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1、3 Coulometry (库仑分析法),3.1 Faradays Law,The relationship between the electrochemical reaction and the coulometry of electricity in the circuit can be expressed as: The mass of the subject reacting on the electrode is in direct proportion to the amount of electricity consumed in the system. Consuming s
2、ame amount of electricity, the mass of the subject reacting on the electrode is in direct proportion of its M/n,Discussion of the Law,Meaning of each item Application for analysis Electrogravimetry Coulometry Conditions for the Law 100% current efficiency (exhaustive electrolysis),3.2 Controlled pot
3、ential electrolysis (控制电位库仑分析法),Ue = （Ea + a）- （Ec + c） + iR Ohmic potential drop （iR） (电压降), overpotential（）(超电位),Electrolysis,Cu2+(1.0 mol/L) solution：,cathode： Cu2+ + 2e Cu Eo=+0.34 anode： 2H2O - 4e O2 + 4H+ Eo= -1.23 Net reaction: 2H2O+2Cu2+=2Cu+O2+4H+,Enet = 0.34- 1.23 = -0.92 (V) Ee=-Enet +iR+
4、=1.23-0.34+0.47=0.92+0.47=1.39(V) a= 0.47,Another sample：0.01mol/L Ag+, 1.00mol/L Cu2+ being electrolyzed Standard potential： cathode anode,If Ag+=10-7mol/L，then,The electrolysis potential is depended on the standard potential and the concentration of the analyte reacting on the electrode Control th
5、e potential of the electrode for getting the selected electrolysis And the controlling of potential can separate selected component in the mixture sample.,Controlling method,Why and How The relationship between the potential and the electrolysis Changing potential of the electrodes Adding the “refer
6、ence electrode”, making three electrodes system,Schematic diagram of controlled potential electrolysis,Measuring the quantity of the electricity-Coulometer,Silver coulometer (银库仑计) Titration coulometer (滴定库仑计) Gas coulometer (气体库仑计) Electronic integrator (电流积分库仑计),Titration Coulometer,Pt Cathode 2H2
7、O+2e-2OH-+H2 Ag Anode 2Ag+2Br-2e2AgBr,In the standard condition, the 1 Q of the electricity passing through the solution produce 0.1741 mL total volume of gas,Gas Coulometer,Anode：H2O-2e O2 +2H+,Cathode：2H+2eH2,3.3 Coulometric Titrations (controlled-current coulometry) （库仑滴定, 恒电流库仑分析）,3.3.1 Example
8、For the determination of Fe2+ ， Anode: Fe2+Fe3+ + e （ferrous to ferric ion ) As the reaction took place, Fe2+，the anode potential , then 2H2OO2+4H+4e If adding large number of Ce3+， Anode: Ce3+= Ce4+e Solution: Fe2+Ce4+= Fe3+Ce3+,3.3.2 Principle of Coulometry Titration,Adding mediator The mediator w
9、as electrolyzed to produce titrant, which reacting with the analyte according to stoichiometric(化学计量） relationship. When the analyte was consumed by proper indicating method, stop the electrolyzing and calculating the amount of analyte through the Faradays Law.,3.3.3 Indicating of the end-point,The
10、regent-indicator Potentiometry Others EA (electroanalysis) methods,3.3.4 Apparatus for Constant-Current Coulometry,Electrode: Working Auxiliary Indicators,3.3.5 Characters of Coulometric Titration,3.3.6 Application,卡尔.费休（Karl Fisher）法测定微量水,基本原理：利用I2 氧化SO2 时，水定量参与反应。 I2+SO2+2H20 = 2HI+H2SO4 卡尔.费休试剂 ：
11、由碘、吡啶、甲醇、二氧化硫、水按一定比例组成。 以上反应为平衡反应，需要破坏平衡：,电解产生碘。 1g水对应10.722毫库仑电量。,（1）吡啶：中和生成的HI （2）甲醇：防止副反应发生,A constant current of 0.800 A is used to deposit copper at the cathode and oxygen at the anode of an electrolytic cell. Calculate the number of grams of each product formed in 15.2 min, assuming no other redox reaction. The two half-reactions are,Cu2+ + 2e Cu(s) 2H2O - 4e +O2 (g)+ 4H+,
