Page 108 - 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

186 Electrochemistry

3S 8 8e 0.6 V vs SCE 8S 3 4. Hydrazines and Amines
These substrates are directly oxidized in a base-free
2
K D , 1.1 10 M 1 matrix (Me 2 SO or MeCN) at platinum or glassy-carbon
electrodes with the potential primarily determined by the
2 RN-H bond energy and secondarily by the basicity of the
4S 6
(147)
substrate,
e
2. Sulfur Dioxide (SO 2 ) 2 PhNHNHPh E p,a , 0.6 V vs SCE PhNNHPh R NH 2
(R NH)
In aprotic solvents SO 2 undergoes a reversible one- e PhN NPh R NH 2
electron reduction, R NH
(152)
−
SO 2 + e − SO · E 1/2 , −0.75VvsSCE. (148)
DMF 2 e
2 PhNH 2 E p,a , 0.9 V vs SCE PhNH PhNH 3
However, the product species interacts with excess SO 2 to
2 PhNH 2
give a blue complex that is significantly more difficult to e
PhNHNHPh PhNH 3
oxidize.
2e 2 PhNH 2

SO 2 2 SO 2 (SO 2 ) 2 S(O)O

PhN NPh 2 PhNH 3 .
e E p,a , 0.25 V vs SCE
(153)
In contrast, when a hydroxide ion HO is present it
−
(149)
3SO 2
is more easily oxidized than the amine substrates. In
−
Complete electrolysis yields SO ·, which dimerizes to MeCN, in the absence of substrate, HO − is oxidized
2
dithionite ion (colorless), at +0.7−0.9 V vs SCE. However, with hydrazines and
amines present, the N H bonds are homolytically cleaved
− − − 1 −1
−
2SO · O 2 SSO K D , 2.4 × 10 M . (150) by the HO· product of HO oxidation. The latter’s oxida-
2 2
DMF
tion potential is shifted by the difference in the HO H
Because SO 2 is electrophilic (acidic), it is extremely and RN H bond energies (− G BF ). Thus, the oxidation
resistant to direct electron-transfer oxidation. However, in of PhNHNHPh is shifted by −1.7 V when HO becomes
−
aqueous solutions at pH 1, SO 2 facilitates the oxidation of the electron-transfer mediator; with PhNH 2 the shift is by
+
H 2 O[2H 2 O → HO·+ H O + e ; E , +2.42 V vs SCE −1.1V.
−
◦
3
(pH 1)] at gold electrodes via covalent-bond formation to

e
give sulfuric acid [(HO) 2 S(O) 2 ] in an ECE process. The PhNHNHPh HO E p,a , 1.1 V vs SCE PhNHNHPh H 2 O
peak current is proportional to the SO 2 concentration and
e
its diffusion coefficient, which makes this anodic process PhN NPh H 2 O
HO
suitable as a voltammetric monitor for dissolved SO 2 or
(154)
gas-phase SO 2 via a gas-permeable membrane.
e
PhNH 2 HO PhNH H 2 O
3. Nitric Oxide (·NO) 0.2 V vs SCE

HO , PhNH 2
Reduction of the oxides of nitrogen (·NO, ·NO 2 , and N 2 O) e
PhNHNHPh H 2 O
usually involves the addition of hydrogen atoms that are
(155)
electrogenerated. The use of a mercury electrode inhibits
+
◦
the reduction of H OtoH 2 (E , −2.2 V vs SCE at pH 5),
3
but allows formation of H· when it couples with a substrate
5. Carbon Dioxide (CO 2 )
via covalent-bond formation,
Whereas the reduction of CO 2 at a gold electrode is a one-

N O H 3 O (pH 5.0) e [NO H] E 1/2 , 0.9 V vs SCE
electron per CO 2 process on a voltammetric time scale,
2X at mercury it is a sequential two-electron process. In both
cases the overall reduction is two electrons per CO 2 . The
[HON NOH] N N O H 2 O
(N 2 O) products for anhydrous conditions are CO 2− and CO and,
3
−
−
(151) with H 2 O present, HOC(O)O and HC(O)O ,