页数：20页文件大小：2.05 MB来源： 仪器分析ppt课件
本课件以英文讲解库仑分析法，涵盖电解与库仑分析的关系、控制电位与控制电流电解的差别，以及库仑滴定、微库仑分析与滴定终点的指示方法。
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 in Coulombsm = =MQ96487nMnit96487•Where is mass of analyte in or of deposited substance on the electrode during electrolysis, is the number ofelectrons transfer red per mole of analyte, is Faraday’sconstant(96487C. mol-1), is molar mass. Coulomb isalso equivalent to an A•s, thus for constant current,i, thecharge is given as =
Where is the electrolysis time, if current varies with time,then the total charge is given by =∫i dt0t∫i dtit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
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）+ iRWherewis over potential
Example0.01mol.L-1Ag+and 1mol.L-1Cu2+,sulfate medium, [H+] =1mol.L-1EoAg+/Ag= +0.800V, EoCu2+/Cu= +0.345VOn cathode:Ag++ e-→ AgOn anode:2H2O-4e-→ O2+ 4H+EoO2/H2O=+1.23VWhen Ag starts to deposit, cathodal potential is:EAg+/Ag= 0.800 + 0.059lg0.01= 0.682V∴U分=(1.23+0.47)-0.682=1.02VOverpotentialof anode Over potential of metalelectrode is neglectable Because is very small, Ris 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) = 0.386VU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。Potentio-meter Electro magnetic stirrer Reference electrode
§6-3 Controlled Potential Coulometry Coulometer Potentiometer Apparatus setup of controlled potential coulometry is the same as controlled potential electrolysis. Becausecoulombic analysis is carried out by determining the coulomb passing an electrolytic cell during electrolyzing,anseries-wound(串联）accurate coulometer is necessary.
Coulometer (库仑计）
Silver Coulometer (weight Coulometer)Netty Pt cathode Pt wire anode AgNO3solution On cathode:Ag++ e-→ AgOn 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。
Titration Coulometer
Gas Coulometer (hydrogen-oxygen Coulometer)121.Thermostat, 2. electrodes Electrolysis solution: 0.5mol.L-1K2SO4or Na2SO4Cathode:2H2O +2e-→2OH-+H2Anode:4OH-+-e-→O2+ 2H2O0.1742m of mixed gas of H2andO2is obtained per Coulomb ofcharge at standard state The mass of deposited substance is given bym = / (0.1742×96487n)= / (16779n)
If using 0.1mol.L-1(N2H5)2SO4to substitute K2SO4, then we mayget hydrogen-nitrogen Coulometer
Current integral Coulometer (electronic Coulometer)∫i dtit Problems During controlled potential Coulometric operation, sometimes, you must bubble inert gas (eg. N2) or even keep gentle N2flow over electrolysis system, or even carry out pre-electroysis at more negative potential than the decomposition potential of analyte, please give explanation for its necessity.
§6-4 Constant current Coulometric titration恒电流库仑滴定简称库仑滴定，是建立在控制电流电解基础上的一种分析方法。它可按以下两种类型进行：
被测定物质直接在电极上起反应。
在试液中加入大量某种物质，使此物质经电解反应后产生一种试剂，然后被测定物质与所产生的试剂起反应。事实上，单纯按照第一种类型进行分析的情况很少，一般都是按照第二种类型进行的，也有按两种类型混合的分析方法。按第二种方法进行，不但可以测定在电极上不能起反应的物质，而且还易于使电流效率达到100％。
由于C3+离子是过量存在的,因而就稳定了阳极电位并防止了氧的析出.从反应可知,阳极上虽然发生了铈离子的氧化反应,但所产生的高价铈离子同时又将Fe2+离子氧化为Fe3+离子,因此,电解时所消耗的总电量与单纯Fe2+离子完全氧化为Fe3+离子的电量是相当的.由此典型例子可以看出应用第二种类型的优越性.它不仅可以稳定工作电极电位而避免副反应发生,而且由于用于电解产生试剂的物质可以大量存在,使本法可以在较高的电流密度下进行电解(可以高达20mA/cm2),进而提高测定速度.
电解完成后,根据通过的电流和电解时间应用下式计算出被测物质的质量(g):
Constant current source45～90V乙型干电池串联可变高电阻晶体管恒电流源
电流强度测量用电位计测定流经与电解池串联的标准电阻R上的电压降iR而得。
时间测量可用计时器(如电子计数式频率计)或停表测量。
§６－６Advantages and Applications of Coulometric titration methods§6-7 Automatic Coulometric assay Self-study Problems
What advantages do Coulometric titration have ?
Please state the principles of mass fraction determination of carbon in steel sample and determination of H2S content inatmosphere using automatic Coulometric assay
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