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DUAL PHASE OSCILLATORY BEHAVIOR
203
range, do not completely merge with the first-phase
oscillations (Fig. 2(e) and (f)). The time lag between
the two phases decreased when the substrate concen-
tration was increased from 0.01 to 0.04 M.
In the uncatalyzed bromate system the oscillations
are in a single phase only (Fig. 3). Moreover, the plat-
inum potential profile of the uncatalyzed system bears
a close resemblance to the corresponding first-phase
in the Mn(II) as well as Ce(IV) system in respect of
number frequency, amplitude, and potential range.
Such a resemblance is observed in the BrϪ potential
profile as well. With ferroin as the catalyst the system
shows single-phase oscillation (Fig. 4).
The reactivity of vanillin is such that the bromi-
nation step is a highly favored one in the overall re-
action and, hence, the first-phase of oscillation in-
volves bromination of vanillin as the predominant
reaction. Further, the longer time per oscillation in the
second-phase rules out its participation in this phase
of the profile. Therefore, the substrate in the second-
phase is a derivative of vanillin which could probably
be vanillic acid formed by the oxidation of vanillin by
Ce(IV) or Mn(III)
The reactivity of vanillic acid is less compared to
vanillin. Experiments were carried out by employing
vanillic acid as the substrate in the uncatalyzed and
catalyzed systems. Vanillic acid is characterized by ill-
defined oscillations in the uncatalyzed bromate sys-
tem. The platinum potential profiles of Mn(II) and
Ce(IV) system with vanillic acid as the substrate have
a striking resemblance to the second-phase oscillation
in the Mn(II) and Ce(IV) system, respectively (Fig.
5(a) and (b)). This lends further support to the above
conclusions regarding the reactions taking place in the
first-and second-phase.
Figure 1 Dual-phase oscillatory behavior of vanillin—
Mn(II) system. (a), (c), and (e) potential profiles recorded
Ϫ
with platinum electrode. (b), (d), and (f) corresponding Br
potential profiles. Concentration conditions: [Vanillin] ϭ
0.03 M, [H2SO4] ϭ 0.6 M; [KBrO3] ϭ 0.09 M; Acetonitrile
ϭ 20% (v/v); and Temperature ϭ 30 Ϯ 0.1ЊC [MnSO4] ϭ
0.001 M (a and b), 0.005 M (c and d), and 0.03 M (e and
f).
The decrease in the time interval between the two
phases as the metal ion and substrate concentration
increases, is because of the faster build up of the ox-
idized product. The oxidation of vanillin by Ce(IV) or
Mn(III), has been confirmed by following the variation
in the potential of Mn(III) and Ce(IV) in presence of
acidified vanillin under nonoscillatory conditions.
There is a drastic decrease in the potential of a plati-
num electrode dipped in a solution of acidified Ce(IV)
or Mn(III) on the addition of vanillin. However, such
a decrease in potential is not observed when acidified
ferrin is employed.
tial profile has the same frequency as the platinum
profile in both the phases (Fig. 1(b)). The amplitude
of BrϪ oscillations is small indicating a small change
in the concentration of BrϪ between the base and peak.
An increase in the concentration of Mn(II) to 0.005 M
leads to a decrease in the time lag between the two
phases (Fig. 1(c)) and an increase in the amplitude of
the platinum potential profile. An increase in the con-
centration of Mn(II) to 0.03 M results in the merging
of the two phases (Fig. 1(e)). A similar trend is ob-
served in the BrϪ potential profile with respect to the
frequency and amplitude of oscillations (Fig. 1(d) and
(f)).
The oscillatory behavior of this system can be ra-
tionalized in terms of the following steps involved in
the overall reaction [9].
The decrease in time lag between the first-and sec-
ond-phase of oscillations is a characteristic feature of
the Ce(IV) system also (Fig. 2). However, even at the
highest concentration of Ce(IV) employed, the sec-
ond-phase oscillations, being at a higher potential
Ϫ
BrϪ ϩ BrO3 ϩ 2Hϩ EF HOBr ϩ HBrO2 (1)
BrϪ ϩ HOBr ϩ Hϩ EF Br2 ϩ H2O
(2)