Page 104 - 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

182 Electrochemistry

This activated platinum electrode system is the equiv- sensors for ·O 2 · are the most common analytical method-
alent of an NHE and, therefore, conforms to the Nernst ology for its assay in blood, gas streams, biotreaters, and
expression of Eq. (116). Rearrangement gives process streams.

2
+
=−E Pt (2/0.059) + log H O(aq) , (127)
3 1. Molecular Oxygen
log P H 2
which responds to the overall reaction 3 −
The ground state of molecular oxygen (·O 2 ·, ) has
g
two unpaired electrons in degenerate 2π g orbitals and is re-
K eq

∗
+ 2Pt (s) 2Pt–H (s)
H 2 P H 2 ferred to as dioxygen by most contemporary biologists and
biochemists. When dioxygen is reduced by electron trans-
−2e −
+
2H O(aq) + 2 Pt (s). (128) fer, a series of intermediate basic dioxygen and monooxy-
H 2 O 3
gen species are produced that may take up one or two hy-
Although there is a long-standing tradition to write an dronium ions (H O) from the media (O ·, HOO·, HOO ,
+
−
−
3 2
electron-transfer sequence with electron removal (ioniza- HOOH, ·O ,HO·,HO , and H 2 O). The thermodynamics
−
−
tion)fromthehydrogenatomtoproduceaproton[Pt–H(s) for the various reduction steps in aqueous solutions have
→ Pt + H + e ] followed by hydration to give the ob- been evaluated by numerous techniques, but all are fun-
−
+
+
+
served hydronium ion [H + H 2 O → H O(aq)], the ion- damentally based on the calorimetry associated with the
3
ization potential for a free hydrogen atom (H·) is 13.6 eV reaction
and even greater for bound hydrogen [Pt–H (s)]. Electron
·O 2 · (g) + 2H 2 (g) 2H 2 O (l) − G = nE ◦ F.
◦
removal from an aqueous solution at pH 0 with a GCE cell
(free of H 2 and Pt–H) occurs via the lowest energy path,
(131)
which is oxidation of the solvent
If redox potentials are relative to the normal hydrogen
− H 2 O
−e + +
H 2 O H O· H O + HO· electrode,
3
2
+
E , +2.72 V vs NHE. (129) 2H O + 2e − H 2 (g) E ◦ ≡0.000, (132)
◦
3 H O/H 2
+
3
In the presence of Pt–H (s) (and H 2 ), this process is facil- then the standard redox potential for the four-electron re-
itated via direct formation of an H–OH bond, duction of dioxygen (E ◦ ) can be calculated from
O 2 /H 2 O
the calorimetric data for the reaction of Eq. (131) under
−e −
◦
H 2 O + Pt–H (s) Pt (s) + H O + HO–H E , 0.00 V standard-state conditions,
+
3
H 2 O
+
(− G BF ) = (2.72 − 0.00) 23.1 kcal (eV) −1 ·O 2 · (g) + 4H O(aq) + 4e − 2H 2 O (l)
3
= 63 kcal mol −1 (130) E ◦ O 2 /H 2 O = E ◦ cell − E ◦ + =+1.229 V vs NHE.
H O/H 2
3
(133)
Addition of the Pt–H (s) bond energy [ (− G BF ) =
−1
48 kcal mol ] to the differential bond-formation energy
The reduction potentials for ·O 2 · and various interme-
for H–OH gives an electrochemical measure of the HO–H diate species in H 2 O at pH 0, 7, and 14 are summarized
bond energy [− G BF = 111 kcal mol −1 (literature value, in Table IV; similar data for ·O 2 · in MeCN at pH −8.8,
−1
111 kcal mol )].
10.0, and 30.4 are presented in Table V. For those couples
that involve dioxygen itself, formal potentials are given
3
in parentheses for ·O 2 · at unit molarity (∼10 atm; [·O 2 ·]
B. Dioxygen Species (·O 2 ·, HOO·/O ·, HOOH)
−
2 ≈1mMat1atmpartialpressure).Thereductionmanifolds
Although the electrochemistry of hydronium ion and for ·O 2 · (Tables IV and V) indicate that the limiting step (in
molecular hydrogen is fundamental, the electrochemi- terms of reduction potential) is the first electron transfer
−
cal characterization of oxygen species (·O 2 ·, HOO·,O ·, to ·O 2 · and that an electron source adequate for the reduc-
2
−
HOOH, HOO , ·O·,HO·,O ·,H 2 O, HO ,O 3 ,O ·) and tion of ·O 2 · will produce all of the other reduced forms of
−
−
−
3
of the oxygen component of molecules (e.g., M x O y , oxy dioxygen (O ·, HOO·, HOOH, HOO ,HO·,H 2 O, HO )
−
−
−
2
anions and radicals, quinones) is its most important and via reduction, hydrolysis, and disproportionation steps.
unique application. In general, electrochemical measure- Thus, the most effective means to activate ·O 2 · is the addi-
ments provide the only direct means for the evaluation of tion of an electron (or hydrogen atom; H O + e → H·),
+
−
3
the electron-transfer thermodynamics of oxygen species which results in significant fluxes of several reactive oxy-
and oxygen-containing molecules. Also, amperometric gen species.