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1 Chapter 22 An Introduction to Electroanalytical Chemistry 전기화학 1) 배경 * 물질은본질적으로전기적이다. 따라서모든화학이전기화학과관련이있다. * Electrochemistry (Ionics and Electrodics) * Babylonains as early as 500 BC used the galvanic cell 50년전에 König가 Bagdad 부근에서발굴 The formations of electrochemistry are to be found in the late 18th - century investigation by Galvany and Volta. Galvani frog leg 실험, 1786년 ( 전기화학의시작 ) Alecssandro Volta

2 <Fundamentals of Electrochemistry> 1) Basic concepts: A Redox rxn. involves transfer of electrons from one species to another. -An oxidizing agent -An reducing agent ex) Fe 3+ + Cu + = Fe 2+ + Cu 2+ (1) Oxidizing Agent Reducing Agent Fe 3+ + e = Fe 2+ Cu + - e = Cu 2+

3 2) Relation Between Chemistry and Electricity Electrical Measurement i) Charge is measured in Coulombs, C - C of single electron: C -One mole of electrons has a charge of C Faraday constant, F q = n F Coulombs = (Mole) (Coulombs/Moles) ii) Current : The quantity of charge flowing each second through a circuit. 1 A = 1 C/1 sec

4 iii) Voltage, Joule and Free Energy One J of energy is gained or lost when one coulomb of charge is moved through a potential difference of one volt. Work = E g Joules = Volts Coulomb Work = -ΔG at constant T, P reversible chemical reaction ΔG = - Work = - E q ΔG = - nfe Ohm's law I = E/R Power (P) P = Work/s = (E q)/s = E (q/s) = E I [W]

5 4) Galvanic Cells i) Half reactions ii) Anode : The electrode at which oxidation occurs. iii) Cathode : The electrode at which Reduction occurs. iv) Salt bridge We adopt the convention that the left-hand electrode of each cell is connected to the negative input terminal of the meter. left-hand side : oxidation electrode v) Line notation

6 5) Standard potentials i) Standard reduction potential. ii) SHE NHE Potential assign zero - Using the Nernst equation. Anode : H 2 (g) ---> 2H + + 2e - Cathode : Cd e - ---> Cd(s) E o = Net : Cd 2+ + H 2 (g) ---> Cd(s) + 2H + E(cell) = E o (cell) - ( /2) log ([H + ]/[Cd 2+ ]PH 2 ) ph --> 1 if E=0 --> [H + ] = Solubility Product: [Ag + ] = (K sp ([AgCl])/[Cl - ] = [( )/0.0334] (K --> Eo) E(cell) = E o -(0.0591/n) log Q = M (at any time) O = E o -(0.0591/n) log K (at eq) (0.0591/n) log K = E o or K = 10nEo/ (at 25 C)

7 ex) FeCO 3 (s) + 2e - --> Fe(s) + CO 2-3 E o = V Fe(s) --> Fe2+ + 2e - E o = V FeCO 3 (s) --> Fe 2+ + CO 2-3 (K=Ksp) E o = V Ksp = 10(2)(-0.316)/(0.0591) = Using Cells as Chemical probes a) Eq. between the two half-cells b) Eq. within each half-cell

8 1.1 Electrochemical method의일반성 1 Analytical balance 2 Hot plates 3 Hume hoods 4 Ovens 5 ph meter UV/VIS & IR - spectrophotometer : 50 % 사용 AA - spectrophotometer : 30 % Polarographic analyzer : 12 % Ion - selective electrode : 30 % 사용빈도가적은이유 : 1 교과과정이강조되어있지않다. 2 자동화의난점

9 A) 전기화학적방법의특징 1 Inexpensive 2 Specific for a particular chemical form 3 Concentration보다 activity에감응 B) Electrochemical method의 classification Electrochemical method : 화학적계혹은시료의전기적응답이측정되는것 C) Experimental system l Electrolyte 2 Detector (electrode) 3 Circuit D) Electrochemical method

10 1. Potentiometric 2. All others (voltammetry, coulometry, conductometry, etc.) 1 Potentiometry : J. Willard Gibbs Nernst( 실제로개발 ) System에서전류를끌어내거나전기분해없이계의열역학적평형전위를측정 2 All others : voltage or current가전극에가해져서그때전류를흘리거나혹은 voltage를변화시키거나하여 system을 monitor하는것

12 22A Electrochemical cells 구성 - 2개의반쪽전지의연결 (1) Electrode - Anode oxidation - Cathode reduction 1) Working and indicator electrode : A reaction take place. 2) Reference electrode : 전류의변화에무관하게 constant potential을유지 3) Counter electrode : 참조전류의 internal polarization을피하기위한외부전극 E (working electrode) = (E cell - ir cell - E polarization ) - Electrode의저항이무시되는조건에서전해질의전도성측정 - Electrolyte의저항이무시되는조건에서전극에서발생되는현상측정

13 (1) 종류 * Galvanic cell : chemical En. electrical En. * Electrolytic cell : electrical En. chemical En. Galvanic cell : Zn Zn e - Cu e - Cu

14 (2) 표시법 Zn / Zn 2+ (azn ) Cu 2+ (a 2+ Cu ) / Cu 2+ Left hand electrode : negative pole of cell oxidation process occurs Zn Zn e Right hand electrode : Reduction process Cu e Cu * 동일한상內에서다른화학종표시 Pt, H 2 (p=1atom) / H + (0.1M), Cl - (0.1M), AgCl(satd) / Ag

15 22A-1 Conduction in a Cell Cu(s) Cu 2+ (aq) + 2e - (22-1) Ag + (aq) + e - Ag(s) (22-2) 22A-2 Galvanic and Electrolytic Cells 22A-3 Anodes and Cathodes 22A-4 Cells without Liquid Junctions

17 22A-5 Solution Structure Fig Electrical double layer 1) A compact inner layer (d0 to d1): the potential decreases linearly with distance from the electrode surface. 2) A diffuse layer(d1 to d2): the potential decrease is exponential.

18 22A-6 Faradaic and Nonfaradaic Currents 1) Faradaic process : 전극 - 용액계면을가로질러전류가이동하는 과정으로 Faraday's law 에따르는과정 ( 전하의이동이일어난 때의반응 ) 실제로산화환원반응이일어난다. 2) Non-faradaic process : 전극-용액계면을전하가이동하지못하는경우 condenser 현상으로인한과정 3) Charging current : non-faradaic process의일例어떤전위 E A 에서형성된전기이중층의전위가 E B 로높아졌을때새로운전기이중층을만들기위해전류가흘러야한다. 그때의전류를 charging current.

19 22A-7 Mass Transfer in Cells with the Passage of Current 22A-8 Schematic Representation of Cells

20 22B Potentials in Electrochemical Cells E cell = E right - E left + E lj Figure Measurement of the electrode potential for an M electrode.

21 22-B1. The Thermodynamics of cell potentials Nernst Equation thermodynamic relationship 에근거를둔 potentiometric measurement ΔG = ΔH - TΔS = ΔE + PΔV -TΔS From Vant Hoff reaction equation = de - TdS -SdT + PdV + VdP

23 Ion강도의영향 이온의전하, 활동도대신몰농도를사용 error 유발 A) Standard electrode potential: 표준상태에서, 모든반응물과생성물이 l 의활동도를가질때표준수소전극을기준으로하여측정한그반쪽전지의전위. B) Formal electrode potential: 활동도영향및부반응 ( 용매화, 해리, 회합, 착물형성 ) 에서오는전극전위의편차를부분적으로보상하기위해 swift가산화환원계산에서표준전극전위대신 formal 전극전위값을사용토록제창함. Formal potential사용 계산전위값과실험단위값유사할경우 ( 단, 전해질의종류와농도가크게다른계에서는더큰오차 )

24 @ Effect of complexation on the electrode potential. 침전생성물혹착형성물 -> 전극전위에영향 ex) Zn / Zn 2+ // Cu 2+ / Cu 의전극에 CuSO 4 solution 에 EDTA 첨가하면 -> Cu 2+ + EDTA 4- Cu EDTA 2-

26 22B-2 Liquid - Junction Potentials * 서로접촉하고있는 ionic solution 사이에서두용액의이온이동도의차에 의해생기는전위 A) Diffusion potential Liquid junction potentials, different mobilization & concentrations of ions in electrolytes in contact. B) Donnan potential 두전해질간의계면을가로지르는 1 개혹은많은종류의 ions 의전이의완전한방해로인한전위. 이러한접촉전위를무시하기위해 salt bridge, porous glass Kl : salt bridge. C) Liquid junction 의변수 Transport number Charge Activity of the ions forming the junctions

28 1) Salt bridge 2) Cracked glass bead 3) Ceramic frit 4) Sleeve 5) Gauntly or asbestos fiber, wick 6) Platinum wire 7) Cellulose pulp 8) Glass frit 9) Cellophane 10) Fine capillary drip 이유 : 분석실험에서참조전극의전위는일정해야하고지시전극전위만변해야한다 Liquid junction이무시되어야한다.

29 22C Electrode Potentials E cell = E right E left (22-12) 22C-1 Nature of Electrode Potentials 22C-2 The Standard Hydrogen Electrode 22C-3 Practical Reference Electrodes

30 22C-4 Definition of Electrode Potential 22C-5 Sign Conventions for Electrode Potentials Implications of the IUPAC Convention Half-Cell Potentials

31 22C-6 Effect of Activity on Electrode Potential 22C-7 The Standard Electrode Potential,E 0 22C-8 Measuring Electrode Potentials

33 22C-9 Calculating Half-Cell Potentials from E 0 Values

34 22C-10 Electrode Potentials in the presence of Precipitation and Complex- Forming Reagents

35 22C-11 Some Limitations to the Use of Standard Electrode Potentials Substitution of Concentration for Activities Effect of Other Equilibria Formal Potentials Reactio Rates

36 22D Calculation of cell potentials from electrode potentials

37 22E Currents in an electrochemical cell Cell potential ( 전극전위의대수합 ) 1 Thermodynamic cell potential 2 Liquid junction potential 3 Ohmic potential 4 Polarization potential { 농도편극, 과전압 }

38 1 Thermodynamic cell potential 2 Liquid junction potential

39 관계 ID 가 rid2 인이미지부분을파일에서찾을수없습니다. 3 Ohmic potential : IR drop ( 책 22E-1 내용 ) 전류발생, 전해, 갈바니전지의전위에영향 ( 전지의자체저항으로인해발생 )(1R 강화 ) E = 0.74V If 전지자체저항 4 ohm, 전류가 0.02amp 이면 -0.08V 의 ir drop 발생 전해질의경우 -0.82V 加해야한다 갈바니전지의경우 0.66V 만생긴다 E cell = E right - E left - IR

40 4 Polarization potential concentration polarization ( 책 22E-2 내용 ) overvoltage(kinetic polarization) 전류가흐르는동안표준전극전위값과 IR 강하로부터계산한값에서벗어나게하는인자. Fig Curves for an ideal(a) polarized (b) nonpolarized electrodes 편극에영향을주는인자 : 전극의모양, 크기, 전해질용액의조정, 용액의저어짐 온도, 전류의크기, 반응물, 생성물의물리적상태, 전극물질의조성등.

41 concentration polarization

42 1) Ideally non-polarized electrode. 용액-전극계면을통한전하이동이없는전극 (condenser) 으로작은전류가흐를경우그전극전위는안정하다. 즉, 가역전극과유사 그것의전위는용액중의화학종의활동도에만지배 2) Ideally polarized electrode. 용액-전극계면을통한전하의이동이자유로운전극. 즉, 유사전극은 KCl 용액속에서수은전극 (polarography에응용 ) K + + e K (amalgam), 2Hg + Hg e( 평형농도가낮다 ) 2Cl - Cl 2 + 2e ( 부분압이낮다 ), 2H 2 O + 2e - H 2 + 2OH - ( 수소에대한과전압이높다.) * Capacitance of an electrode 전극표면의전기이중층을 condenser로생각

43 3) Depolarizer : polarized electrode에서산화환원될수있는물질을첨가하므로해서전류를흘릴수있는물질 4) Reversible : 산화환원의속도가너무빨라서전극반응과정이평형상태에있다면이반응은가역적이다. 전류흐름의방향을바꾸면바꾸기전의반응의역반응이일어나는반응가역성 : 전기화학적측정의신속함의상대적속도와전극과정의상대적속도에의존

45 * 종류 : Concentration polarization, charge transfer polarization, kinetic polarization,( 과전압으로표시 ) * 농도편극 : 용액 bulk 의농도와전극표면농도사이의평형속도가 느리고전지를통해흐르는전류의크기의차이에의해생기는현상 * 과전압 : 전해전지에서농도편극이일어나지않을경우에도이론치보다 큰전압이必要이전압차를 overvoltage

46 Fig Electrode Surface layer Solution bulk * 전극과정에포함되는단계 1 Change transfer limiting step : activation polarization 2 Mass transfer limiting step : concentration polarization 3 Chemical reaction or adsorption

47 관계 ID 가 rid2 인이미지부분을파일에서찾을수없습니다. 5) Overvoltage(overpotential) a) Activation overvoltage; Slow electron transfer 의경우 high activation En 要 b) Resistance overvoltage; 전해질용액 전류속이통과한경우발생 ir drop 생김, 전도도유한. 발생 : 전류표면에반응생성물의 adherent layer 가생기기때문에전기전도도를감소 c) Concentration overvoltage; Electrode 의 vicinity 에의한농도변화때문에발생되는 concentration polarized 제거 : stirring 위의 3 가지전극과정에의해평형전위값으로부터전극전위가변하게 된다. 이변위의크기를 overvoltage η = ΔΦ - ΔΦe ΔΦ : Potential difference across the interface ΔΦe : Potential difference across the interface at equilibrium

48 * Non-polarizable electrode : 전극과정이무한히빠르다면 Reference electrode 로사용이가능하며이것을 non polarizable electrode라한다. 1) Activation overvoltage : slow electron transfer 2) Resistance overvoltage ir drop 발생 3) Concentration overvoltage η = ηa + ηc + ηr

49 Sign Conventions & the Nernst Equation Ox + ne - = red R = the molar gas constant(8.314j/mole-k) T = 절대온도 F : coulombs/mole Equilibrium potential : 평형상태에있는전극전위 Or reversible potential : ΔΦe Electrode potential : 전극상이용액상에대하여갖는전위차 ΔΦe η>0 : 산화반응 I >0 I + > I - η<0 : 환원반응 I >0 I = I + + I - I + =n FVOX I - = -nfvred

50 22E-3 Mechanisms of Electrochemical Mass Transport Migration, convection, diffusion 의 3 가지형태가존재. 1) Migration : 전기장내에하전된입자가놓이게되면 electrical gradient은하전된물질로이동이발생 ( 전기적이동 ) 방지 : 바탕전해질 or indifferent electrolytes을사용하면바탕전해질에의해단지농도기울기에의한물질의이전만일어난다. 2) Convection : gross physical movement 에의한전극활성종의이동

51 3) Diffusion : 농도기울기에의해생기는물질의자연적이동 most widely studied dc/dt=k(c-c 0 ), dc/dt= kc

52 22E-4 Charge- Transfer Polarization 1. Overvoltages increase with current density (current density is defined as the current per unit area 22F Types of electroanalytical methods

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untitled Chapter 4 Reactions in Aqueous Solution 3 4.1 Pb(NO 3 ) 2 (aq) + 2KI (aq) HCl (aq) + NaOH (aq) Alka-Selter/Water Cu wire/ag(no 3 ) (aq) 3 4.2 (Solution) KMnO 4 (s) K + (aq)+ MnO 4- (aq) 3 4.3 (Molarity)