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?

André Marie Ampère (1775-1863)

8A Electrolytical Analysis

Pt Pt

+ -

Cu2+

H+

SO42-

OH-

R

A

Figure 8A-1 Apparatus for electrolysis Analysis

IUPAC

Anode: Oxidation reactions

Cathode: Cu2++2e- Cu

Anode: 2H2O 4H++O2+4e-

Cathode: Reduction reaction

So, in CuSO4 solution

Electrolytical cell

Battery

Positive

Negative

Ecell=E 正 -E 负

= 0.31-1.22 = 0.91V

实际分解电压 Eapplied

Overpotential:

理论分解电压 Ecell

Eapplied= E+- E- + iR =(E+ ++) - (E---)+iR = (E+ - E-) + (++-)+ iR = Ecell+ + iR

Ohmic Potential; IR Drop Polarization Effects

8A-2

Ag Cd

0.00mR

- 0.734 V

+

-

-

[Cd2+] = 0.00500 M

[Cl-] = 0.200 M

2.00mR

- 0.764V

+

-

-

Ag Cd

R=15.0Ω

I

Fig. 8-3 An electrolytic cell for determination Cd2+

Ecell = Eright – Eleft = - 0.734V

Eapplied = Ecell – IR = - 0.764V

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: EA c: EC

To separate A and B ,

E = Eb

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 CoulometryCoulometric Titration

Determine the charge Q

Q =0

tidt nA =

Q

nF

nA: the number of moles of the analyten: the number of moles of electrons in the analyte half-reaction

F: Faraday constant, 96487 C/mol

WA = Q

nF Mr

Faraday Laws

8C-1 Controlled-Potential Coulometry

%100%100

impbs

s

T

se iii

i

i

i

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

z

VMm

964851741.0

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.

itnF

MW

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 Titrationsmall E 外

reversible system current the indicator circuitIrreversible system no current

Na2S2O3 titrate I2

i

V(Na2S2O3)

End Point Titration

Cathode I2= 2I- + 2e

Anodic 2I- + 2e= I2

滴定管

Irreversible system Reversible system

双铂电极指示系统 I2

Irreversible system titrate reversible system

Reversible system titrate reversible systemReversible titrate irreversible system

Character and Application

High accuracy High sensitivity 10-5~10-9g/mL

In situ produce unstable regents

No standard solutions

Application

8D Other Coulometric Methods

Microcoulometric analysis

Automated coulometric titration

SensitiveFast speedConvenient

Determination of COD