Page 105 - 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

Electrochemistry 183

TABLE IV Standard Reduction Potentials for Dioxygen Species in Water [O 2 , 1 atm (1 mM)] (Formal Potentials for1MO 2 )
1.66 V vs NHE

pH 0 (1 M H 3 O)
1.13
0.05( 0.12) 1.44 0.80 2.72
O 2 HOO HOOH H 2 O HO 2 H 2 O

0.695 ( 0.78) 1.763

0.73 ( 0.79)
1.229 ( 1.27)

1.20
pH 7
0.64
0.33( 0.16) 0.89 0.38 2.31
O 2 O 2 HOOH H 2 O HO 2 H 2 O

0.281 ( 0.36) 1.349

0.31 ( 0.37)
0.815 ( 0.86)

0.65

pH 14 (1 M HO )
0.09
0.33( 0.16) 0.20 0.03 1.77

O 2 O 2 HOO H 2 O OH 4 H 2 O
0.065 ( 0.02) 0.867

0.03 ( 0.01)
0.401 ( 0.44)

2. Aprotic Media dia decrease. The heat of hydration (− H aq ) for gaseous
−1
−
O · is 100 kcal mol , which is consistent with the unique
2
In the absence of proton sources dioxygen is reversibly
strong solvation of anions by water. Hence, if the E ◦
−
reduced to superoxide ion, O 2 /O ·
2
−
values for the ·O 2 ·/O · couple are affected primarily by
2
−
−
·O 2 ·+ e − O · E , −0.64VvsNHE. (134) the degree of solvation of O · (that is, the solvation en-
◦
2 2
ergy for ·O 2 · is assumed to be small and about the same
The second reduction is an irreversible one-electron
for different solvents), then the relative solvation energies
process,
−
for O · are H 2 O Me 2 SO > DMF > py ∼ MeCN.
H 2 O 2
−
−
O ·+ e − HOO + HO − E p,c , −1.8 V (135)
2
− − −
HO + HOO +·O 2 · 2O ·+ H 2 O 3. Hydrogen Peroxide (HOOH, H 2 O 2 )
2
−9 −1 −1
K, 10 M atm . (136)
The electrochemical reduction of HOOH in pyridine
The reduction potential for the ·O 2 ·/O · couple shifts to yields a superoxide ion. That a reduction process gen-
−
2
more negative values as the solvating properties of the me- erates a species with a higher oxidation state than HOOH