2118 Bull. Chem. Soc. Jpn., 76, No. 11 (2003)
The Potentiometric Effect of As Ion on BZ Oscillation
trode system was a platinum electrode as an indicator and an Ag/
AgCl electrode as a reference electrode, with a laboratory-con-
structed agar salt bridge containing 0.1 M KNO3.
1.3. Procedure. The water-jacketed glass vessel was loaded
with 2 mL of 0.35 M KBrO3, 4 mL of 2 M malonic acid, 0.5 mL
of 0.2 M potassium bromide, and 3 mL of 5 M H2SO4. After
the solution had reached to thermal equilibrium (40 ꢃ 0:1 ꢄC),
the electrodes were inserted into the solution with continuous
and steady stirring. Then, the last species, that is 0.5 mL of 0.15
M Ce(d), was added into the system. To prepare stock solution
of Ce(d), we dissolved 0.8223 g of (NH4)2Ce(NO3)6 in 10 mL
of doubly distilled water. The total volume of reaction mixture al-
ways was 10 mL.
After adding the Ce(d) solution, the oscillations were potentio-
metrically recorded. After the steady state had been reached, the
system was perturbed by injecting a 0.5 mL sample of different
concentrations of As(c) solution. Different stock solutions of
As(c) were prepared from an arsenic(c) ion standard solution.
Changes in the oscillation amplitude, following perturbation,
were used as an analytical signal that was investigated and used
to construct the calibration plot.
Based on the FKN mechanism,15 two species were oscillating,
Brꢂ and Ce(d). Oscillations in Brꢂ concentration can be moni-
tored using a Brꢂ-ion selective electrode versus a reference
electrode. Then, the oscillation is recorded.
Fig. 1. A typical perturbation of As(c). 0.07 M KBrO3, 1 M
malonic acid, 10ꢂ4 M KBr, 0.75 M H2SO4 and 7:5 ꢁ 10ꢂ3
ꢄ
M Ce(d) at 40 C.
ally to the starting value (zone B), after which a new cycle
began. For the effect of an As(c) perturbation to be large,
we found the analyte should be injected at the potential mini-
mum as the system began to return to the starting value.
KBrO3: Increasing the KBrO3 concentration from 0.02 to
0.25 M caused an increase in the amplitude and frequency of
the the oscillations. Variation of the KBrO3 concentration
strongly affected the analyte perturbation also (Fig. 2a). In
0.07 M KBrO3, the sensitivity to As(c) was maximum.
Malonic Acid: The effect of malonic acid was studied over
the range from 0.4 to 1.2 M. The oscillation amplitude de-
creased, but the frequency increased with increasing malonic
acid concentration. As can be seen from Fig. 2b, there is an op-
timum concentration (0.8 M) where the system responded to
the analyte perturbation.
KBr: On the basis of the FKN mechanism,15 Brꢂ is an in-
termediate and plays an important role in the feedback mecha-
nism and oscillation, and appears and disappears during the
reaction. Brꢂ isn’t required as a reactant to start the reaction,
but decreases the induction period and the experiment time.16
As can be seen in Fig. 2c, the effect of Brꢂ on the analyte per-
turbation was studied over the range of 5 ꢁ 10ꢂ5 to 5 ꢁ 10ꢂ2
M. As expected, Brꢂ didn’t strongly affect the amplitude.
However, 0.01 M Brꢂ concentration was chosen as the opti-
mum.
One of cycles in this reaction is:
ð1Þ
This causes variations in the medium potential. This variation in
concentration and potential can be followed with a Pt electrode.
On the other hand, we can measure potential due to redox couple
of (Ce(d)/Ce(c)) on the Pt electrode versus the reference
electrode. We used the second method in this research.
The effect of different variables on the oscillating reaction was
studied in order to establish the optimum working conditions for
determination of interesting species. In order to ensure maximum
sensitivity and precision of the determination, the influence of se-
lected experimental variables in the presence and absence of ana-
lyte were investigated using different methods. For this purpose,
we measured the difference in amplitude between the first oscilla-
tion after analyte injection (A) and the oscillation amplitude imme-
H2SO4: Based on the BZ reaction and FKN mechanism, the
reaction should take place in a strongly acidic medium.16
Therefore, the oscillating properties and sensitivity were inves-
tigated in the range of 0.5 to 2.5 M H2SO4 (Fig. 2d).
Ce(d): The BZ reaction is catalyzed by a metal ion. In this
work, Ce(d) catalyzed the BZ reaction. For this purpose, the
Ce(d) concentration should be optimized. It was investigated
in the range of 10ꢂ3 to 10ꢂ2 M concentrations, with 7:5 ꢁ
10ꢂ3 M chosen as the optimum (Fig. 2e).
diately before injection (A ) and chose this different as the analyt-
ꢄ
ical signal:
ꢀA ¼ A ꢂ A:
ð2Þ
ꢄ
Figure 1 shows typical oscillation profiles obtained for the pro-
posed oscillating chemical system in the absence and presence of
an As(c) perturbation under the above described experimental con-
ditions.
Effect of Temperature: Because of the kinetic mechanism,
temperature strongly affects not only the original system but al-
so the analyte perturbation signal. Therefore, we evaluated the
effect of temperature on the BZ system and analyte signal. As
can be seen in Fig. 3, at lower temperatures analyte injection
caused an induction period that increased with decreasing
temperature. At different temperatures, the analyte signal
2. Results and Discussion
2.1. Effect of Variables on the Oscillation Perturbation.
Point of Injection: In order to ensure accurate and highly sen-
sitive results, where the injection was performed was a crucial
variable. During a typical oscillation cycle (Fig. 1), the poten-
tial dropped to a minimum (zone A) and then increased gradu-