Page 153 - Analytical Electrochemistry 2d Ed - Jospeh Wang

138 PRACTICAL CONSIDERATIONS

Examples

Example 4-1 A rotating ring-disk electrode (1600 rpm) yields a disk current of
12.3 mA for the oxidation of a 2 10 3 M potassium ferrocyanide solution.
Calculate the reduction current observed at the surrounding ring using a
6 10 3 M potassium ferrocyanide solution and a rotation speed of 2500 rpm
(N ˆ 0.33).

Solution From the Levich equation, (4-5), one can calculate ®rst the disk current
under the new conditions:

3
i K 6 10 † 2500† 1=2
D ˆ
12:3 K 2 10 † 1600† 1=2
3
i ˆ 46:125 mA
D

Then, from equation (4-8), we can solve for the ring current:

i ˆ Ni ˆ 0:33 12:125 ˆ 15:221mA
R D
Example 4-2 A rotating mercury ®lm electrode (2 mm diameter) yielded a
stripping peak current of 2.2 mA for a 1 10 8 M lead(II) solution following a
3 min deposition at 1.0 V with a 1600 rpm rotation. Calculate the peak current for
a 2.5 10 8 M lead(II) solution following a 2 min deposition with a 2500 rpm
rotation.

Solution

8 1=2
2:2 ˆ K 1 10 † 3 1600†

K ˆ 1:83 10 6

6 8 1=2
i ˆ 1:83 10 2:5 10 † 2 2500† i ˆ 4:6 mA
p
p

QUESTIONS

1. What are the advantages of using ultramicroelectrodes for electrochemical
measurements?
2. Explain clearly how chemically modi®ed electrodes can bene®t electrochemical
measurements?