Instant Notes: Analytical Chemistry

Since the current depends on the diffusion of the oxygen to the electrode from

the external solution, and this diffusion is proportional to the concentration of

oxygen in the external solution, this electrode may be used to measure dissolved

oxygen.

Amperometric titrationsare used to determine substances by measuring the

limiting diffusion current of a species as a function of the volume of a reagent

added to react with that species. Since Idis proportional to the concentration, it

will decrease as a species is used up, or increase as the excess of a species

becomes greater. For example, for the determination of Pb^2 +with Cr 2 O 72 - :

2Pb^2 ++Cr 2 O 72 - +H 2 O =2PbCrO 4 +2H+

At an applied potential of 0.0 V, and at pH4, dichromate is reduced, but Pb2+is

not, giving the graph shown in Figure 4.

Applications of anodic stripping voltammetry, are chiefly for the determina-

tion of trace amounts of amalgam-forming metals (Fig. 5), while cathodic strip-

ping voltammetry is used for determining species that form insoluble salts with

mercury. The preconcentration stage allows determination in the concentration

range 10-^6 to10-^8 M.

C9 – Voltammetry and amperometry 103

Current

I (

μA)

12

10

8

6

4

2

0

0 5 10 15 20 25 30 35 40

Volume dichromate added (ml)

Fig. 4. Amperometric titration of Pb(II) with dichromate at pH 4 and 0.0 V.

Current

I (

μA)

0

Cu

Pb

Cd

–0.2 –0.4 –0.6

E/V

Fig. 5. Anodic stripping voltammogram for 7.5 ppb Cu(II), 2.6 ppb Pb(II) and trace amounts

of Cd(II).