库伦分析法-英文版-coulometry.ppt

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 * Chapter 6 Coulometry Instrumental analysis Prof. Yang Wu §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 EoAg+/Ag = +0.800V, EoCu2+/Cu = +0.345V On cathode: Ag+ + e- → Ag On anode: 2H2O - 4e- → O2 + 4H+ EoO2/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