西北师范大学：《仪器分析》课程教学资源（PPT课件）Chapter 06 库仑滴定法 Coulometry.ppt_大学文库

Chapter 6 Coulometry Instrumental analysis Prof. Yang Wu

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。阴极电位的控制可由控制外加电压而实现．例如： U分 =（Ea + wa）-（Ec + wc）+ iR Where w is overpotential

Example 0.01mol.L-1 Ag+ and 1mol.L-1Cu2+, sulfate medium, [H+ ] =1mol.L-1 Eo Ag+/Ag = +0.800V, Eo Cu2+/Cu = +0.345V On cathode: Ag+ + e- → Ag On anode: 2H2O - 4e- → O2 + 4H+ Eo O2/H2O=+1.23V When Ag starts to deposit, cathodal potential is: EAg+/Ag = 0.800 + 0.059lg0.01= 0.682V ∴ U分=(1.23+0.47)-0.682=1.02V 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 10-7mol.L-1 , cathodal potential is EAg+/Ag = 0.800+0.059lg(1×10-7 ) = 0.386V U分= 1.23+0.47 – 0.386=1.31V 此时，Cu2+尚未开始析出（Why？）。可见通过控制外加电位，可以实现Ag+, Cu2+分离控制电位电解法

实际分析中，工作电极（通常为阴极）的电位是随着电解过程的进行而不断变化的，而且阳极电位及电解电流也是可变的。因此，凭借控制外加电压来控制阴极电位，从而进行分离存在一定的困难。通常通过下列装置来控制阴极电位。 Potentiometer Electromagnetic stirrer Reference electrode

§6-3 Controlled Potential Coulometry Coulometer Potentiometer Apparatus setup of controlled potential coulometry is the same as controlled potential electrolysis. Because coulombic analysis is carried out by determining the coulomb passing an electrolytic cell during electrolyzing, an series-wound (串联） accurate coulometer is necessary

Coulometer (库仑计） ■Silver Coulometer (weight Coulometer) Netty Pt cathode Pt wire anode AgNO3 solution On cathode: Ag+ + e- → Ag On anode: 2H2O - 4e-→O2 + 4H+ 库仑计本身也是一种电解电池，可以应用不同的电解反应来构成