2216
2 + log
LAMANI et al.
study reveled that the complex is diamagnetic. The
A
compound prepared was analyzed [10] for silver and
periodate by acidifying a solution of the material with
HCl, recovering and weighing the AgCl for Ag and
titrating the iodine liberated when excess KI was
added to the filtrate for IO–4 . The aqueous solution of
DPA was used for the required in the reaction mixture.
During the kinetics a constant concentration, viz.
2.1
1.3
0.5
5
4
3
1.0
10–5 mol dm–3 of KIO4 was used throughout the
×
2
1
study unless otherwise stated. Thus, the possibility of
oxidation of LꢀPhe by periodate was tested and found
that there was no significant interference due to KIO4
under experimental conditions.
0
1
2
3
, min
t
Kinetic Measurements
Fig. 1. First order plots for oxidation of Lꢀphenylalaꢀ
The kinetics was followed under pseudo firstꢀorder
condition where [LꢀPhe] > [DPA] at 27 0.1°C,
unless otherwise specified. The reaction was initiated
by mixing the DPA to LꢀPhe solution, which also conꢀ
tained the required concentration of KNO3, KOH,
and KIO4. The progress of reaction was followed specꢀ
trophotometrically at 360 nm by monitoring the
decrease in absorbance due to DPA with the molar
nine by DPA in aqueous alkaline medium at 27°C;
5
3
[DPA] 10 (mol dm ): (
×
1) 1.0, (2) 3.0, (3) 5.0, (4) 8.0,
(5) 10.0; A is absorbance.
purity of the LꢀPhe sample and with its m.p. 273°C (lit
m.p 275°C). The required concentration of LꢀPhe was
used from its aqueous stock solution. KNO3 and KOH
were used to maintain ionic strength and alkalinity of
the reaction respectively. An aqueous solution of
AgNO3 was used to study the product effect, Ag(I). A
stock standard solution of IO–4 was prepared by disꢀ
solving a known weight of KIO4 (Riedelꢀde Haen) in
hot water and used after keeping 24 h to maintain the
equilibrium. Its concentration was ascertained iodoꢀ
metrically [8] at neutral pH maintained using phosꢀ
phate buffer. The pH of the medium in the solution
was measured by ELICO LI613 pH meter.
absorbancy index
to be 13900 100 dm3 mol–1 cm–1.
ε
It was verified that there is a negligible interference
from other species present in the reaction mixture at
this wavelength. The pseudo first order rate constants,
k
obs, were determined from the logA (A is absorbance)
vs time plots. The plots were linear up to 80% (Fig. 1)
completion of reaction under the range of [OH–] used.
In view of the modest concentration of alkali used
in the reaction medium, attention was also directed to
the effect of the reaction vessel surface on the kinetics.
Use of polythene/acrylic wares and quartz or polyꢀ
acrylate cells gave the same results, indicating that the
surface does not have any significant effect on the
reaction rates.
Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence
of KIO4 as described elsewhere [9]: a mixture of 28 g of
KOH and 23 g of KIO4 in 100 cm3 of water along with
8.5 g of AgNO3 was heated just to boiling and 20 g of
K2S2O8 was added in several lots with stirring and then
the mixture was allowed to cool. It was filtered through
a medium porosity fritted glass filter and NaOH (40 g)
was added slowly to the filtrate, whereupon a volumiꢀ
nous orange precipitate agglomerates. The precipitate
was filtered as above and washed three to four times
with cold water. The pure crystals were dissolved in
water (50 cm3) and warmed to 80°C with constant stirꢀ
ring thereby some solid was dissolved to give a red
solution. The resulting solution was filtered when it
was hot and on cooling at room temperature, orange
crystals separated out and were recrystallised from
water.
Kinetics runs were also carried out under N2 atmoꢀ
sphere in order to understand the effect of dissolved
oxygen on the rate of reaction. No significant differꢀ
ence in the results was obtained under a N2 atmoꢀ
sphere and in the presence of air. In view of the ubiqꢀ
uitous contamination of carbonate in the basic
medium. The effect of carbonate was also studied.
Added carbonate had no effect on the reaction rates,
however fresh solutions were nevertheless used for carꢀ
rying out each kinetic only. The spectroscopic changes
during the reaction are shown in Fig. 2. It is evident
from the Fig. 2 that the concentration of DPA
decreases at 360 nm.
Regression analysis of experimental data to obtain
The complex was characterized from its UV specꢀ
trum, exhibited three peaks at 216, 255, and 362 nm.
These spectral features were identical to those
regression coefficient
r and the standard deviation S,
of points from the regression line, was performed with
reported earlier for DPA [9]. The magnetic moment the Microsoft office Excelꢀ2003 programme.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 83
No. 13
2009