電化學(xué)分析---循環(huán)伏安法

1、Cyclic Voltammetry（循環(huán)伏安法）（循環(huán)伏安法）Figure 1.8Figure 1.9Figure 1.10Reversible system:The peak current for a reversible couple (at 250C) is given by Randles-Sevcik equation: Where n is the number of electrons, A is the electrode area (in cm2), C is the concentration (in mol/cm3), D is the diffusion coeff

2、icient (in cm2/s), and is the scan rate (in V/s).(1.6)(1.7)(1.8)Figure 1.11Q. L. Echegoyen et al, J. Am. Chem. Soc. 1992,114,3978Irreversible and Quasi-reversible Systems: (1.9)(1.10)Figure 1.12 不可逆過程電流和電勢隨掃描速度的變化不可逆過程電流和電勢隨掃描速度的變化Study of Adsorption Processes(1.11)Figure 1.13Figure 1.14(1.12)Study

3、of Reaction MechanismsQuantitative ApplicationsPolarography (極譜極譜) and Voltammetry (伏安法伏安法)的區(qū)別？的區(qū)別？polarographyA classical electroanalytical technique discovered in 1922 by J. Heyrovsky, for which he was awarded the Nobel Prize for Chemistry in 1959. Essentially, it is linear-sweep voltammetry using

4、 a dropping-mercury electrode for working electrode and a large mercury pool as counter electrode.voltammetryAn electrochemical measuring technique used for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions. Voltammetry is a family of techniques w

5、ith the common characteristics that the potential of the working electrode is controlled (typically with a potentiostat) and the current flowing through the electrode is measured. In one of the most common applications of the technique, the potential is scanned linearly in time; this is called the l

6、inear-sweep voltammetry, LSV, or LV. Cyclic voltammetry (CV) is a linear-sweep voltammetry with the scan continued in the reverse direction at the end of the first scan, this cycle can be repeated a number of times.6.7多電子轉(zhuǎn)移的電極過程多電子轉(zhuǎn)移的電極過程對于兩電子還原反應(yīng)，一般的表達(dá)式為：對于兩電子還原反應(yīng)，一般的表達(dá)式為：A+ e BB+ e CFig.6.7 兩電子轉(zhuǎn)移反

7、應(yīng)示意圖兩電子轉(zhuǎn)移反應(yīng)示意圖對于此過程，對于此過程， 可以分為三個(gè)極限情況進(jìn)可以分為三個(gè)極限情況進(jìn)行討論行討論：(1) 第二步在更負(fù)的電位下反應(yīng)， 我們可以觀察到兩個(gè)分開的波。即： kf,2 - kb,2 kb,1B + eC fast(3) 第二步是決速步驟，即：kf,2 - kb,2 kb,1A + e B pre-equilibriumkf,2 kb,1B + eC rate-determining stepDue to pre-equilibrium, the voltammetric wave is steeper than in case 2. The activated comp

8、lex is more sensitive than in case 2 to changes in applied potential.Fig.6.8 Voltammograms for the reduction of species A followingA+e B +e C according to the relative rates of the two steps. IL = IL (A B). (a) Second step much more difficult than the first; (b)First step rate-determining; second st

9、ep fast; ( c)First step pre-equilibrium; second step rate-determining.3.4 特性吸附特性吸附即使電場不存在也能發(fā)生的吸附即使電場不存在也能發(fā)生的吸附- 特性吸附特性吸附特性吸附使零電荷電勢特性吸附使零電荷電勢(EPZC)發(fā)生移動。發(fā)生移動。負(fù)離子使之負(fù)移，正離子使之正移。為什么？負(fù)離子使之負(fù)移，正離子使之正移。為什么？為什么表面活性劑能夠使微分電容在為什么表面活性劑能夠使微分電容在EPZC處變處變化很大？化很大？答案：見吳浩青，李永舫答案：見吳浩青，李永舫電化學(xué)動力學(xué)電化學(xué)動力學(xué)p2426.吸附等溫線吸附等溫線(isoth

10、erm)：(1)Langmuir; (2)Temkin; (3)Frumkin.圖圖3.20特性吸附對電毛細(xì)曲線的影響特性吸附對電毛細(xì)曲線的影響-+-陽離子的活性順序是：陽離子的活性順序是：N(C3 H7)4+ Ti+ K+陰離子的活性順序是：陰離子的活性順序是：S2- I- Br- NO3-圖圖3.21 陰、陽離子吸附對于電毛細(xì)曲線的影響陰、陽離子吸附對于電毛細(xì)曲線的影響圖圖3.22吸附等溫線吸附等溫線(isotherm)：(1)Langmuir; (2)Temkin; (3)Frumkin圖圖3.23 各種吸附等溫線各種吸附等溫線Langmuir isotherm:Langmuir 假設(shè)：

11、假設(shè)：(a)吸附在電極表面的分子彼此沒有相互作用；吸附在電極表面的分子彼此沒有相互作用；(b)表面吸附是均勻的；表面吸附是均勻的； (c) 在高濃度時(shí)電極表面達(dá)到飽和濃度在高濃度時(shí)電極表面達(dá)到飽和濃度(單分子層單分子層)， 用用 s 代表。代表。 = i / s (覆蓋度覆蓋度)。i i = /(1- ) = i /(s - i )Esin-Markov coefficient, 吸附等溫線吸附等溫線(isotherm)：Temkin isotherm: 主要考慮到吸附能是主要考慮到吸附能是覆蓋度有關(guān)。覆蓋度有關(guān)。 i = RT/(2g)ln( i i )g是一個(gè)與吸附物質(zhì)之間相互作用能有關(guān)的

12、參數(shù)。是一個(gè)與吸附物質(zhì)之間相互作用能有關(guān)的參數(shù)。Frumkin isotherm: i = RT/(2g)ln(i i )ln(s - i )/ i g是正值時(shí)表示吸附物質(zhì)之間是吸引作用，是正值時(shí)表示吸附物質(zhì)之間是吸引作用，g是負(fù)值時(shí)表示是負(fù)值時(shí)表示吸附物質(zhì)之間是排斥作用。吸附物質(zhì)之間是排斥作用。常用的研究物質(zhì)在電極表面吸附的電化學(xué)方法有循常用的研究物質(zhì)在電極表面吸附的電化學(xué)方法有循環(huán)伏安法和電勢雙階躍法環(huán)伏安法和電勢雙階躍法循環(huán)伏安法循環(huán)伏安法圖圖3.24吸附物質(zhì)的循環(huán)循環(huán)伏安圖吸附物質(zhì)的循環(huán)循環(huán)伏安圖圖圖3.25圖圖3.26R.H.Wopschall and I.Shain, Anal.Chem.,39, 1514(1967)Why do we view adsorbed neutral Species as being intimately bound to the electrode surface, rather thanBe