voltammetry 伏安分析法. basic principle of voltammetry voltammetry: a group of electrochemical...

29
Voltammetry 伏伏伏伏伏

Upload: bertina-kelly

Post on 18-Jan-2016

298 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Voltammetry

伏安分析法

Page 2: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Basic principle of voltammetry

Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve during electrolysis - only a small amount of sample (analyte) is used

Polarography: Invented by J. Heyrovsky (Nobel Prize 1959). Differs from voltammetry in that it employs a dropping mercury electrode (DME) as Working electrode to continuously renew the electrode surface.

Read: pp. 716 –753 Problems: 25-1,2,3,6,13

Page 3: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve
Page 4: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve
Page 5: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve
Page 6: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Polarographic analysis :Electrolytic analysis carried out

under special conditions.

1. Polarographic analysis process and the con

ditions for polarographic wave formation

specific characteristics :A 、 a polarized electrode and a depolarized electrode are used as working electrodeB 、 No stirring Incomplete electrolysis (only a small amount of analyte is consumed)

Page 7: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

If the electrode potential has great changes wh

en infinite small current flow through the electro

de, such electrode is referred to as polarized el

ectrode. eg. DME ;

If the electrode potential does not change with

current , such electrode is called ideal depolariz

ed electrode. eg. SCE

Polarized electrode and depolarized electrode

Page 8: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve
Page 9: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Three electrode cell: Working

ReferenceCounter/auxilliary

current flows between working and counter electrodes. Potential controlled by potentiostat between working and reference electrodes.

Page 10: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Two special electrodes

Supporting electrolyte : Usually relatively higher concentration of strong electrolytes (alkali metal salts) serves as supporting electrolyte

Dissolved oxygen is usually removed by bubbling nitrogen through the solution

Voltage scanning Under unstirred state, recording voltage - current curve

Page 11: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

① ~ ② residual current

③ electrolytic current

④ ~⑤ limiting diffusion current

Cd 2+ +2e + Hg = Cd(Hg)

2Hg + 2Cl- -2e = Hg2Cl2

Page 12: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

i (A)

0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte

V vs SCE

Working electrode is no yet capable of reducing Cd2+ only small residual current flow through the electrode

Electrode become more and more reducing and capable of reducing Cd2+

Cd2+ + 2e-Cd

Current starts to be registered at the electrode

Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion of Cd2+ does not limit the current yet

All Cd2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd2+ from the bulk solution to the electrode. Thus, current stops rising and levels off at a plateau id

Base line of residual current

Page 13: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

E½ at ½ i

Limiting currentRelated to concentration

Page 14: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

0.5mmol 镉离子极谱图

Page 15: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Limiting diffusion current -- A basis of polarographically quantitative analysis

When the applied voltage exceeds the decomposition voltage, diffusion-controlled current is expressed as:

i = K(C-C0)

When the applied voltage gets more negative, C0 →0,

current becomes only diffusion limited, then

id = KCId reaches a limiting value proportional to ion concentration C in bulk solution, and do not changes with applied voltage longer

Page 16: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

The potential at which the current is equal to one half the limiting current is called the half-wave potential and given the symbol E1/2.

Half-wave potential —polarographic qualitative analysis

Page 17: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

How it works?

▲ The applied voltage is gradually increased, typicall

y by going to a more positive( more negative decomp

osing potential)

▲ A small residual current is observed.

▲ When the voltage becomes great enough, reductio

n occurs at the analytical electrode causing a current.

▲ The electrode is rapidly saturated so current produ

ction is limited – based on diffusion of the analyte to t

he small electrode.

Page 18: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

How it works ?

The reduced species alters the surface of the

mercury electrode.

To prevent problems, the mercury surface is

renewed by “ knocking off ” a drop –providing a

fresh surface.

This results in an oscillation of the data as it is

collected.

Page 19: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

2. The diffusion current theory and polarographic wave equation

In above equations, K is called Ilkovic constant, it is expressed as follows:

id = KC

We have already known:

K = 607 n D1/2m2/3t1/6

Thus,

id = 607nD1/2m2/3t1/6C

Page 20: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

id = 607nD1/2m2/3t1/6C

Average limiting diffusion current denoting average current on mercury drop from drop for

ming to falling (A)

Number of transferring electrons in electrode

reaction(e/mol)

Diffusion coefficient of electroactive analyte in solution(cm

2.sec-1)

Mercury mass flow rate(mg.sec-1)

Drop time (sec)

Concentration of electro-active

analyte(mmol.L-1)

From above equation, we can find that when temperature, matrix solution and capillary characteristic are kept constant, id is proportional to C

Page 21: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

polarographic wave equation:

ii

i

nF

RTEE

d ln2/1

When i = ½ id , log term in above equation is equal to zero, corresponding potential is called halfwave potential E1/2

●E1/2 independent on the concentration●basis of qualitative analysis

Page 22: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

● Residual current

(1) redox reactions of impurities in solution

(2) charging of Hg drop

(non-faradaic current / non-redox current)

● Migration current

The current produced by static attraction of the ele

ctrode to sought-for ions

3. Interference current in classical DC polarography

Page 23: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Complex artifactual phenomenonLess likely at low drop rates, in concentrated electrolyte, or low concentration of electroactive speciesLessened by inclusion of surfactants in medium

● Polarographic Maximum (or malformed peak )

Page 24: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

● Oxygen wave

Dissolved oxygen is easily reduced at many working electrodes. Thus an aqueous solution saturated with air exhibits two distinct oxygen waves.

The first results from the reduction of oxygen to hydrogen peroxide:

O2 + 2H+ + 2e- H2O2

The second wave corresponds to the further reduction of hydrogen peroxide:

H2O2 + 2H+ + 2e- 2H2O

Sparge solutions with high purity N2 or Ar for 5-20 min

Page 25: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve
Page 26: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Factors that affect limiting diffusion current

Characteristics of capillary – hight of Hg

Potential of dropping Hg electrode

Composition of solution

Temperature

Factors that affect half-wave potential

Type and concentration of supporting electrolyte

Temperature

Forming complex

Acidic of solution

Page 27: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Question

Why a reference electrode with large area and a dropping mercury electrode with very small area are used to electrolyze in polarographic analysis ?

Why large amount of supporting electrolyte is added to sample solution?

Why does nitrogen gas pass through the solution before electrolysis ?

In the process of polarographic analysis whether or not to carry out stirring the solution? Why?

Page 28: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

( id ) avg = K·c

●Direct comparison method

●Calibration curve method

●Standard addition method

4. Polarographically quantitative analytical methods

Page 29: Voltammetry 伏安分析法. Basic principle of voltammetry Voltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve

Fundamental studies

Inorganic applications

Organic applications

Applications in pharmaceutical and biochem fields

5. Applications