文档介绍：Chapter 6 Coulometry
Instrumental analysis
Outline
Faradaic electrolysis law & summary of Coulometry
Principle and equipment of controlled potential Coulometry
Principle and equipment setup of constant current Coulometric titration
Advantages and applications of Coulometric titration
Equipment of auto-Coulometric titration
§6-1 Faraday’s law of electrolysis
During an electrolysis, the absolute amount of analyte is proportional to the total charge Q in Coulombs
m = =
MQ
96487n
M
n
i t
96487
•
Where m is mass of analyte in or of deposited substance on the electrode during electrolysis, n is the number of electrons transferred per mole of analyte, F is Faraday’s constant(96487C. mol-1), M is molar mass. A Coulomb is also equivalent to an A• s, thus for a constant current, i , the charge is given as
Q = i t
Where t is the electrolysis time, if current varies with time, then the total charge is given by
Q =
∫
i dt
0
t
∫i dt
i
t
法拉第电解定律有两层含义:
●电极上发生反应的物质的量与通过体系的电量成正比
●通过相同量的电量时,电极上沉积的各物质的质量与其M/n成正比。
电解分析包括:
●电重量分析
●库仑分析
库仑分析要求:
●工作电极上只发生单纯的电极反应
●current efficiency requires 100%
§6-2 Controlled potential electrolysis
不同金属离子具有不同的分解电压,在电解分析中,金属离子大部分在阴极析出,要达到分离目的,就需要控制阴极电位。:
U分=(Ea + wa)-(Ec + wc)+ iR
Where w is overpotential
Example
-1 Ag+ and -1Cu2+, sulfate medium, [H+] =-1
EoAg+/Ag = +, EoCu2+/Cu = +
On cathode: Ag+ + e- → Ag
On anode: 2H2O - 4e- → O2 + 4H+
EoO2/H2O=+
When Ag starts to deposit, cathodal potential is:
EAg+/Ag = + =
∴ U分=(+)-=
Overpotential of anode
Overpotential of metal electrode is neglectable
Because R is very small, iR is neglectable
In fact, as the electrode reaction progresses, applied voltage will be changeable, it can be seen from following result
When [Ag+] reduces to 10--1, cathodal potential is
EAg+/Ag = +(1×10-7) =
U分= + – =
此时,Cu2+尚未开始析出(Why?)。可见通过控制外加电位,可