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1、Electroanalytical methods,Chapter 2,2,Electroanalytical methods,Electrogravimetry (电重量法）,Coulometry （库仑法）,Potentiometry （电位法）,Voltammetry （伏安法） Conductometry (电导法),基本概念,电位分析法 利用电极电位和溶液活度或浓度之间的关系来测定被测物质活度或浓度的一种电化学分析法。 以测量电动势为基础。,3,Potentiometry（电位法）,4,分类,直接电位法,通过测量电池电动势来确定指示电极的电位，然后根据能斯特方程得出被测物质的活度或浓
2、度。,电位滴定法,通过测量在滴定过程中指示电极电位的变化来确定滴定终点，再按滴定中消耗的标准溶液的体积和浓度来计算待测物质含量。,Potentiometry（电位法）,电化学分析法的特点,5,灵敏度、准确度高，选择性好，应用广泛。被测物质的最低量可以达到10-12mol/L数量级。 电化学仪器装置较为简单，操作方便；尤其适合于化工生产中的自动控制和在线分析、无机离子的分析、测定有机化合物也日益广泛。,6,Potentiometry（电位法）,Fundamentals of potentiometry （基本原理）,Reference electrodes（参比电极）,Indicator and
3、 ion selective electrodes（指示电极及离子选择电极）,Instrumentation and measurement of cell electromotive force (e.m.f) （仪器及电动势测量）,7,Fundamentals of potentiometry,When a metal is immersed in a solution containing its own ions, the potential difference is established between the metal and the solution,Oxne Red,8,
4、Fundamentals of potentiometry,M n+ + ne = M, = + (RT/nF) ln Mn+,Nernst equation,9,Fundamentals of potentiometry,Indicator electrode,+,Reference electrode,+,Solution,Cell,10,Fundamentals of potentiometry,M M n+ reference electrode,E = (+) - (-) + L,E = (+) - (-) = r - - (RT/nF) ln Mn+,Liquid junction
5、 potential,11,Reference electrodes,Hydrogen electrode（氢电极）,Calomel electrode（甘汞电极）,Silver silver chloride electrode （银-氯化银电极）,电位已知且恒定 重现性好,液接电位差异小 装置简单,12,Calomel electrode（甘汞电极）,13,Calomel electrode（甘汞电极）,HgHg2Cl2，KCl(xM) Hg2Cl2(s)+ 2e 2Hg(l)+ 2Cl-,Electrode potential, = Hg2Cl2 / Hg + (RT/nF) ln (1
6、/ Cl-2) = Hg2Cl2 / Hg - 0.059 lg Cl-,14,Calomel electrode（甘汞电极）,甘汞电极的电极电位（25 ）,15,Silver silver chloride electrode,16,Silver silver chloride electrode,AgAgCl，KCl(xM) AgCl(s)+ e Ag(s)+Cl-,Electrode potential, = AgCl / Ag + (RT/nF) ln (1/ Cl-) = AgCl / Ag - 0.059 lg Cl-,17,Indicator and ion selective
7、electrodes,Indicator electrode指示电极,-The potential depends on the activity of a particular ionic species which it is desired to quantify,电极电位随待测离子活度变化而变化,18,Indicator and ion selective electrodes,Electrode of the first kind （第一类电极）,Electrode of the second kind （第二类电极）,The glass electrode（玻璃电极）,Crysta
8、lline membrane electrode（晶体膜电极）,Biochemical electrode（生化电极）,Inert electrode（惰性电极）,Metal electrode,Membrane electrode,*,19,Electrode of the first kind,Indicator and ion selective electrodes,-The ion to be determined is directly involved in the electrode reaction （待测离子直接参与电极反应）, = M n+ / M+ (RT/nF) ln
9、 Mn+,Metal M immersed in a solution of M n+ ion （金属M浸于含有M n+ 离子的溶液中）,20,Electrode of the second kind,Indicator and ion selective electrodes,Silver silver chloride electrode,- coating a silver wire with silver choloride,AgCl(s)+ e Ag(s)+Cl-, = AgCl / Ag + (RT/nF) ln (1/ Cl-) = AgCl / Ag - 0.059 lg Cl
10、-,21,Fe 3+ + e Fe 2+, = Fe 3+ / Fe2+ + (RT/nF) ln (Fe3+ / Fe2+),Inert electrode（惰性电极）,-An inert electrode (Pt) is place in a system containing both an oxidizing agent and its reduction product （惰性金属置于氧化态和还原态离子中）,思考题,金属基电极的共同特点是什么？ 上述三类电极分别可用于哪些物质的测定？,22,23,The glass electrode（玻璃电极）,24,The glass elec
11、trode,Composition-组成,SiO2 72% + Na2O 22% + CaO 6%,SiO2 63% + Li2O 28% + Cs2O 2% + BaO 4% + La2O3 3%,25,The glass electrode,Theory,- Ion exchange process （离子交换过程）, glass = K + (RT/nF) ln H+ = K 0.059 pH,26,The glass electrode,27,H+溶液= H+硅胶 E内 = k1 + 0.059 lg( a2 / a2 ） E外 = k2 + 0.059 lg(a1 / a1 ）,Th
12、e glass electrode,E膜 = E外 - E内 = 0.059 lg( a1 / a2）,E膜 = K + 0.059 lg a1 = K - 0.059 pH试液,k1=k2 ， a1 = a2,28,The glass electrode,properties,Can be used in the presence of strong oxidants and reductants （可在强氧化剂和强还原剂存在下使用）,Can be used in viscous media （可用于粘稠介质）,Can be used in the presence of proteins
13、（可在蛋白质存在下使用）,29,The glass electrode,properties,High resistance（内阻较高）,Acid error and alkaline error （酸差和碱差）,30,Crystalline membrane electrode,composition,Lanthanum fluoride eletrode,Crystal of lanthanum fluoride（氟化镧单晶）,+,0.1 mol/L NaF 0.1 mol/L NaCl,+,Silver silver chloride electrode,31,Crystalline m
14、embrane electrode,Theory,Lattice defect（晶格缺陷）, membrane = K - (RT/nF) ln F- = K 0.059 lg F-,32,Crystalline membrane electrode,properties,Detection limit 10-7 mol/L,Interference OH-,pH range 5 - 6,33,Gas sensing electrode,NH3 NH4Cl,CO2 NaHCO3,NO2 NaNO2,34,Biochemical electrode,Urea electrode,CO(NH2)2
15、 + H2O + 2H+ 2NH4+ + CO2,urease,35,Biochemical electrode,组织电极,以动物组织内天然存在的某种生物酶来催化反应，并制成敏感膜,香蕉与碳糊制成的香蕉电极多巴胺测定 猪肝切片夹在尼龙网中紧贴在氨气敏电极上利用其中的谷氨酰胺酶催化谷氨酰胺而释放出氨气测定谷氨酸,36,Biochemical electrode,37,离子选择性电极的膜电位及选择性,膜电位通式, membrane = K (0.059/n) lg ,38,离子选择性电极的膜电位及选择性,选择性系数,能产生相同电位的待测离子活度i和干扰离子活度j之比值,作用：,判断电极好坏,估计误差大小,例： 某硝酸根电极对硫酸根的选择系数：K NO-,SO2-为4.110 -5,用此电极在1.0mol/L硫酸盐介质中测定硝酸根,如果要求测量误差不大于5%,试计算可以测定的硝酸根的最低活度为多少?,解： NO-，SO-（aSO- ）ni/nj /aNO- 5% aNO- 4.110-1.0 /5 aNO- 8.210 -molL-1。 测定的硝酸根离子的活度应大于8.2104mol L-1 。,40,Instrumentation,Determination of pH,41,
