chapter 8
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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