polaro- and voltamanalytical methods Chapter 6

6.1 Introduction

woking electrodes

liquid electrodes

solid electrodes

polarogaphy

voltammetry

6.2 Foundamentals of general polarography

6.2.1 The introduction of the classical polarography

e.g. Polarograms ( i vs E relationships ) of Cd2+ Fig6.2-2

• Design Fig6.2-1

• The half-wave potential is indepent of the concentration

• The specification of polarograms determination

6.2.2 the form of polarogram The polarogram has three current regions:

• The residual current (ir) region

• Diffusion current (id) region

• Limiting current (il) region

6.2.3 The specification of electrode• Polarization electrode• Unpolarization electrode• The advantages of the dropping mercury electrode (DME)

6.3 Diffusion current equation

6.3.1 Ilkovic diffusion current equation

id = k •n •D1/2 •m2/3 •t1/2 •Cox

6.3.2 The factors determining the diffusion current • The analyte concentration

• The characteristics of the capillary

• The effect of dropping mercury potential

• Temperature effect

6.4 Factors affecting the shape of the polargram

• The residual current Fig6.4-1

Fig6.4-2

• The current maxima Fig6.4-3• The presence of oxygen Fig6.4-4

6.5 The quantitative analytical method of polarography

6.5.1 The determination of diffusion current

• Parallel method Fig6.5-1

• Thritangential method Fig6.5-2

6.5.2 The quantitative method of polargraphy

• Directive comparative method• Working curve Fig6.5-3

• Standard addition method Fig6.5-4

6.6 Introduction about new polarography 6.6.1 Polaro-catalysing wave

6.6.2 Stripping analysis method

6.6.3 Linear sweep(dc) oscillo- polarography

• Basic line Fig6.6-1 Fig6.6-2

6.6.4 Circular voltammetry

• Fundamental Fig6.6-3

• Application Fig6.6-4

It is special electrolytic design which consists of:

a. Electrolytic cell

b. Voltmeter V

c. Ampere meter

Its specialties show:

a. Large areas of reference electrode

b. Small areas of working electrodec. The infinite dilution

As the half-wave potential is independent of solution analyte concentration and dependant primary on the nature of analyte.

It can be used for the identification of analyte species taking part in the electrode reaction.

In general, they are ascribed to the convection of the solution layer in the vicinity of the working electrode which is induced by the inhomogeneous charge distribution in the dropping Hg electrode.

Elimination:

The current maxima can be supppressed by the addition of polarographically inactive surfactants, such as gelation, methyl red or other dyes to obtain the desired effect.

Elimination methods:

In neutral or alkaline media by Na2SO3 solution

In solution of all pH by N2 or Ar or H2 etc. In acid solution by ascorbic acid

h = KCx

H = K VCx + VsCs

V + Vs

Then

Cx =

CxVshH (V + Vs) - hV

The principles of qualitative quantitative and analysis Fig.6.6

ip = Kn3/2D1/2V1/2AC

The basis for quantitative analysis:

The basis for qualitative analysis:

Peak potential is only decided by the properties of reducing material under certain condition (temperature, the fixed bottom solution ).

Working electrode, static electrode such as:

Suspending electrode

Stationary electrode

Which was mainly used to study the mechanism of electrode process, such as:

The determination of its reverse:

(mv)Ep = Epa – Epc =56n

Ep > (mv),56n

ipa

ipc= 1 ,

The distance between two peaks is farther, the more irreverent it is.