6

Dr. Professor: Zhang, Chengxiao

E-mail: [email protected]

Tel: 85307567

Part Two

Electrochemical Methods

Chapter 6 Introduction to Electrochemistry

Chapter 7 Potentiometry

Chapter 8 Bulk Electrolysis: Electrogravimetry and Coulometry

Chapter 9 Voltammetry

Analytical Electrochemistry 2004. 10; 2005. 10.31; 2006. 10.23; 2007. 2.10 -4.27;2008. 4.5-6.27.

Instrumental Analysis 2008.9 2009.9

Course Introduction

Bilingual Course

Education: Ph.D. in Chemistry, 1958, Harvard University M.A. in Chemistry, 1956, Harvard University B.Sc. in Chemistry, 1955, City College of New York Hackerman-Welch Regents Chair in Chemistry, The University of Texas, 1985-present. Research Interests: Electro-organic Chemistry Photoelectrochemistry Electrogenerated Chemiluminescence Electroanalytical Chemistry SECMEditor-in-Chief JACS. 20 yearsMa Fen. PhD. Candidate, ( SNNU, Anal. Chem. My Lab).

A conversation with Allen J. Bard

6.1 Characterizing oxidation/reduction reactions

6.2 Electrochemical cells

6.3 Electrode Potentials

6.4 Nernst Equation

6.5 Applications of Standard Electrode Potentials

Chapter 6

Introduction to Electrochemistry

Aim: Master

•Classification of Electrochemical Methods•Advantages of Electroanalysis•Electrochemical Cell•Electrodes•Nernst Equitation •Trends in Analytical Chemistry

Chapter 6

Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. A large part of this field deals with the study of chemical changes caused by the passage of an electric current and the production of electrical energy by chemicalreactions. In fact, the field of electrochemistry encompassesa huge array of different phenomena (e.g., electrophoresisand corrosion), devices (electrochromic displays, electro-analytical sensors, batteries, and fuel cells), and technologies (the electroplating of metals and the large-scale production of aluminum and chlorine). While the basic principles of electrochemistry discussed in this course apply to all of these, the main emphasis here is on the application of electrochemical methods to the study of chemical systems.

Electroanalytical methods are concerned with the interplay between electricity and chemistry, namely the measurements of electrical quantities, such as current, potential, or and their relationship to chemical parameters. Such use of electrical measurements for analytical purposes has been found a vast range of applications, including environmental monitoring, industrial quality, and biomedical analysis.

Electrochemistry Analytical Electrochemistry; Electrochemical Ananlysis;Electroanalysis

Variables affecting the rate of an electrode reaction

Classification of Electrochemical Methods

R

VI

(1) Electrical parameter: R: Conductometry; V: Potentiometry; I: Bulk Electrolysis Electrogravimetry and Coulometry Voltammetry(2) Current Value Zero: Small: Large:

(3) IUPAC recommendation 1976

A. No Involving double layer: Conductometry

B. Involving double layer; No electrode reaction: Potentiometry

C. Involving electrode reaction:

Electrolysis; Voltammetry

Electroanalysis as a representative of wet-chemical method has many attractive advantages, such as

selectivity and sensitivity, notwithstanding its inexpensive equipment;

ample choice of possibilities; and direct accessibility, especially to electronic and hence automatic control even at

distance; automatic data treatment; and simple insertion, if desirable, into a process-regulation

loop. 3S+2A

Advantages of Electroanalysis

In an oxidation/reduction reactions, electrons are transferred from one reactant to another. For example:


Ce4+ + Fe2+ Ce3+ + Fe3+

Cerium Ce4+: an oxidizing agent/oxidant, electron acceptor.Iron Fe2+ : an reducing agent/reductant, electron donor.

Oxidation/Reduction reactions are sometimes called redox reactions. So we can write a generalized equation for a redox reaction as:

(1)Ared reductant, reduced form, having given up electrons, becomes an oxidizing agent, Aox.

(2) Box, the oxidized form of species B, accepts electrons from Ared to form the new reductant Bred.

Ared + Box Aox+ Bred


6.2 Electrochemical cells

Constitute ：

Two conductors (called electrodes), Electrolyte solution , Salt bridge.

Zn ZnSO4(0.0200M) CuSO4(0.0200M) Cu

Ecell=Eright-Eleft

Types of electrochemical cells:

（ 1 ） Galvanic (or voltaic) cells store electrical energy. Batteries.

（ 2 ） Electrolytic cell requires an external source of

electrical energy for operation. Cathodes and Anodes （ galvanic or electrolytic cell ） The cathode ： reduction 阴极 The anode ： oxidation 阳极

What is electrode? First type conductor, Second type conductor.

Half-cell

Figure 6-1

Alessandro Volta (1745-1827), Italian physicist. He was the inventor of the first battery, the so-called voltaic pile (shown on the right). It consisted of alternating disks of copper and zinc separated by disks of cardboard soaked with salt solutions.


The Standard Hydrogen Reference Electrode (SHE)

The Standard Hydrogen Electrode(abbreviated SHE) is the universal ref

erence for reporting relative half-cell potentials. It is a type of gas el

ectrode and was widely used in early studies as a reference electrode.

The SHE is also called the “ Normal Hydrogen Electrode ”

What is Reference Electrode ( RE)?

Figure 6-3

To provide the reference

standard for measuring

potential.

The platinum electrode (Pt) is made of a small square of platinum foil which is platinized (known as platinum black). Hydrogen gas, at a pressure of 1 atmosphere, is bubbled around the platinum electrode. The platinum black serves as a large surface area for the reaction to take place, and the stream of hydrogen keeps the solution saturated at the electrode site with respect to the gas.

The potential of the standard hydrogen electrode is assigned a value of 0.000V at all temperatures.

How to make SHRE?


Calomel electrode 甘汞电极

Ag/AgCl electrode

Standard Electrode Potential (E0)

Pt, H2 (1.00 atm) | H+(a=1.00 ） || Ag+(a=1.00) | Ag

or SHE || Ag+(a=1.00) | Ag

When the silver ion activity( 活度 ) is 1.00, the cell pote

ntial E is the standard electrode potential of the Ag+/Ag.

E0Ag

+/ Ag =0.799V

Electrode Potential of Silver Electrode:

Ecell=Eright-Eleft=EAg-ESHE

= EAg-0.000= EAg


Figure 6-4

Name of electrodes

1. Cathodes and Anodes;

2. Positive Electrode and Negative Electrode.

3. Working electrode WR ( response electrode)

Reference electrode RE

Auxiliary electrode; Counter electrode CE;

supporting electrode

4. Polarized electrode and unpolarized electrode

(nonpolarizable electrode).

0E=E + lno

R

RT

nF

Ox ＋ ne- Red

E0: Standard Electrode Potential

R: ideal gas constant, 8.314J K -1 mol-1

T: temperature, K n: number of moles of electrons

F: the faraday=96,485C ln: natural logarithm=2.303log

6.4 The Nernst Equation

If we substitute numerical values for the constants, convert to base 10 logarithms, and 25 for the temperature. We get:℃

0 0.059E=E + log

o

Rn

6.4 The Nernst Aquation

Walther nernst(1864-1941)received the 1920 Nobel Prize in chemistry for his numerous contributions to the field of chemical thermodynamics. Nernst (far left) is see here with Albert Einstein, Max Planck, Robert A. Millikan, and Max von Laue in 1982.

6.4 The Nernst Equation

Calculation redox equilibrium constants

For example: biological redox systems

The scheme Cytochrome c

Cytochromes are ironheme proteins in which a porphyrin ring is coordinated through nitrogen atoms to an iron atom. They undergo one-electron redox reactions, and their physiological function is to

6.5 Applications of Standard Electrode Potentials

facilitate electron transport. Cytochromes are intimately involved in the formation of water from H2. Reduced pyridine nucleotides

deliver hydrogen to flavoproteins. The reduced flavoproteins are reoxidized by the Fe3+ of Cytochromes b or c. The chain is completed when cytochromes oxidase transfers electrons to oxygen. The resulting oxide ion(O2-) is unstable and immediately picks up two H+ ions to produce H2O.

How to measure the potential?

Trends in Electroanalysis:

Micro-; Nano-

In vive; On line

Bio-

国际电化学会主席 Robert Hillman 教授致开幕

Home Work

Questions and Problems P 628-631

1.Why is it that half-reactions in

electrochemical cells

proceed spontaneously in one direction and

furnish

current?

2. What is the effect of the salt bridge?

3.. What is the effect of ion migration?

4. What is absolute and relative potential

differences?

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