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K. Mukherjee et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 204–208
Fig. 3. Absorption spectrum of unpromoted and promoted reaction mixtures (after
completion of reaction): (a) [D-sorbitol]T = 100 ꢂ 10ꢁ4 mol dmꢁ3
, [Cr(VI)]T = 5 -
Fig. 2c. Scanned absorption spectra of the reaction mixture at regular time interval
ꢂ 10ꢁ4 mol dmꢁ3
,
[H2SO4] = 0.5 mol dmꢁ3
,
(b) [PA]T = 75 ꢂ 10ꢁ4 mol dmꢁ3
, (c)
(2 min). [D-sorbitol]T = 100 ꢂ 10ꢁ4 mol dmꢁ3
,
[Cr(VI)]T = 5 ꢂ 10ꢁ4 mol dmꢁ3
, [H2-
[bipy]T = 75 ꢂ 10ꢁ4 mol dmꢁ3, (d) [phen]T = 75 ꢂ 10ꢁ4 mol dmꢁ3, Temp = 35 °C.
SO4] = 0.5 mol dmꢁ3, [TX-100]T = 1 ꢂ 10ꢁ2 mol dmꢁ3, Temp = 35 °C.
Experimental section
Materials
D-sorbitol (AR, Qualigens), picolinic acid (AR, BDH), 2,20-bipy
(AR, MERCK), 1,10-phen (AR, Spectrochem, India), sodiumdodecyl
sulphate (AR, SRL), TX-100 (AR, SRL) and all other chemicals used
are of highest purity available commercially. Solutions are
prepared in doubly distilled water. Concentrations of above
reagents are as follows: [D-sorbitol] = 100 ꢂ 10ꢁ4 mol dmꢁ3
,
[Cr(VI)]T = 5 ꢂ 10ꢁ4 mol dmꢁ3, [H2SO4] = 0.5 mol dmꢁ3
.
Procedure and kinetic measurements
The reaction is initiated by mixing the requisite amount of the
oxidant with the reaction mixture. Progress of the reaction is mon-
itored by measuring the absorbance of Cr(VI) at different time
interval at 450 nm. The kinetics is monitored for 80% completion
of the reaction by spectrophotometric determination of the decay
of absorbance of chromium(VI) at 450 nm using a Bausch & Lomb
Spectronic-20 spectrophotometer. No other reactant or product
has any significant absorption at this wavelength. Pseudo first
order rate constants (kobs) are evaluated from the linear plot of
log[Cr(VI)]T vs time (t) (Fig. 1) and the half-life (t1/2) values
(Table 1) were also determined.
Fig. 2d. Scanned absorption spectra of the reaction mixture at regular time interval
(30 s). [D-sorbitol]T = 100 ꢂ 10ꢁ4 mol dmꢁ3
,
[Cr(VI)]T = 5 ꢂ 10ꢁ4 mol dmꢁ3
, [H2-
SO4] = 0.5 mol dmꢁ3
,
[TX-100]T = 1 ꢂ 10ꢁ2 mol dmꢁ3
,
[phen]T = 75 ꢂ 10ꢁ4
-
mol dmꢁ3, Temp = 35 °C.
20-bipyridine (bipy), 1, 10-phenanthroline (phen) are well known
as promoter for chromic acid oxidation of organic substances
[19–27]. Our present work will find out the best combination of
promoter and micellar catalyst in presence of which the reaction
occurs most rapidly. The overall stoichiometry of the reaction
may be presented as:
The scanned spectra indicates the gradual disappearance of
Cr(VI) and the appearance of Cr(III) at regular time interval. For
unpromoted and uncatalyzed reaction an isobestic point appears
near about 535 nm (Fig. 2a). The presence of a isosbestic point
indicates that two species (hexavalent chromium and trivalent
chromium) were in equilibrium and formation of a single discrete
species (trivalent chromium) during the reaction [30]. A shift in
isobestic point was observed for both promoted and catalyzed
reaction. In aqueous medium bipy was the best promoter as is evi-
dent from Table 1. For this reaction isobestic point appeared at
523 nm (Fig. 2b) and for TX-100 catalyzed reaction isobestic point
appeared at 523 nm (Fig. 2c). But the rate of the reaction was
fastest in presence of the combination of phen and TX-100 and
in this case isobestic point was observed at 526 nm (see Fig. 2d).
Under the experimental condition the color of the solution after
completion of the reaction is pale blue for unpromoted reaction
(kmax = 582 nm and kmax = 356 nm) and the corresponding transi-
3RCH2OH þ 2HCrOꢁ þ 8Hþ ! 3RCHO þ 2Cr3þ þ 8H2O
4
where
H
HO
H
OH
H
OH
R =
H
OH
CH2OH
.
The product of the reaction is glucose. It is detected and
confirmed by spot tests by using the reagent solution prepared
by dissolving 4 g of urea and 0.2 g of stannous chloride in 10 ml
40% sulfuric acid under heating condition [28,29]. The presence
of isosbestic points in both figures indicates that two species are
in equilibrium.
4
tion are A2g(F) ? 4T2g(F) and 523 nm for 4A2g(F) ? 4T1g(F) of Cr(III)