Page 102 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

180 Electrochemistry

2. Electro-Induced Hydrogenation Often, the first step in the electrochemical characteriza-
tion of a compound is to ascertain its oxidation-reduction
Reductive electron transfer in an electrochemical cell oc-
reversibility. Cyclic voltammetry usually is the most con-
curs by insertion of an electron from the electrode (cath-
venient and reliable technique for this and related quali-
ode) into the solution matrix within the double layer of tative characterizations of a new system. The discussion
−
+
the electrode/solution interface {e.g., H 2 O/(H O)(ClO );
3 4 in earlier sections outlines the specific procedures and re-
I
+
−
[Na (OH 2 ) ](ClO )}.
6 4 lationships. The next step in the characterization usually
e − is the determination of the electron stoichiometry of the
◦
+
H O [H·] + H 2 O(E ) pH 0 , −2.10VvsNHE
3 oxidation-reduction steps of the compound. Controlled-
◦
(E ) pH 7 , −2.51 V. (111) potential coulometry provides a rigorously quantitative
means to such evaluations.
The reductive processes of Eq. (111) may be facilitated by The electrochemical characterization of the oxida-
the presence of substrates to stabilize the H-atom product. tion/reduction chemistry of metals, metal ions, and metal
Forexample,inpH0water,thereductionprocessisshifted complexes has been discussed in Section III. In the follow-
by +4.82 V when hydroxyl radicals (HO·) are present, ing paragraphs, the use of electrochemistry for molecular
characterization is further illustrated.
e −
◦
+
H O + [HO·] HO–H + H 2 O E , +2.72 V
3
−1
− G BF , 111 kcal mol (112) A. Hydronium Ions, Brønsted Acids,
and Molecular Hydrogen
and by +2.10 V at a platinum electrode,
The most fundamental redox process in electrochem-
e −
+
H O + Pt Pt–H + H 2 O E , 0.00 V istry is the reductive transformation of hydronium ion
◦
3
+
−1 [H O(aq)] at a platinum electrode to molecular hydrogen
− G BF , 48 kcal mol (113) 3
[H 2 (g)],
In the presence of benzoquinone (Q), the shift is +2.80 V, Pt
2H O(aq) + 2e − H 2 (g) E , 0.0000 V vs NHE.
◦
+
3
O H
(115)
2e

2H 3 O Q (E°) pH 0 , 0.70 V
+
When properly engineered and with [H O(aq)] at unit ac-
3
1
O H ( ∆G BF , 65 kcal mol ). at unit fugacity, this electrode system is the
tivity and P H 2
(114) thermodynamic reference standard for measurements of
electrochemical potentials and is referred to as the Normal
Hydrogen Electrode (NHE), which is alternatively called
V. ELECTROCHEMICAL the Standard Hydrogen Electrode (SHE),
CHARACTERIZATION OF MOLECULES
2

E = E NHE + 2.30 RT/2F log H O(aq) P H 2
+
◦
3
An increasing number of chemists use electrochem-
◦
(E ◦ ≡ 0.0000 V at all temperatures, 0–100 C)
istry as a characterization technique in a fashion anal- NHE
ogous to their use of infrared, UV-visible, NMR, and [2.30RT/2F = 0.05915/2at25 C]. (116)
◦
ESR spectroscopy. Some of the chemical questions
that are amenable to treatment by electrochemistry in- The latter equation for the NHE also is the defining ba-
◦
clude (1) the standard potentials (E ) of the com- sis for the potentiometric measurement of hydronium-ion
+
pound’s oxidation-reduction reactions, (2) evaluation of activity [H O] and molecular hydrogen fugacity (P H 2 ),
3
the solution thermodynamics of the compound, (3) de- + ◦
pH a ≡−log H O = [(E − E ind )/0.059]
termination of the electron stoichiometry of the com- 3 NHE

pound’s oxidation-reduction reactions, (4) preparation =−E ind /0.059 (E ◦ = 0.000 V;
− log P H 2 NHE
and study of unstable intermediates, (5) evaluation of
= 1.00 atm) (117)
the valence of the metal in new compounds, (6) de-
P H 2
termination of the formulas and stability constants of
2
+
metal complexes, (7) evaluation of M–X,H–X, and = [(E ◦ − E ind )2/0.059] + log H O
NHE 3
log P H 2
O–Y covalent-bond-formation energies (− G BF ), and
+
=−E ind (2/0.059) E ◦ = 0.000 V; H O
(8) studies of the effects of solvent, supporting electrolyte, NHE 3

and solution acidity upon oxidation-reduction reactions. = 1.000 . (118)