14 (双语) 第四章 电位法.ppt

第1页：Potentiometric Methods

Potentiometric Methods These methods rely on the measurement of Ecellfor quantification.A number of common reference and ionselective electrodes are reviewed along withgeneral calculations and analyticalapproaches§4 Potentiometry

第2页：本章主要内容

本章主要内容•电位分析法基本原理•离子选择性电极与膜电位•离子选择性电极的选择性•离子选择性电极的种类与性能•测定离子活（浓）度的方法•电位滴定法•重点掌握电位分析法的基本原理，掌握溶液pH值的计算方法、运用工作曲线法和标准加入法进行离子活度的计算。

第3页：Classification

Classification of electroanalytical methods•Based on the quantitative relationship between some physical chemical properties of the chemical cell and concentration of the solute•Use themutationof the physical chemical properties as indicating method of titration reaction end-point, thatis, electrical volumetry•Transfer the analyte into solid state (metal or oxide),then weigh mass of these metal or oxide deposited toascertain amount of the analyte, that is electro-gravimetry.

第4页：Classification

Classification of electroanalytical methods

第5页：Principle of potentiometry

§4-2 Principle of potentiometry•电位分析法的实质是通过在零电流条件下测定两电极间的电位差(即所构成原电他的电动势)进行分析测定。包括电位测定法和电位滴定法。Theoretical basis of potentiometric analysis is Nernst equation:E =EOx/Red+lnRTn FaOxa Red For metal base electrode, Red= 1, hence,E =EMn+/M+lnRTn FaMn+

第6页：实际测量时

实际测量时，都是通过测定由indicating electrode和referenceelectrode组成的Cell的电动势（electromotive force）来完成的。Reference electrodes通常由以下几种电极来充当，它们的电位在一定条件下是恒定的，与待测离子浓度无关。甘汞电极Ag/AgCl电极氢电极pH玻璃电极Setup of potentiometric methods Frame of reference electrodes in common use(滞后作用，磁滞现象）)

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第16页：measurement of solution

§4-3 measurement of solution by potentiometry

第17页：Innersolution

Innersolution(constant)Outer solution(varied)Membrane bulk Membrance Inner surface Outer surface Inner solution Outer solution Ion-selective membrane structure

第18页：当玻璃电极被浸入被测溶液时

当玻璃电极被浸入被测溶液时，玻璃膜处于内部溶液(氢离子活度为a H+,内)和待测溶液(氢离子活度为a H+,试)之间，这时跨越玻璃膜产生一电位差DEM(称为膜电位)，它与氢离子活度之间的关系符合能斯特公式：DEM=lg2.303RT H+,试F H+,内2.303RT Flg H+.试= +2.303RT FpH试= K-

第19页：For following cell Ag│AgCl

For following cell Ag│AgCl,0.1mol.L-1HCl│glass membrane│ test solution║KCl (saturated) , Hg2Cl2│Hg = ESCE–EGlass= ESCE-(EAg Cl/Ag+DEM)+DEAS+DELDEAS称为不对称电位，是由于玻璃膜内外表面的情况不完全相同而产生的。其值与玻璃的组成，膜的厚度，及吹制条件和温度等有关液接电位(液体接界面电位)。是由于浓度或组成不同的两种电解质溶液接触时，正负离子扩散速度不同，破坏了界面附近原来溶液正负电荷分布的均匀性而产生的。也称为扩散电位。通常用盐桥连接两种电解质溶液来减小或消除

第20页：Denote K’ = ESCE–EAg

Denote K’ = ESCE–EAg Cl/Ag+DEas+DEL–KE = ESCE–EGlass= ESCE-EAg Cl/Ag+DEAS+DEL2.303RT FpH试-K +2.303RT FpH试E = K’ +由于K’无法准确测定，所以实际测量时，是先用已知准确pH值的标准缓冲溶液对pH计进行定位，然后再测定。pH试=E–K’2.303RT/FpH标=E标–K’2.303RT/FpH试=E–E标2.303RT/F+ pH标

第21页：Ion-selective electrode

§4-4 Ion-selective electrode and membrane potential离子选择性电极是一种以电位法测量溶液中某些特定离子活度的指示电极。由于所需仪器设备简单，轻便，适于现场测量，易于推广，对于某些离子的测定灵敏度可达ppm级（个别可达ppb数量级），特效性较好，因此发展极为迅速。

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第23页：Innersolution Glass bulk

Innersolution Glass bulk Testsolution Hydratedlayer HydratedlayerE内E试Schematic diagram of form at ion of membrane potential of glass electrodeDEM= E试-E内E试=k1+lg2.303RT H+,试F a’H+,试E内=k2+lg2.303RT H+,内F a’H+,内DEM= E试–E内

第24页：Supposed two

Supposed two hydrated layers (in and out) of glass membrane is symmetrical, thenk1=k2, a’H+,试= a’H+,内Membrane potential is re-expressed as follows:DEM=E试-E内=2.303RT H+,试F H+,内lg Consideringa H+,内= constant, above equation can be re-writed as DEM= K+2.303RT Flga H+,试In fact, for any ion, above express is vivid

第25页：DEM= K±

DEM= K±2.303RT FlgaXObviously, membrane potential of ion selective electrode is directly proportional to logarithm of activity of analyte ion undercertain conditions“+” for cation“-” for an ion

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第31页：Selectivity of ion

§4-5 Selectivity of ion selective electrodes Considering co-existence ion’s response to membrane potential,following equation is reasonable:DEM= K±2.303RTni Flg ai+Ki,j(aj)ni/nj In above equation, “+”, or “-” symbol, respectively, correspondsto the cation or an ion. And Ki,jis selectivity coefficient ofinterferentjto analytei When ni=nj, this equation reduces to simpler form as follows:DEM= K±2.303RTni Flg ai+Ki,jaj

第32页：As an example

As an example, interference by ions such as Na+on pHmeasurement is expressed DEM= K+2.303RT Flg H++KH+,Na+a Na+Above equation says that sodium activity contributes to the measured potential, and the contribution is proportional tothelogarithm of the selectivity coefficient: smaller selectivity coefficient indicates less interference. KH+,Na+ values varydepending on the type of glass composing the electrode.As an example, if Ki,j= 10-2, andni= nj=1, then above equation indicates that the Na+ contribution will equal that from H+whena Na+= 100a H+.

第33页：选择性系数Ki

选择性系数Ki,j它可理解为在其它条件相同时提供相同电位的欲测离子活度ai和干扰离子活度aj的比值(ratio of the concentrations of analyte and interferent yielding identical cell potentials)：Ki,j= ai/ (aj)ni/nj Example借助选择性系数，可以估算某种干扰离子对测定造成的误差：Relative error =Ki,j×(aj)ni/njai×100%Eg. Ki,j=10-2, ai=aj, and ni= nj=1, then Relative error=10-2×100%=1%选择性系数的测定方法参见D. Harvey,Modern analyticalchemistry, Mc-Graw Hill, Singapore,2000,p477

第34页：Category

§4-6 Category and capability (种类和性能) ionselective electrodes•晶体膜电极( crystalline membrane electrode)•均相膜电极(homogeneous membrane electrode )•非均相膜电极( heterogeneous membrane electrode)•非晶体膜电极( non-crystalline membrane electrode )•刚性基质电极(rigid matrix electrode)•活动载体电极(electrode with mobile carrier)•敏化电极(sensitized electrode)•气敏电极(gas sensing electrode)•酶(底物)电极(enzyme (substrate) electrode)

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第36页：Crystalline membrane

Crystalline membrane electrodes The membranes are usually made in corresponding in soluble salts by pressing or drawing (加压或拉制）them to single crystal ,poly-crystals or mixed crystals.Based on different preparation processes, crystallinefilms are divided into homogeneous and in homogeneous phases.均相膜电极的敏感膜一般由一种或几种化合物的均匀混合物的晶体构成，如硅橡胶、聚氯乙烯、聚苯乙烯、石蜡等，其中电活性物质对膜电极的功能起决定作用。电极的机制是，由于晶格缺陷（空穴）引起离子的传导作用。接近空穴的可移动离子移动至空穴中，一定的电极膜，按其空穴大小、形状、电荷分布，只能容纳一定的可移动离子，而其它离子则不能进入。晶体膜就是这样限制了除待测离子外其它离子的移动而显示其选择性。因为没有其它离子进入晶格，干扰只是由于晶体表面的化学反应而引起的。

第37页：Crystalline membranes

Crystalline membranes:Single crystal-LaF3for F-Polycrystalline or mixed crystal-Ag2S for S2-and Ag+

第38页：电极外管

12341电极外管（玻璃或塑料）2内参比溶液3内参比电极4氟化镧单晶膜Schematic diagram of fluorideion Selective electrode, in LaF3singlecrystal movable ion is F-DEM= K-2.303RT Flga F-Linear range: 1 ~ 10-6mol.L-1Detection limit is determined by single crystal’s solubilityproduct Optimal range: 5 ~ 6?LaF3+ 3OH-= La(OH)3+3 F-2F-+ H+→HF2-La3+的强络合剂也会影响电极的响应时间和响应范围

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第41页：Solid state electrodes

Solid state electrodes The Cl-electrode will produce concentration dependentresponse for both Cl-and Ag+.Both can be primary absorbed ions.Interferences are observed for S2-,Br-, I-, CN-andammonia.

第42页：Ag2S membrane Ag

Ag2S membrane Ag contact point Epoxy resin packing Shieldedlead(导线)离子接触型全固态型Ag2S membrane electrodes self-study

第43页：Non-crystalline membrane

Non-crystalline membrane electrodes(rigid matrix electrodes)For example: Cationic IS Es Self-study

第44页：Electrodes with mobile

Electrodes with mobile carrier离子交换剂中性载体大环内酯化合物、大环聚醚、大环酰胺，树形化合物、卟啉等

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第48页：Cu2+BF4

Cu2+BF4-

第49页：Sensitized electrodes

Sensitized electrodes

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第51页：NH3+ H+NH4+

NH3+ H+NH4+

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第53页：CO2气敏电极

CO2气敏电极

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第55页：Progress

Progress: chemical sensors, biosensors,biological tissue electrodes参看Anal. Chem., annual reviews,司士辉编著，生物传感器，化学工业出版社，2003

第56页：离子敏场效应晶体管

离子敏场效应晶体管（ion sensitive field effective transistor）Self-study§4-7 Methods to measure ion activity (or concentration)For cell composing of an indicating electrode, referenceelectrode and analyte solution, according to previous discussion,following expressions are true:E = K-2.303RTn Flga阴离子E = K+2.303RTn Flga阳离子Where is constant depending on the temperature, properties of membrane, inner reference solution, potentials of inner and outer reference electrodes and liquid junctionpotential etc.

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第58页：Calibration graph method

Calibration graph method(standard curve, working curve)Place ISE and reference electrode into series of standardsolutions which accurate concentrations are known, measuretheir cell potentials and then plot graph of cell potential Eversuslogarithm of activity (or concentration) of analyte ions Elogailogci离子强度控制：

当试样中含有一种含量高而且浓度基本恒定的非欲测离子时，可使用“恒定离子背景法”，即以试样本身为基础，用相似的组成制备标准溶液。

如果试样所含非欲测离子及其浓度不能确知或变动较大，则可使用加入“离子强度调节剂”的办法。离子强度调节剂是浓度很大的电解质溶液，它应对欲测离子没有干扰，将它加到标准溶液及试样溶液中，使它们的离子强度都达到很高而且近乎一致，从而使活度系数基本相同。在某些些情况下，此种高离子强度的溶液还含有pH缓冲剂和消除干扰的络合剂。

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第60页：Standard add it ion

Standard add it ion methodE1= K’ +2.303RTn Flg x1g1CxWhere x1is molar fraction of dissociate/ uncomplexed ionsC2=CxV0+ VsCsV0+Vs≈Cx+CD

第61页：Standard add it io（续）

E2= K’ +2.303RTn Flg x2g2C2= K’ +2.303RTn Flg x2g2()Cx+CDCD=VsCsVs+ Vo∵Vs« V0,∴g1≈g2, x1≈x2CD=VsCsVoDE = E2–E1=2.303RTn Fx2g2( )x1g1CxCx+CDlg

第62页：Standard add it io（续）

DE =2.303RTn Flg CxCD1 +Denoting = 2.303RT/FDE =Snlg CxCD1 +Rewriting above equation as:Cx= CD(10nDE/S-1)-1Vo ,(*)

第63页：Standard add it io（续）

D

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第65页：Gran graphical

Gran graphical determination Re-express equation (*) as following form(V0+ Vs)10E/S=k(cxV0+ cSVS)So plot of the left hand side in above equation versus Vs will be straight line. On the point of intersection(交点) of the straight lineandx-axis,(V0+ Vs)10E/S=k(cxV0+ cSVS)=0Cx=-cSVSV0

第66页：下图

下图

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第68页：Factors influencing

§4-8 Factors influencing determination

Temperature Slope: 2.303RT/nFIntercept:K’ = ESCE–EAg Cl/Ag+DEas+DEL–KSo during measurement, the temperature must be kept constant

Measurement of electromotive force = +RTn Fln CDifferentiate above equation, we obtain:

第69页：That isd =RTn FdCCDE

That isd =RTn FdCCDE =RTnFDCC Where R=8.314J.K-1.mol-1, F=96487C, when T=298.15K, cell potential is calculated using above equation :DE =0.2569nDCC•×100Rearranging above equation we obtain:%Relative error =×100% = ≈4nDEDC CnDE0.2569

第70页：一价离子的电极电位每产生±1mV测定误差

一价离子的电极电位每产生±1mV测定误差，将产生约±4%的浓度相对误差；对两价离子响应的电极的误差为±8％，三价离于则为±12％。说明用宜按电位法测定，误差一般较大，对价数较高的离子尤为严重。对于高价离子，将其转变为电荷数较低的络离子后测定是较为有利的，例如将B(III)转化为BF4-离子后可用BF4-离子薄膜电极测定。测定S2-时，可加入过量Ag+离子使之形成Ag2S沉淀，再测定剩余的Ag+离子。其测定误差将符合n=1时的关系。离子电极一般适用于较低浓度的测定，测试较高含量组分的含量时宜采用电位滴定法。对于直接电位测定法，要求测量电位酌仪器必须具有高的灵敏度和相当的准确度

第71页：Interference ions

Interference ions

Influence the determination due to interaction with electrodemembraneeg.LaF3(s) + Ct3-(aq) = LaCt(aq) + 3F-(aq)SCN-(aq) + AgBr(s) = AgSCN(s) + Br-

Influence activity of sought-for ions by affecting ion strengthof the solution

Interfere the determination by reaction with determinand,for example,6F-+ Al3+= AlF63-+2Cu2++ 4I-= 2Cu + I2Measurements eliminating interferences:Masking agents, separating interference ions

第72页：p of the solution

p of the solution•Concentration of analyte (sought-for ion)•Response time•Sluggish effect（迟滞效应）Self-study

第73页：Apparatus pair electrodes

§4-9 Apparatus pair electrodes (an indicating electrode and referenceelectrode), container of test solution, stirrer and apotentiometer.Requirement for potentiometer:

High inputimpedancetoensure the measurement on null-current condition

Enough high detection precision, eg 2% precision ~ 0.5m • Enough stability

第74页：Application of ISE

§4-10 Application of ISE methods Advantages of ISE analysis

Simple and rapid High selectivity and rapid response, simple instrumental setup, direct determination of colored, feculent(浑浊的)and dope solutions

Easy to carry out continuous operation and automation Shortcoming

big error

limited selectivity

poorerreproducibility, poorer linear than spectrophotometry

第75页：potentiometric titration

§4-11 potentiometric titration Data processing of potentiometric titration

第76页：SCE2 indicating

123456781 SCE2 indicating electrode Ag electrode3 buret（滴定管）4 electromagnetic valve5 salt bridge6 test solution, Cl-solution7 saturated KCl solution8 automatic potentiometrictitrator Schematic diagram of setup of autokinetic potentiometric titratorburette stand（滴定管架）

第77页：Applications

§4-12 Applications of potentiometric titrimetryand selection of the indicating electrodes Self-study Problems

What characteristics does potentiometric titration method have?

How to select an indicating electrode for your potentiometric analysis?

What problems should you consider when youchoose and use an indicating electrode and reference electrode?