Chemistry Reference
In-Depth Information
20
10
5
3
1
0.5
0.3
0.1
3.3
3.5
3.7
3.9
10
3
/T
°
K
Figure 6 The oxidation rate of ascorbic acid at low temperatures in undercooled (
m
) and
frozen solutions: dilute (
'
) and concentrated (
K
). Broken lines denote
subfreezing temperatures. Redrawn after Hatley et al.
59
example, worthy of detailed study, concerns the complex effects of
freezing on the acid-catalysed mutarotation kinetics of glucose.
17,58
The observed rate constant initially rises steeply during freeze-concen-
tration; it increases to a maximum at -81C to decline with further
decrease in temperature. The study also shows how any so-called inert
additives, e.g. salt, significantly affect the mutarotation kinetics during
freezing.
For a comparison between the effects of undercooling and freezing on
chemical rates, the oxidation kinetics of ascorbic acid by hydrogen
peroxide over the temperature range
þ
30 to -201C, shown in Figure 6,
can serve as a good example.
59
It is seen that the effects of cooling, but
with the avoidance of freezing, follow the normal Arrhenius type of
behaviour. On the other hand, if the solution is allowed to freeze, then
the oxidation rate increases during freeze-concentration, probably to
assume Arrhenius-type behaviour at some high degree of supersatura-
tion, but probably not with the same activation energy to that observed
Search WWH ::
Custom Search