Page 79 - Analytical Electrochemistry 2d Ed - Jospeh Wang

64 CONTROLLED-POTENTIAL TECHNIQUES

TABLE 3-1 Functional Groups Reducible at the Dropping Mercury Electrode
Class of Compound Functional Group E 1=2 V† a
Azo NˆN 0.4
Carbon±carbon double bond b CˆC 2.3
Carbon±carbon triple bond b C C 2.3
Carbonyl CˆO 2.2
Disul®de S S 0.3
Nitro NO 2 0.9
Organic halides C X(X ˆ Br, Cl, I) 1.5
Quinone CˆO 0.1
a Against the saturated calomel electrode at pH 7.
b Conjugated with a similar bond or with an aromatic ring.

several reducible organic functionalities common in organic compounds are given in
Table 3-1. Compounds containing these functionalities are ideal candidates for
polarographic measurements. (Additional oxidizable compounds can be measured
using solid-electrode voltammetric protocols.) Since neutral compounds are
involved, such organic polarographic reductions commonly involve hydrogen ions.
Such reactions can be represented as

R ‡ nH ‡ ne „ RH 3-6†
‡

n
where R and RH are oxidized and reduced forms of the organic molecule. For such
n
processes, the half-wave potential will be a function of pH (with a negative shift of
about 59 mV=n for each unit increase in pH, due to decreasing availability of
protons). Thus, in organic polarography, good buffering is vital for generating
reproducible results. Reactions of organic compounds are also often slower and more
complex than those of inorganic cations.
For the reduction of metal complexes, the half-wave potential is shifted to more
negative potentials (vs. the true metal ion), re¯ecting the additional energy required
for the decomposition of the complex. Consider the reversible reduction of a
hypothetical metal complex, ML :
p
ML ‡ ne ‡ Hg „ M Hg†‡ pL 3-7†

p
where L is the free ligand and p is the stoichiometric number. (The charges are
omitted for simplicity.) The difference between the half-wave potential for the
complexed and uncomplexed metal ion is given by (2):

1=2
RT RT RT D free
E † E † ˆ ln K p ln‰LŠ‡ ln 3-8†
1=2 c 1=2 free d
nF nF nF D
c