Instant Notes: Analytical Chemistry

The reaction that occurs is:

Cu^2 ++H 2 O =Cu(s) +^1 ⁄ 2 O 2 +2H+

The copper is then deposited on the cathodeand oxygen is evolved at the

anode. This is the basis for electrogravimetry, where the copper is completely

deposited from solution and the increase in weight of the cathode determined.

The analysis may be conducted using either a controlled potential or a

controlled current.

Coulometric methodsof analysis involve measuring the quantity of electricity

in coulombs needed to convert the analyte to a different oxidation state. If the

electrolysis occurs at 100% efficiency, Faraday’s laws may be applied and each

96485 C will bring about the reaction corresponding to 1 mole of electron transfer.

For example, using a silver anode, the passage of a current produces silver

ions, which react with any chloride in the solution. Bromine and acids may also

be generated coulometrically.

Polarographic methods employ a microelectrode, often a dropping mercury

electrode (DME), as the working electrode, plus a reference electrode (SCE) and a

mercury pool as auxillary electrode. The simplest potential-time regime, where

the potential increases regularly (linear potential sweep dc voltammetry)is

applied to the cell containing the analyte and a supporting electrolyte to carry

the majority of the current.

In these methods, the transport of ions to the electrodes depends on three

factors: diffusion, convection or stirring, and conduction. The effects of conduc-

tion of the ion that reacts at the electrode is minimized by using a concentration

of supporting electrolyte such as KCl about 50-times higher than that of the

analyte. Stirring and convection are minimized. The resulting polarographic

curve shows three regions.

(i) If a potential difference is applied across a cell and no reaction occurs, only

the residual current Irwill flow.

(ii) If a reducible ion, say Cd^2 +, is present, it will migrate to the dropping

mercury cathode. If the applied potential exceeds its decomposition poten-

tial, ED, it will be reduced to the metal which dissolves in the mercury:

Hg

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

C9 – Voltammetry and amperometry 99

Potentiostat

Nitrogen in Nitrogen out

Reference electrode

Auxillary electrode

Stirrer

Working

electrode

Sample

solution

V A

Fig. 1. Basic voltammetric cell.