1
72
SUBBA RAO ET AL.
EXPERIMENTAL
RESULTS AND DISCUSSION
Ϫ3
A known concentration of (0.1 mol dm ) iron(III)
in aqueous perchloric acid medium was prepared
and standardized according to the method described
elsewhere [10,11]. Fluka sample of sodium do-
decyl sulphate (SDS) has been used in the prepara-
Determination of Binding Constants
For determining binding constants of 2,2Ј-bipyridyl,
aniline, thiourea, and ascorbic acid with SDS mi-
ϩ
Ϫ3
Ϫ3
celles at [H ] ϭ 5.0 ϫ 10 mol dm and ϭ 0.1
Ϫ3
mol dm adjusted with NaClO , spectra of these
Ϫ3
4
tion of 0.1 mol dm solution. The purity of the
substances have been scanned at different SDS con-
centrations above cmc and under the conditions,
sample was tested by determining the critical
Ϫ3
Ϫ3
micelle concentration (cmc) (8.0 ϫ 10 mol dm )
12] conductometrically. The cmc of SDS corre-
sponding to the present experimental conditions
[SDS] ϾϾ [substrate] or [bipyridyl]. A suitable
[
wavelength is chosen ( ϭ 255 nm for aniline, 250
nm for thiourea, 220 nm for ascorbic acid, and 240
Ϫ3
(
1
ϭ 0.1 mol dm ) has been reported to be
O
W
nm for 2,2Ј-bipyridyl). AM and A , the absorbances
Ϫ3
Ϫ3
Ϫ3
.0 ϫ 10 mol dm [13,14]. 0.05 mol dm solu-
in the presence of micelle and its absence, respec-
tively, have been determined for various SDS concen-
trations. The binding constants have been determined
using the equation [15].
tions of aniline, perchloric acid, thiourea, and
bipyridyl were prepared and estimated by standard
methods.
The products of oxidation of aniline, thiourea, and
ascorbic acid have been found to be azobenzene [7],
formamidine disulphide [8], and dehydroascorbic
acid [9], respectively, along with tris(2,2Ј-bipyridyl)-
iron(II), the latter having a molar extinction coeffi-
O
O
O
1
/(A Ϫ A ) ϭ 1/(A Ϫ A ) (1 ϩ 1/K C)
M W M W s
O
A is limiting absorbance in the presence of micelle.
M
O
W
1/(A Ϫ A ) is plotted against C (i.e., stoichiomet-
M
Ϫ1
Ϫ1
3
cient of 8750 cm mol dm ( ϭ 510 nm). In the
ric concentration of SDS-cmc) and from slopes and
intercepts of these plots, the binding constants of
2,2Ј-bipyridyl, aniline, thiourea, and ascorbic acid
have been found to be 20.0 Ϯ 2.0, 70.0 Ϯ 3.0,
presence of SDS, the molar extinction coefficient of
2
ϩ
Ϫ1
Ϫ1
Fe(2,2Ј-bipyridyl)
decreases to 8250 cm mol
3
3
dm at the cmc and remains constant with further in-
crease in [SDS]. This shows that at [SDS] Ն cmc, the
product is completely bound by the micelle. The re-
action mixture, in each case, has been found to initi-
ate the polymerization of acrylo nitrile showing that
the reaction involves the formation of free radical in-
termediates.
3
Ϫ1
7.0 Ϯ 2.0, and 28.0 Ϯ 2.0 dm mol , respectively,
at 30°C.
Interestingly with each of the substrates, the same
kinetic pattern has been observed: the reaction obeys
first-order kinetics in iron(III) (Fig. 1) and substrate
(Table I) but the rate is directly proportional to
2
The course of reaction is followed by measur-
ing the absorbance of tris(bipyridyl)-iron(II) formed
at various times at 510 nm using Milton Roy
[bipyridyl] over a ten-fold variation of bipyridyl con-
2
centration. Plot of k vs. [bipy] (Fig. 2) is a straight
line passing through origin. The nonobtainment of in-
tercept shows that the contribution to the overall rate
(Spectronic 1201) spectrophotometer with kinetic
3
ϩ
attachment. The rate of oxidation of ascorbic acid
is considerably higher and hence the course of
the reaction is followed using stopped flow spec-
trophotometer (SFS-Biologic) at the same wave-
length as in the case of oxidation of aniline or
thiourea. At this wavelength, all the other mate-
ials concerned have negligible absorbance. All
the kinetic runs were carried out keeping [SDS]
in large excess such that the micellar surface
covered by the binding of products is negligible. In
the kinetic runs, the ionic strength was maintained
by path involving Feaq can be neglected. Further,
2
the direct proportionality between k vs. [bipy] rules
3
ϩ
out the involvement of the 1:1 complex, Fe(bipy) ,
as the active oxidizing species. The rate is propor-
ϩ 2
ϩ
tional to 1/[H ] over a ten-fold variation of H ion
(Fig. 3). The reaction is markedly accelerated by SDS
(Table II). The acceleration in the presence of the mi-
celle may be due to [16]: (i) the binding of the reac-
tants in a small volume of Stern layer of the micelle
thus leading to considerable concentration effect and
(ii) due to the possibility that the transition state is
stabilized more than the initial state in the micellar
pseudo-phase leading to the lowering of the activa-
tion energy. Though SDS catalysis has been observed
in each case, the acceleration was maximum with ani-
line as substrate (thirty times in the presence of SDS
Ϫ3
constant at 0.1 mol dm with sodium perchlorate.
ϩ
The concentration of H is at least 100 times more
than iron(III) and no change in pH has been ob-
served during the course of the reaction. Duplicate
kinetic runs were performed in each case and the
rate constants were found to be reproducible
within Ϯ 5%.
Ϫ3
Ϫ3
concentration of 5.0 ϫ 10 mol dm ) whereas the
extent of acceleration in the oxidation of ascorbic