Page 73 - Analytical Electrochemistry 2d Ed - Jospeh Wang

58 STUDY OF ELECTRODE REACTIONS

Solution From equation (2-19)

2
Df ˆ K Dmf =A
0
2
5 ˆ K2 5 =1 K ˆ 0:1
Under the new experimental conditions, Df is given by

2
Df ˆ 0:1† 4 8 =0:5 ˆ 51:2Hz
Example 2-4 A potential-step spectroelectrochemistry experiment using a reactant
concentration of 2 mM generated a product with an absorbance (sampled after 25 s)
of 0.8. Calculate the reactant concentration that yielded an absorbance of 0.4 upon
sampling at 16 s.

Solution From equation (2-17),

A ˆ KC t 1=2 0:8 ˆ K2 25† 1=2 K ˆ 0:08
0
Accordingly,

1=2
0:4 ˆ 0:08C 16† C ˆ 5mM
0 0

QUESTIONS
1. Explain and demonstrate clearly how spectroelectrochemistry can provide useful
information about a reaction mechanism involving a redox process followed by
a chemical reaction (EC mechanism), involving decomposition of the reaction
product. Draw an absorbance±time plot for different rate constants of the
decomposition reaction.

2. Which voltammetric technique can be used for estimating the surface coverage
of an adsorbed molecule? How?
3. Draw an EQCM (mass±potential) pro®le for a metal deposition-stripping
process during a cycling voltammetric scanning.
4. A cyclic-voltammetric peak current of 12.5 mA was observed for the reversible
reduction of a 1.5 mM lead solution using a 1.2 mm-diameter disk electrode and
a50 mV s 1 scan rate. Calculate the lead concentration that yields a peak
1
current of 20.2 mA at 250 mV s .
5. Explain clearly how SECM images the microdistribution of the electrochemical
activity of composite electrode materials.