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Chapter 22 An Introduction to Electroanalytical Chemistry 전기 화학 1) 배경

Chapter 22 An Introduction to Electroanalytical Chemistry 전기 화학 1) 배경 * 물질은 본질적으로 전기적이다 . 따라서 모든 화학이 전기 화학과 관련이 있다 . * Electrochemistry (Ionics and Electrodics) * Babylonians as early as 500 BC used the galvanic cell ← 50 년 전에 König 가 Bagdad 부근에서 발굴

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Chapter 22 An Introduction to Electroanalytical Chemistry 전기 화학 1) 배경

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  1. Chapter 22 An Introduction to Electroanalytical Chemistry 전기 화학 1) 배경 * 물질은 본질적으로 전기적이다. 따라서 모든 화학이 전기 화학과 관련이 있다. * Electrochemistry (Ionics and Electrodics) * Babylonians as early as 500 BC used the galvanic cell ← 50년 전에 König가 Bagdad 부근에서 발굴 The formations of electrochemistry are to be found in the late 18th -century investigation by Galvani and Volta. Galvani → frog leg 실험, 1786년 (전기 화학의 시작) ↓ Alecssandro Volta

  2. ◉ Volta Cell (battery) led to the beginning of physical electrochemistry. ◉ Ostwald and his associates developed the electrochemical techniques and theories quite rapidly. ◉ William Nicholson (1753-1815) and Anthony Carlisle(1768-1840) used a pile to electrolyze water and various solutions. ◉ Jones Berzelius (1779-1848) and William Hisinger (1766-1852) at 1803 electrolyze salts. ◉ Davy(1778-1848) and Faraday(1791-1867) invented electric motor, generator, and the transformer. Fused salt electrolysis (1807). ◉ In 1805, ChristiaGrotthus (1785-1822) explains why electrolysis products appear only at the electrodes. ◉ Determination of transference numbers were beginning to open up.

  3. ◉ Henry Cavendish (1731-1810) explains relation conductances of water and salt solutions as early as 1776. ◉ In 1847 Eben Horsfoed (1818-1893) studied the effects of electrode polarization. ◉ Friedrich Kohlrausch (1840-1910) in 1869 introduce techniques for the ac method. ◉ Savante Arrehenius (1859-1927) developed his theory of electrolytic dissociation. ◉ In the period 1853 to 1859 Johann Hittorf (1824-1914); the analysis, after electrolysis, of the solutions around one or both of the electrodes. -Λo values has been subjected to considerable theoretical study and correction, notable by Peter Debye (1884-1966) and Erich Huckel (1896-1980) and by Lars Onsagar(1903-1976)

  4. ◉ Electrical energy - Basic were the investigations of these relationships carried out by James Joule(1818-1889) • ◉ Ostwald, Father of physical Chemistry(1853-1932), 1884, Professor of Chemistry • He interested in the emfs of the cells, (He thought that it should be possible to employ a DME as the basis of a system of absolute potentials) → factors that govern the emf of cells • ◉ Walther Nernst (1864-1941) : assistants of Ostwald in 1889. Nernst's announcement of the relationship that is the basis of electorlytic potentiometry - Nernst equation. • ◉ Theodore Richards studied with Ostwald in 1894. • Major contributions to fields such as precise coulometry.

  5. ◉ Max Le Blanc(1865-1943) - ① Studied decomposition voltages of solutions at Pt electrodes. ② In HCl, halogen can replace oxygen as an anodic product. ③ Separately determine the anodic and the cathodic potentials. ◉ Wilhelm Bottger (1871-1949) used the hydrogen electrode to study the potentiometric titration curves of acids and bases. ◉ Standardization of Potentials. The need for standard electrode potentials increase interest in potentiometry. In 1890 Ostwald introduced the calomel electrode. Nernst chose the NHE, assigning to it a potential of zero. ◉Gilbert Lewis(1875-1946) - The concept of ionic activity in 1907. Lewis used this concept to set up a listing of electrode potentials based on the standard hydrogen electrode. In 1953, international agreement confirmed the hydrogen scale.

  6. <Preparative Electrochemistry> -Electrolytical producing Aluminum is quite expensive by chemical means. -Cryolite - Alumina electrolytic process in 1886 by C.M.Hall in the U.S.A and by P. Herault in France. ◉ Humphry Davy - potassium, sodium, and other reactive metals in 1807. Fused-salt electrochemistry - Aluminum production is representative ◉ Synthesis of inorganic compounds by electrochemical means contains more than 4,000 ◉ Bromine winning form brine.

  7. <Organic Electrochemistry>  ◉ Faraday, Schoenbein & Kolbe as the founders ex) - Electroreduction of nitrobenzene - dye stuffs precursors. - Controlled - potentital electrolysis - Fritz Haber introduce it. - Various alkaloids are formed in living system. - Large-scale, adiponitrile - nylon intermediate. <Some Other Aspects of Industrial Electrochemistry> - Modern electrodeless sensing system. - Copper - electrorefining - Electrochemical Machining involves the controlled removal of metal

  8. <Instrumentation>   A basics requirement is the ability to measure electrical quantities such as potential, current, & resistance or conductance. ① Oersted's 1820 discovery of the deflection of a magnetic needle by an adjacent current carrying wire. ② William Thomson designed the galvanometer. - Ink-jet printing. ③ Poggendorf in 1841 - null-point method - Wheatstones. <The move toward the Electronic Age> - Galvanometric pen recorders - 1890.후반 - X-Y recorder - in 1951. - Photographic recorder - 1925. ◉ Development such as that of the vacuum triode, cathode ray tube and, transistor and solid-state IC. ◉ Automation

  9. <Electroanalytical Chemistry> Emeritus professor I. M. Klothoff He is the scientific ancestor of generations of electroanalytical chemist. 1) Electrogravimetry; ① Szabadvay devotes an entire chapter ② 1st Electrogravimetric Determination - Wolcott Gibbs (1864) in 1865, C. Luckow(German) 2) Potentiometry; ① Ion selective electrodes. ② Glass electrode The 1st potentiometric titration - R. Behrend in 1893. The 1st monograph, by E. Mueller (1923)

  10. 3) Conductometry; The 1st analytical determination by this method occurred in 1895 (I. M. Kolthoff) 4) Coulometry; Faraday's law - 1834. Silver coulometer, other chemical coulometer. The first major development occurred in 1938, Hungarian, L. Szebeooedy & Z. Somoggi described the titration of HCl with OH - a good example is the technique of spectroelectrochemistry

  11. 5) Voltammetry; The discovery of polarography - DME - In 1952. stripping voltammetry - Pulse technique(1958) - Cyclic voltammetry - diagnosis of electrode reaction(1964) 6) Amperometric titration; Titration at one indication electrode was first reproved by Heyrovsky & Berezicky in 1929 - I. M. Kolthoff continued intensively.

  12. <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) Fe3+ + Cu+ = Fe2+ + Cu2+ (1) Oxidizing Reducing Agent Agent Fe3+ + e = Fe2+ Cu+ - e = Cu2+

  13. 2) Relation Between Chemistry and Electricity • Electrical Measurement • i) Charge is measured in Coulombs, C • - C of single electron: 1.6021892×10-19C • One mole of electrons has a charge of 9.648456×104C • 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

  14. 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]

  15. 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

  16. 5) Standard potentials i) Standard reduction potential. ii) SHE → NHE ← Potential assign zero - Using the Nernst equation. Anode : H2(g) ---> 2H+ + 2e- Cathode : Cd2+ + 2e- ---> Cd(s) Eo = -0.402 ----------------------------- Net : Cd2+ + H2(g) ---> Cd(s) + 2H+ E(cell) = Eo(cell) - (0.05916/2) log ([H+]/[Cd2+]PH2) pH --> 1 if E=0 --> [H+] = 1.6×10-7 Solubility Product: [Ag+] = (Ksp([AgCl])/[Cl-] = [(1.8×10-10)/0.0334] = 5.4×10-9M (K --> Eo) E(cell) = Eo-(0.0591/n) log Q (at any time) O = Eo-(0.0591/n) log K (at eq) (0.0591/n) log K = Eo or K = 10nEo/0.05916 (at 25 C)

  17. ex) FeCO3(s) + 2e- --> Fe(s) + CO32- Eo = - 0.0756 V Fe(s) --> Fe2+ + 2e- Eo = - 0.440 V ----------------------------------------------------------------- FeCO3(s) --> Fe2+ + CO32-(K=Ksp) Eo = - 0.316V Ksp = 10(2)(-0.316)/(0.0591) = 2×10-11 - Using Cells as Chemical probes a) Eq. between the two half-cells b) Eq. within each half-cell

  18. <밧데리와 축전지> 1. 건전지 : Leclanche 전지 or dry cell (1.5V) (ZnCl2. NH4Cl, MnO2, Zn, C) Anode : Zn(s) ---> Zn2+ + 2e- Cathode : 2NH4+ + 2e- ---> 2NH3(g) + H2(g) 전극 : Carbon - 폭발 방지 2MnO2(s) + H2(g) ---> Mn2O3(s) + H2O(l) Zn2+(aq) + 2NH3(g) + 2Cl- ---> Zn(NH3)2Cl2(s)   2MnO2(s) + 2NH4Cl + Zn(s) ---> Zn(NH3)2Cl2 + H2O + Mn2O3(s) 2. 알칼리 밧데리. (1.54V) Zn(s) + 2OH- ---> ZnO(s) + H2O(l) + 2e- 알칼리 용액 2MnO2(s) + H2O(l) + 2e- ---> Mn2O3(s) + 2OH- -->기체발생 없음

  19. 3. 수은 밧데리 (1.35V) Anode : Zn(s) + 2OH-(aq) ---> ZnO(s) + H2O(l) + 2e- Cathode : HgO(s) + H2O(l) + 2e- ---> Hg(l) +2OH-(aq) 4. 축전지 - 납 축전지: 다공성 납+불용성 산화 납(10) in C-H2SO4 Anode : Pb(s) + SO42-(aq) ---> PbSO4(s) + 2e- Cathode : NiOOH(s) + H2O(l) + e- ---> Ni(OH)2(s) + OH-(aq) 5. Fuel Cell Anode (H2의산화): 2H2(g) + 4OH-(aq) ---> 4H2O(g) Cathode (O2의 환원): O2(g) + 2H2O(l) + 4e- ---> 4OH-(aq) 70 - 140 C에서 정상적으로 작동 Output ---> 0.9V

  20. <전기 분해> Fused salt Anode : 2Cl- ---> Cl2(g) + 2e- Eo = - 1.36V Cathode : Na+ + e- ---> Na(s) Eo = - 2.71V --------------------------------- Net 2Cl- + 2Na+ ---> 2Na(s) + Cl2(g) 1) Aluminum 초기 : 3Na(s) + AlCl3(s) ---> Al(s) + 3NaCl(s) 전기 분해 : Anode: O2발생 Cathode: Al 2) Na 제조 : Humphrey Davy NaOH ---> Na 3) Cl2 and NaOH 제조 NaCl ---> Cl2 + 2e- Na+ + e- + Hg ----> Na(Hg) 2Na(Hg) + 2H2O(l) ---> 2NaOH(aq) + H2(g) + Hg(l)

  21. { Corrosion } Fe ----> Fe2O3.H2O 25%부식으로 소모 2Fe(s) + 3O2(g) ---> 2Fe2O3 4Cu(s) + O2(g) ---> 2Cu2O(s) 4Al(s) + 3O2(g) ---> 2Al2O3(s) Anode : M(s) ---> Mn+ + ne- Cathode : 2H+(aq) +2e- ---> H2(g) 2H2O(g) + 2e- ---> 2OH-(aq) +H2(g) O2(g) + 2H2O(l) + 4e- ---> 4OH-(aq) 철의 부식(산소) Anode : Fe(s) ---> Fe2+(aq) + 2e- Cathode : 2H2O(l) +2e- ---> 2OH-(aq)+ H2(g) -------------------------------------------------- Net : Fe(s) + 2H2O(l) ---> Fe2+(aq) + 2OH-(aq) + H2(g) Fe(OH)2(g) - Fe2O3

  22. 물 + 산소 존재 시---> 산소 없을 때보다 100배 빨리 일어남. Anode : 2[Fe(s) ---> Fe2+(aq) + 2e-] Cathode : O2(g) + 2H2O(l) + 4e- ---> 4OH-(aq) --------------------------------------------------- Net : 2Fe(s) + O2(g) + 2H2O(l) ---> 2Fe(OH)2(s) 6Fe(OH)2(s) + O2(s) ---> 2H2O(l) + 2Fe2O3·H2O ---> 2Fe3O4·H2O(s) + 4H2O(l) 녹색. 자황색 Fe3O4·H2O(s) ---> H2O(l) + Fe3O4(s) 흑색. 자철광 Cl-이온의 부식에 대한 영향 - Anodic inhibition : 산화 반응을 제한 또는 방지 ex) 2Fe(s) + 2Na2CrO4(aq) + 2H2O ---> Fe2O3 + Cr2O3 + 4NaOH(aq) 철의 표면을 Na2CrO4로 산화. - Cathodic protection : 더 빨리 산화되는 다른 금속에 연결하여 금속이 음극이 되도록 함. ex)양철

  23. 1.1 Electrochemical method의 일반성 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 % 사용 빈도가 적은 이유 : ① 교과 과정이 강조되어 있지 않다. ② 자동화의 난점

  24. A) 전기 화학적 방법의 특징 ① Inexpensive ② Specific for a particular chemical form ③ Concentration보다 activity에 감응 B) Electrochemical method의 classification Electrochemical method : 화학적 계 혹은 시료의 전기적 응답이 측정되는 것 C) Experimental system ⓛ Electrolyte ② Detector (electrode) ③ Circuit D) Electrochemical method

  25. Potentiometric • All others (voltammetry, coulometry, conductometry, etc.) • ① Potentiometry : J. Willard Gibbs → Nernst(실제로 개발) • System에서 전류를 끌어내거나 전기분해 없이 계의 열역학적 평형 전위를 측정 • ② All others : voltage or current가 전극에 가해져서 그 때 전류를 흘리거나 혹은 voltage를 변화시키거나 하여 system을 monitor하는 것

  26. 22A Electrochemical cells 구성 - 2개의 반쪽 전지의 연결 (1) Electrode - Anode → oxidation - Cathode → reduction 1) Working and indicator electrode : A reaction take place. 2) Reference electrode : 전류의 변화에 무관하게 constant potential을 유지 3) Counter electrode : 참조 전류의 internal polarization을 피하기 위한 외부 전극 → E(working electrode) = (Ecell - iRcell - Epolarization) - Electrode의 저항이 무시되는 조건에서 전해질의 전도성 측정 - Electrolyte의 저항이 무시되는 조건에서 전극에서 발생되는 현상 측정

  27. (1) 종류 * Galvanic cell : chemical En. → electrical En. * Electrolytic cell : electrical En. → chemical En. Galvanic cell : Zn ⇔ Zn2+ + 2e- Cu2+ + 2e- ⇔ Cu

  28. (2) 표시법 Zn / Zn2+(aZn2+)∥Cu2+(a Cu2+) / Cu Left hand electrode : negative pole of cell  oxidation process occurs Zn → Zn2+ + 2e Right hand electrode :  Reduction process Cu2+ + 2e → Cu * 동일한 상 內에서 다른 화학 종 표시 Pt, H2(p=1atom) / H+(0.1M), Cl-(0.1M), AgCl(satd) / Ag

  29. 22A-5 Solution Structure Fig. 22-3 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.

  30. 22A-6 Faradaic and Nonfaradaic Currents • Faradaic process : 전극-용액 계면을 가로질러 전류가 이동하는 과정으로 Faraday's law에 따르는 과정 (전하의 이동이 일어난 때의 반응) 실제로 산화 환원 반응이 일어난다. • 2) Non-faradaic process : 전극-용액 계면을 전하가 이동하지 못하는 경우 condenser 현상으로 인한 과정 • 3) Charging current : non-faradaic process의 일 例 • 어떤 전위 EA에서 형성된 전기 이중층의 전위가 EB로 높아졌을 때 새로운 전기 이중 층을 만들기 위해 전류가 흘러야 한다.  그 때의 전류를 charging current.

  31. 22B Potentials in Electrochemical Cells Ecell = Eright - Eleft + Elj

  32. 22B1. Thermodynamic cell potential Nernst Equation ← thermodynamic relationship에 근거를 둔 potentiometric measurement ΔG = ΔH - TΔS = ΔE + PΔV -TΔS From Vant Hoff reaction equation = dE - TdS -SdT + PdV + VdP

  33. Ion강도의 영향 ∝ 이온의 전하, 활동도 대신 몰 농도를 사용 → error 유발 A) Standard electrode potential: 표준상태에서, 모든 반응물과 생성물이 l 의 활동도를 가질 때 표준 수소 전극을 기준으로 하여 측정한 그 반쪽전지의 전위. B) Formal electrode potential: 활동도 영향 및 부반응 (용매화, 해리, 회합, 착물 형성)에서 오는 전극 전위의 편차를 부분적으로 보상하기 위해 swift가 산화 환원 계산에서 표준 전극 전위 대신 formal 전극 전위 값을 사용토록 제창함. Formalpotential사용 → 계산 전위 값과 실험 단위 값 유사할경우 (단, 전해질의 종류와 농도가 크게 다른 계에 서는 더 큰 오차)

  34. @ Effect of complexation on the electrode potential. 침전 생성물 혹 착 형성물 -> 전극 전위에 영향 ex) Zn / Zn2+ // Cu2+ / Cu 의 전극에 CuSO4 solution 에 EDTA 첨가하면 -> Cu2+ + EDTA4- ⇔ Cu EDTA2-

  35. 22B-2 Liquid - Junction Potentials *서로 접촉하고 있는 ionic solution사이에서 두 용액의 이온 이동도의 차에 의해 생기는 전위 A) Diffusion potential Liquid junction potentials, different mobilization & concentrations of ions in electrolytes in contact. B) Donnan potential 두 전해질 간의 계면을 가로지르는 1개 혹은 많은 종류의 ions의 전이의 완전한 방해로 인한 전위. 이러한 접촉 전위를 무시하기 위해 salt bridge, porous glass Kl : salt bridge. C) Liquid junction의 변수 ┏ Transport number ┃ Charge ┗ Activity of the ions forming the junctions

  36. 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 : 분석 실험에서 참조 전극의 전위는 일정해야 하고 지시 전극 전위만 변해야 한다 ∴Liquid junction이 무시되어야 한다.

  37. 22E Currents in an electrochemical cell Cell potential (전극 전위의 대수 합) ① Thermodynamic cell potential ② Liquid junction potential ③ Ohmic potential ④ Polarization potential {농도 편극, 과전압}

  38. ① Thermodynamic cell potential ② Liquid junction potential

  39. ③ Ohmic potential : IR drop 전류 발생, 전해, 갈바니 전지의 전위에 영향 (전지의 자체 저항으로 인해 발생)(1R강화) E = 0.74V If 전지 자체 저항 4 ohm, 전류가 0.02amp 이면 -0.08V의 iR drop 발생 전해질의 경우 -0.82V加 해야 한다 갈바니 전지의 경우 0.66V만 생긴다

  40. ④ Polarization potential ┏ concentration polarization ┗ overvoltage(kinetic polarization) 전류가 흐르는 동안 표준 전극 전위 값과 IR강하로부터 계산한 값에서 벗어나게 하는 인자. Fig. 22-6 Curves for an ideal(a) polarized (b) nonpolarized electrodes 편극에 영향을 주는 인자 : 전극의 모양, 크기, 전해질 용액의 조정, 용액의 저어짐 온도, 전류의 크기, 반응물, 생성물의 물리적 상태, 전극 물질의 조성 등.

  41. 1) Ideally non-polarized electrode. 용액-전극 계면을 통한 전하이동이 없는 전극(condenser)으로 작은 전류가 흐를 경우 그 전극 전위는 안정하다. 즉, 가역 전극과 유사 ∴ 그것의 전위는 용액 중의 화학 종의 활동도에만 지배 2) Ideally polarized electrode. 용액-전극 계면을 통한 전하의 이동이 자유로운 전극. 즉, 유사 전극은 KCl 용액 속에서 수은 전극 (polarography에 응용) K+ + e ⇔ K (amalgam), 2Hg+ ⇔ Hg22+ + 2e( 평형 농도가 낮다) 2Cl- ⇔ Cl2 + 2e (부분압이 낮다), 2H2O + 2e- ⇔ H2 + 2OH- (수소에 대한 과전압이 높다.) * Capacitance of an electrode 전극 표면의 전기 이중층을 condenser로 생각

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