物理化学研究方法3.ppt

Potential Step（Chronocoulometry),OCP,Current Step,Voltammetric Analysis,EIS,Polarography,RDE RRDE,Cyclic voltammetry,Cyclic potential sweep,Resulting cyclic voltammogram,(initial potential and switching potential),Nernstian (reversible) systems,Peak current is linear with square root of scan rate No effects of scan rate on peak potential Reductive peak current is equal to oxidative peak current Value of peak potential difference is 58 mV/n,Peak current and scan rate,At 25, ip is,Totally irreversible systems,Three-electrode cell and notation for the different electrodes,Scan rate,In LSV, the potential is scanned from a lower limit to an upper limit Unit of scan rate(): V/s or mV/s Effects of scan rate on charging current:,Differential Pulse Voltammetry/Polarography,Differential Pulse Voltammetry/Polarography,Electrode types in Voltammetric Analysis,Static Mercury Drop Electrode,Dropping Mercury Electrode,多功能汞工作台VA663,Sampled Current Polarography,Sampled Current Polarography,Sampled Current Polarography,Potential step,Signal,Resulting i-t curve,Concentration profiles for various times into the experiment,Potential wave form for chronoamperometry and chronocoulometry,Change Parameters dialog box for chronoamperometry/ chronocoulometry,Current-time response for a double-potential step chronoamperometry experiment,Potential step under diffusion control,Cottrell equation: Instrumental and experimental limitations: (1) Potentiostatic limitations, (2) limitations in the recording device, (3) limitations imposed by Ru and Cd, (4) limitations due to convection: the available “time window” for Cottrell measurements lies approximately between 20 sec and 200 sec.,Limitations imposed by Rs and Cd,Note that for a potential step input, an exponentially decaying current is obtained, with a time constant, =RsCd, hence, the current for charging the double-layer capacitance drops to 37% of its initial value at t=,Chronocoulometry,At t=0, the potential is shifted to E2, which is sufficiently negative to enforce a diffusion-limited current. The following equation describes the chronocoulometric response:,Charge-time response for a double-potential step chronocoulometry/chronoamperometry experiment,Effects of the additional components on chronocoulometric response,Where Qdl is the capacitive charge and nFAquantifies the faradaic component given to the reduction of of adsorbed species:,Bulk electrolysis methods,The bulk electrolytic methods are characterized by large A/V conditions and as effective mass transfer conditions as possible. Current efficiency:,Current excitation signal for a double step chronopotentiometry experiment,Current Step,Change Parameters dialog box for chronopotentiometry,Change Parameters dialog box for double step chronopotentiometry,Typical potential response for a chronopotentiometry experiment,Current Step,When the RsCd circuit is charged by a constant current i, for a current step, assuming a constant Cd, the potential increases linearly with time.,Sand equation,The measured value of at known i can be used to determine C* or D. A lack of constancy of the transition time constant, i1/2/C*, with i or C* indicates complications to the electrode reaction from coupled homogeneous chemical reactions, adsorption, double-layer charging or the onset of convection.,Rotating disk electrode,Rotating disk electrode,RDE is rather simple to construct and consists of a disk of the electrode material imbedded in a rod of an insulating material. RDE is rotated at a certain frequency, f(revolutions per second), but the parameter of interest is the angular velocity, (sec-1), where =2f.,Schematic resultant streamlines,Theoretical treatment of convective systems,Within the layer, 0x, no solution movement occurs and mass transfer takes place by diffusion. Thus the convection problem is converted to a diffusional one in which the adjustable parameter is introduced. In this model, it is assumed that convection maintains the concentrations of all species uniform and equal to the bulk values up to a certain distance from the electrode, ,Levich equation,This equation applies to the totally mass-trasfer-limited condition at RDE and predicts that the limiting current is proportional to C* and 1/2. is called the kinematic viscosity and has units of cm2/s, for water and dilute aqueous solutions near 20, is about 0.01 cm2/s.,The concentration profile,General current-potential curves at the RDE,Rotating ring-disk electrode,Radius of disk: r1, Radius of insulator: r2, Radius of ring:r3,The collection efficiency,The collection efficiency can be calculated from the electrode geometry, since it depends only on r1,r2,and r3, and is independent of ,C*,D, etc.,RRDE apparatus: Biopotetiostat,理论 欧姆定律给出直流电位和直流电流的简单关系: E = i R (1),当有交流参与时，关系式为: Eac = iac Z (2), 其中Z为阻抗,Electrochemical Impedance Spectroscopy,Electrochemical Cell,+ - + - + - + - + -,WE,CE,RE,POTENTIOSTAT GALVANOSTAT,Diff. ampl.,s,D,E,F,G,A,B,C,H,I,+ - + - + - + - + -,Solution Resistance,Capacitor double layer,D,E,+ - + - + - + - + -,Polarization Resistance,Rsol,Rct,Cdl,D,E,F,HIGH FREQUENCY,LOW FREQUENCY,Bode图: lg(Z)lg(f) 和 lg(f),Nyquist图: Z -Z”,Spectroelectrochemistry (Ultraviolet Visible Spectroscopy),研究物质在紫外、可见光区的分子吸收光谱的分析方法称为紫外-可见分光光度法。 紫外可见分光光度法是利用某些物质的分子吸收200 800 nm光谱区的辐射来进行分析测定的方法。这种分子吸收光谱产生于价电子和分子轨道上的电子在电子能级间的跃迁，广泛用于无机和有机物质的定性和定量测定。,Ultraviolet Visible Spectroscopy,Types of electronic transitions,Ultraviolet Visible Spectroscopy,s to s*,alkanes(烷烃) 150 nm,p to p*,h to s*,(含硫化合物),h to p*,Choice of Solvent,分子光谱的吸收是带光谱,I0,It,定量公式：,分子吸光度测定标尺量程设计,T,A,0 25% 50% 75% 100%, 0.602 0.301 0.125 0,取代基或溶剂的影响,红移现象：由于取代基或溶剂的影响使最大吸收峰向长波方向移动的现象称为红移现象。 蓝移现象：由于取代基或溶剂的影响使最大吸收峰向短波方向移动的现象称为蓝移现象。 增色效应：使值增加的效应称为增色效应 减色效应：使值减少的效应称为减色效应。,物质颜色和吸收光的关系,紫外可见吸收光谱示意图,末端吸收,最强峰,肩峰,峰谷,次强峰,max min ,A,吸收带吸收峰在吸收光谱上的波带位置,（1）R 吸收带： n*跃迁 特点：a 跃迁所需能量较小，吸收峰位于 200400nm b 吸收强度弱， 102 （2）K 吸收带： 共轭双键中*跃迁 特点：a 跃迁所需能量较R带大，吸收峰位 于210280nm b 吸收强度强， 104 随着共轭体系的增长，K 吸收带长移， 210 700nm 增大。,例： max 1-己烯 177 104 1.5-己二烯 178 2104 1.3-己二烯 217 2.1 104 1.3.5-己三烯 258 4.3 104 K 吸收带是共轭分子的特征吸收带，可用于判断共轭结构应用最多的吸收带,B吸收带：有苯环必有B吸收带 230270nm之间 有一系列吸收峰，中吸收，芳香族化合物的特征吸收峰。,苯环上有取代基并与苯环共轭，精细结构消失,E 吸收带：*跃迁 E1=185nm 强吸收 104 E2=204 nm 较强吸收 103,苯在乙醇中的紫外吸收光谱,苯在185 nm和204 nm处有两个强吸收带，分别称为E1和E2吸收带，是由苯环结构中三个乙烯的环状共轭体系的跃迁产生的，是芳香族化合物的特征吸收。 在230270 nm处有较弱的一系列吸收带，称为精细结构吸收带，亦称为B吸收带。B吸收带的精细结构常用来辨认芳香族化合物。,精细结构：,小结： R带 n* 弱吸收 K带 *强吸收 共轭 B带 *中吸收 E带 *强吸收,苯环,卟啉在紫外可见区域具有特征吸收：420 nm 附近的强吸收峰称为Soret