Chemistry Reference
In-Depth Information
The problem can be overcome by boosting for a few milliseconds the power of
the lamp providing the analyzing light (with a 450-W xenon lamp about tenfold).
Thus, the
I
0
value of the analyzing light is now much higher than the intensity
of the
erenkov light (f (fluorescence) and signals ref lect with sufficient accuracy
the absorption properties of the intermediates formed during the pulse (laser
f flash). Recently, attention has been drawn to pitfalls by stray light (Czapski et
al. 2005) and solute absobance (especially in laser f flash photolysis; von Sonntag
1999). Data acquisition and storage are computerized which allows multiple-
signal averaging. The dose in a pulse radiolysis experiment may be determined
(Butler and Land 1996) by the thiocyanate dosimeter (Schuler et al. 1981), more
recently revised (Buxton and Stuart 1995;
G
(SCN)
2
•
−
Č
×
ε
475 nm
= (2.59
±
0.05)
×
10
−4
m
2
J
−1
).
In many free-radical reactions, neutral radicals give rise to charged species.
For example, neutral peroxyl radicals may release HO
2
•
/O
2
•
−
(p
K
a
(HO
2
•
) = 4.8;
Chap. 8.11). The equivalence conductance of H
+
and OH
−
is 315 and 175
Ω
−
1
mol
−
1
cm
−1
, respectively. Monoanions and monocations have values in the range of
45-60
−1
mol
−1
cm
−1
. When the neutralization is completed, the signal of the
change of conductance produced by the charged species will be large and posi-
tive at pH below pH 7 [expression (20)], but smaller and negative above pH 7
[expression (21)]. The neutralization reaction must not push the pH out of the
basic range. The lowest pH at which an experiment can be carried out in basic
solution is hence approximately pH 9.
Ω
Acid solution
:
Pulse
H
+
+ X
−
= + 315 + 45
−1
mol
−1
cm
−1
→
Ω
(20)
−1
mol
−1
cm
−1
∆κ
= + 360
Ω
Basic solution
:
Pulse
H
+
+ X
−
= + 315 + 45
−1
mol
−1
cm
−1
→
Ω
(21)
Neutralization: H
+
+ OH
−
−1
mol
−1
cm
−1
→
H
2
O =
−
315
−
170
Ω
−1
mol
−1
cm
−1
∆κ
=
−
125
Ω
The available set-ups are very sensitive, and a pH range between 2.5 and 12 is
accessible. Obviously, the presence of buffers will have a considerable effect on
the signal height, but in favorable cases a computer analysis may allow the quan-
tification of the various contributions to the conductance signal even under such
conditions (Das et al. 1987; Schuchmann et al. 1989).
For dosimetry, the reaction of the reaction of
•
OH with DMSO which yields
methanesulfinic acid (92%; Veltwisch et al. 1980; Chap. 3.2) is usually used. This
allows one to put the conductance signals on a quantitative basis (calculation
of
G
values), and the rates of reactions that are kinetically of first order can be
determined for the time dependence of the signal evolution. DMSO dosimetry
yields only a relative dose. For the determination of second-order rate constants,
however, the exact dose must be known, and this can be determined by the 'zero
conductivity change dosimetry' or 'neutralization kinetics dosimetry' (Schuch-
mann et al. 1991).
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