode: 2H2O 4H++O2+4e- OH- H+ Pt SO42- Cu2+ Cathode: Cu2++2e- Cu 8A Electrolytical Analysis IUPAC Anode: Oxidation reactions Cathode: Reduction reaction So, in CuSO4 solution Electrolytical cell Battery Positive Negative Figure 8A-1 Apparatus for electrolysis Analysis

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

  6. + - - - 0.734 V + - 0.00mR - - 0.764V 2.00mR Ag Cd R=15.0Ω Cd Ag An example for overpotential I [Cd2+] = 0.00500 M Ecell = Eright – Eleft = - 0.734V [Cl-] = 0.200 M Eapplied = Ecell – IR = - 0.764V Fig. 8-3 An electrolytic cell for determination Cd2+

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

  8. 8B Electrogravimetric Methods8B-1Controlled-Current Electrolytical Analysis WE: large-surface-area platinum gauze CE: plane Pt Constant current A two-electrode eletrolytical cell Low selectivity Figure 8B-1 Apparatus for electrodeposition of metals without cathode-potential control.

  9. 8B-2 Controlled-Potential Electrolytical Analysis 控制阴极电位 Instrument 自动控制阴极电位电解分析实验装置示意图 自动调节E外 three-electrode system constant negative potential

  10. Figure 8B-2 Curve i-E of separation ion A and ion B. 8B-3 Choice of Negative Potential a: EA c: EC To separate A and B , E = Eb

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

  12. three-electrode system

  13. 8C Coulometry Determine the charge Q Controlled-PotentialCoulometry Coulometric Titration

  14. nA = WA= Mr Q = 8C-1 Controlled-Potential Coulometry Faraday Laws nA: 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

  15. 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%。

  16. Determination of charge 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。 氢氧库仑计示意图

  17. 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

  18. 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.

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

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

  21. End Point Titration Na2S2O3 titrate I2 Irreversible system Reversible system i V(Na2S2O3) 滴定管 Cathode I2= 2I- + 2e Anodic 2I- + 2e= I2 I2 双铂电极指示系统

  22. Reversible titrate irreversible system Reversible system titrate reversible system Irreversible system titrate reversible system

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

  24. Application

  25. 8D Other Coulometric Methods Automated coulometric titration Determination of COD Microcoulometric analysis Sensitive Fast speed Convenient

More Related