Voltammetry

伏安分析法

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

Basic principle of voltammetry

Voltammetry:

A group of electrochemical methods based on measuring current (i) electrolysis

applied

potential curve during - 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 renew the electrode surface.

to continuously

1.

Polarographic analysis process and the conditions for polarographic wave formation

Polarographic analysis

： Electrolytic analysis carried out under special conditions.

specific characteristics

： A 、 a polarized electrode and a dep olarized electrode are used as work ing electrode B 、 No stirring Incomplete electrolysis (only a s mall amount of analyte is consumed)

Polarized electrode and depolarized electrode

If the electrode potential has great changes when infinite small current flow through the electrode, such electrode is referred to as polarized electrode . eg. DME ; If the electrode potential does not change with current , such electrode is called ideal depolarized electrode . eg. SCE

Three electrode cell: Working Reference Counter/auxilliary current flows between working and counter electrodes.

Potential controlled by potentiostat between working and reference electrodes.

Two special electrodes Supporting electrolyte : Usually relatively higher conc entration of strong electrolytes (alkali metal salts) serv es as supporting electrolyte Dissolved oxygen is usually removed by bubbling nitrogen through the solution Voltage scanning Under unstirred state, recording voltage - current curve

①～ ② residual current Cd 2+ +2e + Hg = Cd(Hg) ③ electrolytic current 2Hg + 2Cl -2e = Hg 2 Cl 2 ④ ～ ⑤limiting diffusion current

i

(  A) 0.001 M Cd 2+ in 0.1 M KNO 3 supporting electrolyte Electrode become more and more reducing and capable of reducing Cd 2+ Cd 2+ + 2e Cd Current starts to be registered at the electrode Working electrode is

E ½

no yet capable of reducing Cd 2+  only small residual current flow through the electrode All Cd 2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd 2+ bulk solution to the electrode. Thus, current stops rising and Current at the working electrode continue to rise as the electrode become more reducing and more Cd 2+ around the electrode are being reduced. Diffusion of Cd 2+ does not limit the current yet from the Base line of residual current -0.2

-0.4

-0.6

V vs SCE -0.8

-1.0

-1.2

-1.4

Limiting current Related to concentration E ½ at ½ i

0.5mmol

镉离子极谱图

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-C

0 )

When the applied voltage gets more negative, C 0 current becomes only diffusion limited, then →0,

i

d = KC

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

Half-wave potential —polarographic qualitative analysis The potential at which the current is equal to o ne half the limiting current is called the half-wa ve potential and given the symbol

E

1/2 .

How it works?

▲

The applied voltage is gradually increased, typically by going to a more positive( more negative decomposing potential)

▲

A small residual current is observed.

▲

When the voltage becomes great enough, reduction occurs at the analytical electrode causing a current.

▲

The electrode is rapidly saturated so current production is limited – based on diffusion of the analyte to the small electrode.

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.

2.

The diffusion current theory and polarographic wave equation

We have already known:

i

d = KC

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

K = 607 n D

1/2

m

2/3

t

1/6

Thus,

i

d

= 607nD

1/2

m

2/3

t

1/6

C

i

d

= 607nD

1/2

m

2/3

t

1/6

C

Concentration of electro-active analyte(mmol.L

-1 ) Drop time (sec) Diffusion coefficient rate(mg.sec

-1 ) Average limiting diffusion current denoting average Number of transferring current on mercury drop from drop forming to falling (  A) 2 .sec

-1 ) electrons in electrode reaction(e/mol)

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

polarographic wave equation

：

E



E

1 / 2



RT nF

ln

i d i



i

When i = ½ i d , log term in above equation is equal to zero, corresponding potential is called

halfwave potential E 1/2

● E 1/2 independent on the concentration ● basis of qualitative analysis

3. Interference current in classical DC polarography

●

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 electrode to sought-for ions

●

Polarographic Maximum (or malformed peak )



Complex artifactual phenomenon



Less likely at low drop rates, in concentrated electrolyte, or low concentration of electroactive species



Lessened by inclusion of surfactants in medium

●

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: O 2 + 2H + + 2e  H 2 O 2 The second wave corresponds to the further reduction of hydrogen peroxide: H 2 O 2 + 2H + + 2e  2H 2 O  Sparge solutions with high purity N 2 or Ar for 5-20 min

Factors that affect limiting diffusion current

Characteristics of capillary – hight of Hg Potential of dropping Hg el ectrode Composition of solution Temperature

Factors that affect half-wave potential

Type and concentration of supporting electrolyte Temperature Forming complex Acidic of solution

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?

4. Polarographically quantitative analytical methods

（ id ） avg = K

·

c ●

Direct comparison method

●

Calibration curve method

●

Standard addition method

5. Applications

Fundamental studies Inorganic applications Organic applications Applications in pharmaceutical and biochem fields