国外电化学讲义

Section 5: Voltammetric Methods

Voltammetric Methods •Historical

•Electrolysis at DME -1920’s

•Usually 3-electrode cells

•Measurement of current that results from the application of potential.

•Different voltammetric techniques are distinguished primarily by the potential function that is applied to the working electrode and by the material used as the working electrode.

Types of Voltammetry

Polarography

Linear sweep and Cyclic Voltammetry Hydrodynamic Voltammetry

Pulsed methods

AC Voltammetry (not here)

ip=2.69 x 105n3/2A D O1/2v1/2C O

It is instructive to start with Polarography

Voltammetry at a dropping mercury electrode

Polarography uses mercury droplet electrode that is regularly renewed during analysis.

Applications:

Metal ions (especially

heavy metal

pollutants) -high

sensitivity.

Organic species able

to be oxidized or

reduced at electrodes:

quinones, reducing

sugars and derivatives,

thiol and disulphide

compounds, oxidation

cofactors (coenzymes

etc), vitamins,

pharmaceuticals.

Alternative when

spectroscopic methods

fail.

History

Jaroslav Heyrovskýwas the inventor

of the polarographic method, and the

father of electroanalytical chemistry,

for which he was the recipient of the

Nobel Prize. His contribution to

electroanalytical chemistry can not

be overestimated. All modern

voltammetric methods used now in

electroanalytical chemistry originate

from polarography.

On February 10, 1922, the "polarograph" was born as Heyrovskýrecorded the current-voltage curve for a solution of 1 M NaOH.Heyrovskýcorrectly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ions, forming an amalgam.

Typical polarographic curves (dependence of current I on the voltage E applied to the electrodes; the small oscillations indicate the slow dropping of mercury): lower curve -the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve -the same after addition of small amount of thallium.

Swedish king Gustav Adolf VI awards the

Nobel Prize to Heyrovský

in Stockholm on 10.12.1959

Potential Ramps

Linear sweep Polarography

In order to derive the the current response

one must account for the variation of drop area with time:

A = 4%(3mt/4%d)2/3= 0.85(mt)2/3

Density of

Mass flow rate of drop drop

We can substitute this into Cottrell Equation (see Mass Transport Notes)

i(t) = nFACD1/2/ %1/2t1/2