Adv Analytical Chemistry I
Adv Analytical Chemistry I CEM 834
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Date Created: 09/19/15
Chapter 1 Introduction of Electrochemical Concepts Electrochemistm concerned with the interrelation of electrical and chemical effects Reactions involving the reactant the electron Chemical changes caused by the passage of current An electrochemical system is not homogeneous but is heterogeneous Broad Field electroanalysis sensors energy storage and conversion devices corrosion electrosynthesis and metal electroplating Electroanalytical Chemistry Electroanalytical chemistry encompasses a group of quantitative analytical methods that are based upon the electrical properties of an analyte solution when it is made part of an electrochemical cell These methods make possible the determination of a particular oxidation state of an element Ox ne39 lt gt Red There are two general types of electrochemical methods potentiometric no current equilibrium potential and voltammetric current measured as a function of the applied potential Electrochemical Cells Electrochemical cells consist of two electrodes an anode the electrode at which the oxidation reaction occurs and a cathode the electrode at which the reduction reaction occurs Cus Zn2 lt gt Cu2 Zns Cus lt gt Cu2 2e39 oxidation Zn2 2e39 lt gt Zns reduction There are two types of electrochemical cells galvanic ones that spontaneously produce electrical energy and electrolytic ones that consume electrical energy Electrochemical Cells Conduction V 1 Metals Vollmcler 2 Solution ion migration 3 Electrode rxns at interfaces 00100 M 00100 M Cuso4 solution Zns Zn aq 2 Cu2ltaq 2e Cus a2n20010 acu20010 Amde Cathode Figure 221 A galvanic electrochemical cell with a salt bridge A potential difference between two electrodes represents a ten y i u Electrochemical Potentials The potential that develops in a cell is a measure of the tendency for a reaction to proceed toward equilibrium TABLE 221 Standard Electrode Potentials Reaction E0 at 25 C V E E0 log C12g2e392Cl 1359 013 4m 4c 2H10 1229 f n F Br2aq Ze39 28139 1087 M d Br2l 2e 3 231quot 1065 easure Ag e Ags 0799 E vs Ref Fe3 e F1 077139 I 2 31 39 0536 Cuquot 2639 3 Cus 0337 ax Y X HggCle 2c 2mm 2c1 0263 AgCl e A30 Clquot 0222 5263 e Ago 25202 001o 2H 2e 1123 0000 Standard red uction reactions all relative Asks 39 Aw 1 151 to the H2H reaction 298 K u n it activities 2ff 22 quot 50 for all species and pH 0 hum2 0363 See Appendix 3 for n more extensive lisL Electrochemical Potentials We use concentrations in the Nernst equation but really activities are the proper term The activity of a species can be defined as the ability of a species to participate an equilibrium reaction involving itself eg Fe3 e39 lt gt Fe2 FeC2 etc Depends on ionic strength Ecell E cathode Eanode Aern 39 r1FEceII Key equations AC3an RTInKeq Reference Electrodes ctrodes EC 1 1 AgCIs e39 lt gt Ags Cl I l E E0 0059nog1C39 H2 e quot1 m 2 HgZC2s 26 lt gt 2CI 2Hgl 322222222125 2211ng E E 00592Iog1CI12 cathode AgC1s wt Aj39ozq C1 aq A 1 A I x a 1 5 1 Solid Age Figure 223 A galvanic cell without a liquid junction n number of electrons transferred per mole 2303 RTF 0059 V Electrochemical Cells Voltmetcr Figure 225 De nition of the stan dam Electrode potenual for M hm um 100 aMzo 100 2e 1 M q 5 g Cu2 H2g lt gt Cus 2H ZnZnso4 aZn2 100Cuso4 aCu2 100Cu Anode oxidation Cathode reduction This shorthand is not always used in your textbook Electrochemical Cells and Reactions Electrode conductor Electrolyte ionic solution Electrode Solution solid solution OX 6 Red Electrodes Pt Au Pd C Hg Electrolyte solutions low ohmic resistance ionic solutions NaCI molten salts and ionic polymers Nafion Electrode reaction kinetics are affected by the electrode surface cleanliness surface microstructure and surface chemistry Electrochemical Cells and Reactions Two electrified interfaces but only one of interest Power supply V JC measure of the V6 r energy available to drive charger externally between electrodes AgBr Figure 113 Schematic diagram of the electrochemical cell PtHBr1 MAgBrAg attached to power supply and meters for obtaining a current polential i E curve Rate of oxidation Rate of reduction Reference electrode AgBr e39 lt Ag Braq E0 0071V vs NHE Electrochemical Cells and Reactions 9 Potential 6 V Potential G Figure 112 Representation of a reduction and lb oxidation process of a species A in solution The molecular orbitals MO of species A shown are the highest occupin MO and the lowest vacant MO These correspond in cm approximate way to the E03 of the AA39 and AVA couples respectively The illustrated system could represent m aromatic hydrocarbon eg 910diphenylamhmcene in an aprotiu solvent eg acetonitrile at a platinum electrode Electrode l Energy level 0 electrons Electrode Energy level 0 6 EC I Ol lS l Solution Vacant MO 9 39 Occupied MO AeA gtA a Solution Vacant MO 6 Occupied l MO A e m b Electrode A z Electrode Solution H Solution agnitude of the p tential controls the direction and rate of c arge transfer 5 a potential is moved n gative the species t at will be reduced fist assuming all are r pid is the oxidant cceptor in the couple ith the least negative 0 Electrochemical Cells and Reactions There are two types of current flow 1 Faradaic charge tranferred across the electrified interface as a result of an electrochemical reaction Q nF 2 Nonfaradaic charge associated with movement of electrolyte ions reorientation of solvent dipoles adsorptiondesorption etc at the electrodeelectrolyte interface This is the background current in voltammetric measurements Electrochemical Cells and Reactions PtH Br391 MAgBrAg Cathodic 39 PtH Br 1 MAgBrAg Current Onset cl H reduction on Pt l i 39 15 05 o o5 Onset of Br oxidation on Pi Anodic current cathodic current anodic potential left potential right AAAA Cell Potential Figure 114 Schematic currentpotential curve for the cell PtH L Br39l MAgBrAg showing the limiting proton reduction and bromide oxidation processes The cell potential is given for the Pt electrode with respect to the Ag electrode so it is equivalent to Ep V vs AgBr Since E AgAEBI 007 V vs NHE the potential axis could be convened to E1 V vs NHE by adding 007 V to each value of potential Br2 2639 gt ZBr E0 109 V VS NHE 2H 2e39lt gt H2 E 000 V VS NHE Ecell EC 39 Ea Electrochemical Cells and Reactions Hng39 Bra mAgBrAg Cathodic HgH Br39 1 MAgBrAg Kinetical y fast reactions have significant faradaic current flow near E0 while sluggish quotquot assistedquot l i395 reactions have little current mm flow except at large overpotentials Current Onset of H reduction Anodic Potentlal V us NHE Figure 115 Schematic currentpotential curve for the Hg electrode in the cell HgH lit l MAgBrAg showing the limiting processes proton reduction with a large negative overpotential and mercury oxidation The potential axis is de ned through the process outlined in the caption to Figure 114 H92ng Ze39lt gt2Hg ZBr E0 014iv vs NHE 2H 2e39 lt gtH2 E0 V VS Electrochemical Cells and Reactions HgH Br 1 M Cd21mMAgBrAg HgH Br 1 M Cd21mMAgBrAg Cathodic Onset of H reduction 2 CdBr4239 2e HodHg 4339quot Onset of Cd 39 reduction E a 39 39 0 o 2 04 0 6 08 1 Onset of Hg oxidatlon Anod Potential V vs NHE Figure 116 Schematic currentpotential curve for the Hg electrode in the cell HgIF Br 1 MCd210 3 MAgBrAg showing reduction wave for Cd HgZBr2 Zequot 392Hg ZBr E0 014 V VS NHE 2H 2639 lt gtH2 E 000 V VS NHE Electrochemical Cells and Reactions G Possible Possible 9 reduction oxldation reactions reactions Ea V vs NHE V vs NHE o25 ALZ 2 Ni Approximate 0 2H 22 3 H2 potential for 0 zero current Au 015 1422 Snz Han sit k 015 I2 22 g 054 E F92 Fes elt FLZ39 077 Approximate patentlal or zem current oz AH 4 lt QHEO 123 Alf 3 t 150 Zia2243 1 o41 ci39 Mac 21 2e 6 H2 Kinetically slow Approxlmate potentlal for zsm current 5 v w NHE c Figure 117 a Polentials for possible reductions at a platinum electrode initially at l V vs NHE in a solution of 0m M each of Fe3 Ni in 1 M HCL b Potentials for possible oxidation reactions at a gold electrode initially at NOJV vs NHE in a solution of 001 M each of Sn2 d Fe in 1 M HI c Potentials for possible reductions at a mercury electrode in 001 M Cr and Zn2 in 1 M HCl The arrows indicate the directions of potential change discussed in the 16th Electrified Interfaces Ideally polarizable electrode IPE no charge transfer across the interface Ions move in and out of the interfacial region in response to potential changes The interface behaves as a capacitor charge storage device 7 Excess electrons on one G 1 plate and a deficiency on il ng 55ij the other r r H mum onclpackmb a I g b emu cpacmmubmuy Changing the potentialrE Causes the charge stored Q to change according to the relationship Qcoulombs Cfarads x Evots Electrified Interfaces qmetal qsolution 39Dl uhollyor Q Solvalodca on thermal agitation 39 9 Mml quot L Speci cally W mien l O Schom moicub I q 1 Figure 113 Proposed model of the v vgt doublelayer region under conditions charge neutrality CompaCtLayer inner and outer Helmholtz planes electrostatic forces are very strong Diffuse Layer gradient of charge accumulation cmeta39 qmeta39larea uCcmz ya when mien are speci cally adsorbed The excess charge on a metal is confined to the near surface region However the balancing charge on the solution side of the interface extends out into the solution with some thickness ionic zones in sol Electrified Interfaces Cymetal GHP CSOHP Cydiffuse Structure of the electric double layer has d llldel effect on electiuu39e reaction PM 5 Is 39 quot quot a kinetics Faradaic reaction rates Rip 3 Solvated cation 6 Species not specifically adsorbed 3quotGh l39f l 39lld M IOHP nonfit wis u39 ii approach the OHP DZ Cls is wasted CDm Cls is potential felt by analyte 2 quot q Figure 124 Potential pro le across the double layer region in the absence of speci c adsorption of ions The variable 4 called the inner potential is discussed in detail in Section 22 A more quantitative x1 2 3 representation of this pro le is shown in x 39gt Figure 1236 M dy a Field strength is critical Electrified Interfaces The solution side of the interface consists of a compact layer inner and outer Helmholtz layers plus a diffuse layer Diffuse layer extends from the OHP to the bulk solution Ionic distribution influenced by ordering due to coulombic forces and disorder caused by random thermal motion Qm QCL QDL 0 Qm 39 QCL QDL Q CE C Farad E voltage difference QDL CdIAEEpzc A area cm EIOZC point of zero charge H F 1CTOT 1CCL 1CDL Smallest value dominates the interfacial capacitance CCL CDL Electrochemical Experiment and Variables in Electrochemical Cells External variables Temperature T Electrode Variables Pressure P Material ms m Surface area A Geomet Electrical variables 393 Surface condition 7 T Potential E Mass transfer Current i variables Quantity of electricity Q Mode diffusion convection Surface cbpcentrations Adsorption Solution variables Bulk concentration of electroactlve species Co CR Concentrations of other species electrolyte pH Solvent Figure 132 Variables affecting the rate of an electrode reaction Electrode pretreatment matters a great deal Electrochemical Experiment and Variables in Electrochemical Cells Electrode surface region Electrode 39 I Chemical reactions Electron gr o39ada 45 Bulk solution Mass transfer W 039 gti gauntmobulk 1 Mass transfer of reactantproduct to and away from the electrode interface Electron transfer at interface Preceeding or followup chemical reactions Surface processes adsorptiondesorption transfer VI I 09 R ads Wag Chemical k reactions We 939 35112 3 W abulk Working Electrode Indicator Electrode Figure 136 Pathway of a general electrode reaction Electrochemical Experiment and Variables in Electrochemical Cells amperes Tmoulombss rr Vr1 a if 7 amp coulombs nFcoulombsmol N moles eleCtrC Iyzed dN Rate moIscmZ i Rate mols F nF Mass Transport Modes of Mass Transport 1 Migration movement of charged body under influence of an electric field 2 Diffusion movement of species under the influence of a concentration gradient an x 3 Convection stirring or Figure 141 Concentration pro les solid lines and diffusion layer approximation dashed lines x 0 corresponds to the electrode surface and 60 is the diffusion layer thickness 39 a m 390 39 areshown at quot quot r 39 1 where Cox 0 F r is about can 2 where Cox 0 a o andi i J D dCix ziF D C dClgtx C iX i dx RT i i dx Mx Jix flux of l molscm2 D diffusion coeff cm2s C conc molcm3 M potential gradient 2 charge on species ux velocity cms Mass Transport 39 I 20012 1 2 39a 4 ca Diffusion layer thickness Coltx0 39 1 I I Figure 145 Growth of the 811 502 803 8a x diffusionlayer thickness with time Umt Do dCodxx0 Dmt Do 00 CoXO50 Do 00 Cox080 inFA DrCrxO g5r CurrentVoltage Curve Shapes Figure 151 Effect of an irreversible following homogeneous chemical reaction on nemstian i E curves at a rotating disk electrode 1 Unperturbed curve 2 and 3 Curves with following reaction at two rotation rates where the rotation rate for 3 is greater than for 2 Chapter 2 Thermodynamics and Potentials Thermodynamics encompass systems at equilibrium Chemical Reversibility chemical reactions associated with the electrochemical reaction are reversible PtHZH Cl39lAgCIAg standard states Eright Eleft EC Ea 0222 0000 0222 V H2 2AgC lt gt 2Ag 2H 2CI39 Thermodynamic Reversibility Infinitesmal driving force causes the reaction to move forwardreverse direction Essentially at equilibrium Practical Reversibility Actual processes occur at finite rates therefore they cannot proceed at true equilibrium However processes can be carried out in a manner in which thermodynamic considerations apply E E0 2303RTnF ogOxRed Link between E and concs at surface removal of weightslow or fast E cell Thermodynamic Quantities Aern nFErxn AGorxn nFE0rxn AS aAGaTp nF 9EDm8TIo AHAg1AsnanammDp Em RTanKmQAG nFEm emf relates to the direction of the reaction Reduction potential OR couple vs emf for the reduction or oxidation reaction Formal Potentials Standard conditions for E0 298 K pH 0 a 1 Nonstandard conditions or conditions where the activities of Ox and Red are affected by the medium then formal potentials are used EO The formal potential incorporates the standard potential and some activity coefficients E E 2303RTnFogOxRed E0 E0 2303RTnFog vexWed Reference Electrodes Reversible and obeys the Nernst Eq chemical and echem Constant E with time Returns to original potential with passage of small currents Exhibits little hysteresis with temperature cycling E E0 2303RTnF logOxRed Reference Electrodes AgCIs e39lt gt Ags C39aq 1 M NaCl or KCI Eref E0 00591 3909 1C39 Eref 0222 V VS NHE AgCI coated on Ag if CI39 1 M Porous glass frit Quasireference electrodes are sometimes used Pt or Ag Electrode Potential Measurement Two electrodes two interfaces are required to make a potential measurement Dlstance s M Metal Electrolyte Vacuum Figure 223 Potential pro le through the system shown in Figure 222 Distance is measured radially from the center of the metallic sphere Interfacial Potentials Interactions Between Phases qM C13 0 Distance AL ps P E lectrolyte I 7 cpl l I Metal I v I quid solid cpM cps interfacial potential difference depends on charge imbalance and size of interface gt Changes in potential of a conducting phase can be affected by altering the charge distribution on or around the phase gt If phase undergoes a change in excess charge the excess charge will be distributed over the entire boundary of the phase Measurement of Potential Differences gt Electrons can be pumped into or out of a metal electrode using a power supply gt Acp controls the relative energies of charged species on either side of the interface Acp controls relative electron affinities of the two phases the direction of the reaction CuZnZn2 Cl39lAgCllAgCu gt E06 is sum of several interfacial potentials and none can be measured independently Cu Electrolyte pCu pCu gt Maintain constant interfaCIal potentials at all interfaces then change in E must be due to changes at electrodesolution interface Znelectrolyte Distance Electrochemical Potentials Junction Potentials gt Zn2 energy state depends on local chemical and electrical environment CuZnan2Cu2Cu Eceu Wmpf cpCu pa ch p0 ch pa is the diffusion potential or junction potential between two solutions 1 Two solutions of same electrolyte at different concentrations 2 Two solutions of same concentration with different electrolytes having a common ion 3 Two solutions not satisfying 1 or 2 Junction Potentials Zl ti 1 Transference number fraction of current carried by ion t t 1 KAI Conductance ohm1 K F 2 zi uiCi Conductivity ohm1 cm1 I zi uiCi v cms u cm2Vs 39 Zzj uJCJ E VCm