Chemistry Reference
In-Depth Information
•
OH + HCO
3
−
H
2
O + CO
3
•
−
→
(9)
The CO
3
•
−
radical is characterized by a strong absorption at 600 nm (
2000 dm
3
mol
−
1
cm
−
1
) (Weeks and Rabani 1966; Zuo et al. 1999). This absorption does not
change between pH 0 and 13 (Czapski et al. 1999; Zuo et al. 1999; see, however,
Eriksen et al. 1985). Nevertheless, the p
K
a
value of HCO
3
•
[equilibrium (10)] con-
tinues to be debated, and p
K
a
values between 7 and 9.6 have been reported (Chen
and Hoffman 1972; Chen et al. 1973; Eriksen et al. 1985; Zuo et al. 1999). Howev-
er, there is now increasing evidence (Bisby et al. 1998; Czapski et al. 1999) that it
must be much lower, <0 (Czapski et al. 1999), and hence it is more acidic than its
parent [p
K
a
(H
2
CO
3
) = 3.5]. For a detailed discussion, see Czapski et al. (1999).
ε
≈
(10)
10
10
dm
3
mol
-1
s
-1
], but also the reaction of e
aq
−
with nitrate leads to
•
NO
2
via a short-lived ad-
duct, NO
3
•
2
−
[reaction (12);
k
= 9.7
The reaction of
•
OH with nitrite yields
•
NO
2
[reaction (11);
k
≈
1
×
10
9
dm
3
mol
−
1
s
−
1
]. The latter subsequently
decays into
•
NO
2
[reaction (13);
k
= 4.6
×
10
3
s
−
1
; Alfassi et al. 1998].
×
•
OH + NO
2
−
OH
−
+
•
NO
2
→
(11)
e
aq
−
+ NO
3
−
NO
3
•
2
−
(12)
→
•
NO
2
+ 2OH
−
NO
3
•
2
−
+ H
2
O
→
(13)
The sulfate radical anion, SO
4
•
−
, can be formed from peroxodisulfate, S
2
O
8
2
−
,
photolytically [reaction (14)] or by its reaction with e
aq
−
[reaction (15);
k
= 1.2
×
10
7
dm
3
mol
−
1
s
−
1
]. For the
photolytic generation, one has to take into account that peroxodisulfate absorbs
only weakly in the UV with absorption coefficients very close to that of H
2
O
2
(Chap. 2.4). However, its decomposition can be sensitized by triplet acetone (ace-
tone reacts only slowly with SO
4
•
−
). As measured by photoacoustic calorimetry,
the reaction volume and enthalpy changes for reaction (14) are 8.9 ml mol
−
1
and
120 kJ mol
−
1
, respectively (Brusa et a l. 2000). Compared to H
2
O
2
, peroxodisulfate
has a rather weak O
10
10
dm
3
mol
−
1
s
−
1
] and
•
H [reaction (6);
k
= 1.4
×
O bond, and this is ref lected by its ready cleavage which can
also be induced thermally (Strasko et al. 2000) at temperatures, where H
2
O
2
does
not yet show any noticeable decomposition (Chap. 2.4).
−
S
2
O
8
2
−
+ h
2SO
4
•
−
(14)
ν
→
SO
4
•
−
+ SO
4
2
−
S
2
O
8
2
−
+ e
aq
−
(15)
→
S
2
O
8
2
−
+
•
H
SO
4
•
−
+ HSO
4
−
(16)
→
The SO
4
•
−
radical is one of the strongest oxidants (cf. Table 5.2), and in the pres-
ence of Cl
−
it is in equi librium w it h Cl
•
[reaction (17);
K
= 2.9 (Buxton et al. 1999),
K
= 1.5 (Yu et al. 2004)].
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