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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Electroanalytical Chemistry,Lecture#6,An Introduction to Electrochemical Methods(contd),Electroanalytical ChemistryLec,Q:What Experiment is This?,Name of experiment,type of excitation,Response,i,_,slope,Deficiency,Excitation,Response,time,time,E,I,t,o,t,o,t,Q:What Experiment is This?Na,What Experiment Is This?,Name of experiment,Type of excitation,Response,Q,_,intercept,slope,Excitation,Response,time,time,E,Q,t,o,t,o,t,Q,dl,What Experiment Is This?Name o,Q:What Is This Experiment?,Name of experiment,Excitation,Response,i,_,E,p,_ of,E=_,Time,s,E,app,V,Excitation,E,1,E,2,E,app,V,I,A,E,p,E,1,E,2,Response,E,o,E,o,X,X,Q:What Is This Experiment?Na,Cyclic Voltammetry(CV),Important parameters:,E,pa,and E,pc,i,pc,and i,ac,E,D,E=|E,pa,-E,pc,|,Time,s,E,app,V,Excitation,E,1,E,2,E,app,V,I,A,E,pa,E,pc,E,1,E,2,Response,forward,reverse,R-ne,-,=O,Cyclic Voltammetry(CV)Importa,For Nernstian CV,D,E,p,=|E,pa,-E,pc,|=59/n mV at 25,0,C,independent of,n,E,o,=(E,pa,+E,pc,)/2,I,pc,/I,pa,=1,For Nernstian CVDEp=|Epa-E,For Nernstian Process,Potential excitation controls R/O as in Nernst equation:E,app,=E,0,-0.059/n log R/O,if E,app,E,0,O _ R and ox occurs,if E,app,59 mV and E,p,increases with increasing,iR can mascarade as QR system,Irreversible:,chemically-no return wave,slow ET-2 waves do not overlap,Quasi-reversible or Irreversib,EXAMPLE:Electrocatalytic Oxidation of Guanine in DNA,Top:non-faradaic contribution,Bottom:shape and magnitude of redox waves,P.M.Armistead;H.H.Thorp,Anal.Chem.,2000,72,3764-70.,EXAMPLE:Electrocatalytic Oxi,EXAMPLE:UMEs in Sol-Gels,Q:Identify the waves in the CVs shown at left,Top:UME-slow scan rate(sigmoidal shape),Bottom:UME-fast scan rate,Annette R.Howells,Pedro J.Zambrano,and Maryanne M.Collinson*;Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses,Analytical Chemistry;2000;72(21);5265-5271.,EXAMPLE:UMEs in Sol-GelsQ:,UMEs:,0.1,m,Fe,3+,Fast scan rates30 V/s,planar diffusion,Slow scan rates5 mV/s,radial diffusion,0.1,m,UMEs:0.1 mFe3+Fast scan ra,UMEs Radial vs.Planar Diffusion,Radial Diffusion,Redox wave:,sigmoidal shape,I,ss,=4nFrD,o,C,o,*,I,ss,scan rate independent,D,o,C,o,*,Planar Diffusion,Redox wave:,normal shape,I,p,1/2,D,o,1/2,C,UMEs Radial vs.Planar Diffus,EXAMPLE:UMEs in Sol-Gels,Learn D,o,from CA,Obtain C,o,*,from slow scan rate CV(I,ss,),Annette R.Howells,Pedro J.Zambrano,and Maryanne M.Collinson*;Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses,Analytical Chemistry;2000;72(21);5265-5271.,EXAMPLE:UMEs in Sol-GelsLea,EXAMPLE 2:Look Ma,No Electrolyte!,S,2,Mo,18,O,62,4-,+e,-,=S,2,Mo,18,O,62,5-,+e,-,=S,2,Mo,18,O,62,6-,BAS 100-A,3-electrode cell:,GC macrodisk/Pt wire/Pt wire,ACN with no electrolyte,Alan M.Bond,*Darren C.Coomber,Stephen W.Feldberg,Keith B.Oldham,and Truc Vu;Analytical Chemistry;2001;73(2);352-359.,20 mV/s,20 mV/s,100 mV/s,EXAMPLE 2:Look Ma,No Electr,Applications of CV,Many organic functional groups are reducible:C=OC=CC=NN=NS-S,see,Handbook of Organic Compounds,Applications of CVMany organic,Applications of CV,Many functional group are not reducible so we can derivatize these groups,convert them into electroactive groups by chemical modification,EXAMPLES:,alcohols+chromic acid=aldehyde group,phenyl+nitration=nitro group,Applications of CVMany functio,Adsorption Phenomena,Non-specifically adsorbed,No close-range interaction with electrode,Chemical identity of species not important,Specifically adsorbed,Specific short-range interactions important,Chemical identity of species important,Adsorption PhenomenaNon-specif,CV and Adsorption,If electroactive adsorbed species:,E,p,=E,o,-(RT/nF)ln(b,o,/b,R,),i,p,=(n,2,F,2,/4RT),A,o,*,If ideal Nernstian,E,pa,=E,pc,and E,p/2,=90.6 mV/n at 25,0,C,E,app,I,90 mV,CV and AdsorptionIf electroact,EXAMPLE 2:Oxidation of Cysteine at BDD,Nicolae Sptaru,Bulusu V.Sarada,Elena Popa,Donald A.Tryk,and Akira Fujishima*;Voltammetric,Determination of L-Cysteine at Conductive Diamond Electrodes,Analytical Chemistry;2001;73(3);514-519.,EXAMPLE 2:Oxidation of Cyste,Stripping Analysis or Stripping Voltammetry,2 Flavors:,Anodic(ASV),Good for metal cations,Cathodic(CSV),Good for anions and oxyanions,Stripping Analysis or Strippin,Stripping Voltammetry-Steps,1.Deposition,2.Concentration,3.Equilibration,4.Stripping,Stripping Voltammetry-Steps1,Example of ASV:Determination of Pb at HDME,Deposition(cathodic)reduce Pb,2+,Stir(maximize convection),Concentrate analyte,Stop stirring=equilibration/rest period,Scan E in anodic sense and record voltammogram,oxidize analyte(so redissolution occurs),E,app,I,Pb,Pb,2+,+2e,-,I,p,Example of ASV:Determination,Stripping Voltammetry-Quantitation,I,p,C,o,*,Concentrations obtained using either,Standard addition,Calibration curve,Stripping Voltammetry-Quanti,HDME ASV,Usually study M with E,o,more negative than Hg,EX:Cd,2+,Cu,2+,Zn,2+,Pb,2+,Study M with E,o,more positive than Hg at GC,EX:Ag,+,Au,+,Hg,Can analyze mixture with,D,E,o,100
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