American Journal of Analytical Chemistry, 2010, 1, 25-30
doi:10.4236/ajac.2010.11003 Published Online May 2010 (http://www.SciRP.org/journal/ajac)
Kinetic Studies on Hexavalent Chromium Reduction
Ankita Basu, Bidyut Saha*
Department of Chemistry, University of Burdwan, Golapbag, India
Received January 10, 2009; revised February 21, 2009; accepted February 23, 2009
Abstract
Cr(VI) is a known human carcinogen. It is a wide spread environmental contaminant as it is extensively used
in different industry. The kinetic study of reduction of Cr(VI) by a known organic substance, 1-butanol in
micellar media have been studied spectrophotometrically. The reduction of Cr(VI) to Cr(III) occurs in a micro-
heterogeneous system in cell cytoplasm. As micelles are considered to mimic the cellular membranes, the
reduction process occurring in the micellar system is considered as a model to obtain insight in to the reduc-
tion process prevailing in body systems. Micellar media is also a probe to establish the mechanistic paths of
reduction of Cr(VI) to Cr(III) and the effects of some electrolytes common to a biological systems are stud-
ied to establish the proposed reaction mechanism strongly. The overall reaction follows a first order depend-
ency on substrate and hexavalent chromium and second order dependency on hydrogen ion. Suitable surfac-
tant & suitable concentration of electrolyte enhance the rate of the reaction.
Keywords: Pollution, Carcinogen, Cr(VI), Oxidation; Kinetics, Surfactant, Electrolyte
1. Introduction
Water pollution by chromium is of considerable concern,
as this metal has found widespread use in electro plating,
leather tanning, metal finishing, nuclear power plant, tex-
tile industries and chromate preparation [1]. The effluents
from these industries contain Cr(III) and Cr(VI) at con-
centrations ranging from ten to hundreds of mg/L [2]. The
hexavalent form is 500 times more toxic than the trivalent
[3]. Though chromium exists in nine valence states rang-
ing from –2 to +6, Cr(III) and Cr(VI) are of major envi-
ronmental significance because of their stability in the
natural environment [4]. The chromate anion is highly
soluble and therefore can overcome the cellular perme-
ability barrier, entering via sulphate transport pathways
since it bears structural similarity with [5-7]. It has
been reported that hexavalent chromium causes lung can-
cer, chromate ulcer, perforation of nasal septum and kid-
ney damage in humans and it is also toxic to other organ-
ism as well [8,9]. Chromium in its trivalent form is an
essential micronutrient for many microorganisms, rela-
tively insoluble in water.
2
4
SO 
A number of treatment methods for removal of metal
ions from aqueous solutions have been reported mainly
reduction, ion exchange, solvent extraction, reverse os-
mosis, chemical precipitation and adsorption [10]. In the
reduction followed by chemical precipitation method
[11], Cr(VI) is reduced to Cr(III) first, then lime is added
to precipitate chromium as hydroxide.
In this paper, we studied kinetics and mechanism of
Cr(VI) reduction to Cr(III) by an alcohol in presence of
micelle and electrolyte because micelle [12-14] and elec-
trolyte [15] substantiate the reaction mechanism. The
outcome of such an exercise will certainly influence the
detoxification methods.
2. Experimental
2.1. Materials and Reagents
Butan-1-ol (AR, Merck, India), K2Cr2O7 (AR, BDH, In-
dia), N-cetyl pyridinium chloride (CPC) (AR, SRL, In-
dia), Sodium dodecyl sulphate (SDS) (AR, SRL, India),
TX-100 (AR, SRL, India), NaCl (AR, Merck, India),
NH4Cl (AR, Ranbaxy, India) and other chemicals used
were of highest purity available commercially. Solutions
were prepared in double distilled water.
2.2. Procedure and Kinetic Measurements
Under the kinetic conditions, solutions of the oxidant and
mixtures containing the known quantities of the sub-
strate(s) (i.e., butan-1-ol) (under the conditions [S]T >>
[Cr(VI)]T), acid and the other necessary chemicals were

A. BASU ET AL. 26
separately thermostated (±0.1℃). The reaction was initi-
ated by mixing the requisite amounts of the oxidant with
the reaction mixture. Zero time was set when half of the
required volume of the oxidant had been added. The
progress of the reaction was followed by monitoring the
decay of oxidant [Cr(VI)] at 415 nm at different time
intervals (2 minutes) with a UV-VIS spectrophotometer
[UV-2450(SHIMADZU)]. Quartz cuvettes of path length
1 cm were used. The observed pseudo-first-order rate
constants [kobs(s-1)] were determined from the linear part
of the plots of ln(A415) versus time(t). Reproducible re-
sults giving first-order plots (co-relation co-efficient, r ≥
0.998) were obtained for each reaction run. A large ex-
cess (≥ 15-fold) of reductant was used in all kinetic runs.
No interference due to other species at 415 nm was veri-
fied. Under the experimental conditions, the possibility
of decomposition of the surfactants by Cr(VI) was inves-
tigated and the rate of decomposition in this path was
found to be kinetically negligible.
2.3. Product Analysis and Stoichiometry
Under the kinetic condition (i.e., [butanol]T >> [Cr(VI)]T)
butanol oxidizes to butanal and estimation of the reaction
products was carried by gravimetrically as 2,4-dinitro-
phenylhydrazone. In a typical experimental set, 10ml of
0.06 mol·L-1 Cr(VI) in 1.0 mol·L-1 H2SO4 was added to
40 mL of 0.2 mol·L-1 butanol and the reaction was al-
lowed to proceed to completion. Completion of the reac-
tion was indicated by the disappearances of Cr(VI) color.
Then the reaction mixture was added slowly with stirring
to 60 mL of a saturated solution of 2,4-dinitrophenylhy-
drazine in 2.0 mol·L-1 HCl. After storing for about 1 h in
an ice-bath, the precipitate was collected weighed sin-
tered glass crucible, washed with 2.0 mol·L-1 HCl fol-
lowed by water and dried to a constant weight at
100-105℃and recrystallized from ethanol. The hydra-
zone showed melting point 123-125℃ [16]. The found
ratio, [Cr (VI)]T/[Carbonyl compound]T ≈ 2/3 (from 3
independent determinations) supports the following Stoi-
chiometry:
+
322 4
3CHCHCHOH + 2HCrO 8H


322 2
3CHCHCHCHO 2Cr(III) 8HO
3. Results and Discussion
3.1. Dependence on Cr(VI)
Under the experimental condition, [Butan-1-ol]T >>
[Cr(VI)]T, the rate of disappearance of Cr(VI) shows a
first order dependency on Cr(VI). This dependence is
also maintained in the presence of cationic surfactant
(CPC), anionic surfactant (SDS) and neutral surfactant
(TX-100) (Figure 1).
3.2. Dependence on [Substrate]T i.e., [Butan-1-ol]T
From the plot of kobs vs [Butan-1-ol]T (Figure 2), it is
established that the path shows a first order dependency
on [substrate]T, i.e., [butan-1-ol]T.
kobs = ks[S]T.
The above first order dependence on [substrate]T is
also maintained in the presence of surfactant like CPC,
SDS, TX-100.
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
0510
time(min)
-l nA
15
D
A
C
B
Figure 1. Some representative first-order plots to evaluate
the pseudo-first-order rate constants(kobs)for the Cr(VI)
oxidation of 1-butanol at 30℃.Dependence of absorbance of
Cr(VI) on time (min) for the oxidation of 1-butanol at 30℃.
[Cr(VI)] = 5 × 10-4 mol·L-1, [1-butanol] = 1500 × 10-4 mol·L-1,
[H2SO4] = 0.50 mol·L-1. A: [SDS]T = 0 mol·L-1, [CPC]T = 2 ×
10-3 mol·L-1, [TX-100]T = 0 mol·L-1. B: [SDS]T = 0 mol·L-1,
[CPC]T = 0 mol·L-1, [TX-100]T = 2 × 10-2 mol·L-1. C: [SDS]T
= 0 mol·L-1, [CPC]T = 0 mol·L-1, [TX-100]T = 0 mol·L-1. D:
[SDS]T =2 × 10-2 mol·L-1, [CPC] = 0 mol·L-1, [TX-100]T = 0
mol·L-1.
0
2
4
6
8
10
12
05001000 150020002500
[1-butanol]
T
(mol dm
-3
)
10
4
k
obs
(s
-1
)
A
B
C
D
Figure 2. Dependence of kobs on [1-butanol] for the Cr(VI)
oxidation of 1-butanol at 30℃. [Cr (VI)] = 5 × 10-4 mol·L-1,
[H2SO4] = 0.50 mol·L-1. A: [SDS]T = 2 × 10-2 mol·L-1, [CPC]T
= 0 mol·L-1, [TX-100]T = 0 mol·L-1. B: [SDS]T = 0 mol·L-1,
[CPC]T = 0 mol·L-1, [TX-100]T = 2 × 10-2 mol·L-1. C: [SDS]T
= 0 mol·L-1, [CPC]T = 0 mol·L-1, [TX-100]T = 0 mol·L-1. D:
[SDS]T = 0 mol·L-1, [CPC] = 2 × 10-3 mol·L-1, [TX-100]T = 0
mol·L-1.

A. BASU ET AL.27
3.3. Dependence on [H+]T
The acid dependence was followed in aqueous HClO4
medium at fixed Cr(VI) and [substrate]. From the ex-
perimental fit (Figure 3), the observation is
kobs = kH[H+]2
.
Thus the acid variation shows a second order depend-
ence on [H+] ion. The above second order dependency is
also maintained in presence of surfactants.
3.4. Effects of Surfactants
3.4.1. Effects of CPC
Cetyl Pyridinium Chloride (CPC, a representative cati-
onic surfactant is found to retard the reaction path. Plot
of kobs vs [CPC]T (Figure 4) shows a continuous de-
crease and finally it tends to level off at higher concen-
tration of CPC. The observation is identical to that ob-
served by Bunton and Cerichelli [17] in the oxidation of
ferrocene by ferric salt salts in the presence of cationic
0
3
6
9
12
15
00.511.5
[HClO
4
]
2
10
4
k
obs
(s
-1
)
T
(mol dm
-3
)
A
B
Figure 3. Dependence of kobs on [H+] for the Cr(VI) oxida-
tion of 1-butanol 30℃. A: [Cr (VI)] = 5 × 10-4 mol·L-1,
[1-butanol] = 500 × 10-4 mol·L-1, [SDS] = 2 × 10-2 mol·L-1. B:
[Cr (VI)] = 5 × 10-4 mol·L-1, [1-butanol] = 500 × 10-4 mol·L-1,
[SDS] = 0 mol·L-1.
0.002 0.004 0.006 0.008 0.010
0.00000
0.00005
0.00010
0.00015
0.00020
0.00025
Fig 5
kobs(s-1)
[CPC]T(mol dm-3)
Figure 4. Dependence of kobs on [CPC]T for the Cr(VI) oxi-
dation of 1-butanol at 30℃. [Cr(VI)] = 5 × 10-4 mol·L-1,
[1-butanol] = 1000 × 10-4 mol·L-1, [H2SO4] = 0.50 mol·L-1.
surfactant cetyl trimethyl ammonium bromide (CTAB).
3.4.2. Effects of SDS
Sodium dodecyl sulphate(SDS, a representative anionic
surfactant) accelerate the reaction path. The plot of kobs
vs [SDS]T (Figure 5) shows a continuous increase up to
the concentration of SDS.
3.4.3. Effects of TX-100
Triton X-100(TX-100, a representative neutral surfactant)
accelerates the reaction path. But the acceleration rate in
TX-100 is less than that of SDS. The plot of kobs vs
[TX-100]T
Figure 6 shows a continuous increase up to the con-
centration of TX-100.
3.4.4. Test for Acrylonitrile Polymerization
Under the experimental conditions, the existence of free
radical was indicated by polymerization of acrylonitrile
under a nitrogen atmosphere.
2
2.5
3
3.5
02468
10
4
[SDS]
10
10
4
k
obs
(s
-1
)
T
(mol dm
-3
)
(mol·L-1)
(mol·L-1)
Figure 5. Dependence of kobs on [SDS] T for the Cr(VI) oxi-
dation of 1-butanol at 30℃. [Cr(VI)] = 5 × 10-4 mol·L-1,
[1-butanol] = 500 mol·L-1, [H2SO4] = 0.50 mol·L-1.
0
1
2
3
4
0510
10
2
[TX-100]
T
(mol dm
-3
)
10
4
k
obs
(s
-1
)
15
(mol·L-1) (mol·L-1)
Figure 6. Dependence of kobs on [TX-100]T for the Cr(VI)
oxidation of 1-butanol at 30℃. [Cr(VI)] = 5 × 10-4 mol·L-1,
[1-butanol] = 500 × 10-4 mol·L-1, [H2SO4] = 0.50 mol·L-1.

A. BASU ET AL. 28
3.4.5. Mechanism and Interpretation
1CH
3222 Cr
O
O
O H
H
+2
Neutral Ester
Butan-1-ol
H O
2
+
K
CH3
CH
2
CH
2
CH
2OH+HCrO
4+H+K
CH CH CH O
CH
3222Cr
O
O
H
Neutral Ester
+
CH CH CH O
O
CH CHCH CH
3222
OCr
O
OOH2
+
CH3 CH CH CH
22
O
Cr
O
O
OH 2
+
H
CH CH CH
CH
322O
Butanal
+
CrIV kH
+
+
Scheme 1. Cr(VI) reduction by butan-1-ol.
Scheme 1 leads to the flowing rate law
kobs = (2/3) kK1K2[S]T[H+]2
The pseudo-first-order rate constants (kobs) in the
presence of various concentrations of different types of
surfactants, SDS, CPC, TX-100 are represented in Fig-
ures 4-6. The pseudo phase ion-exchange (PIE) [18]
model is applied most widely in micellar catalysis. The
basic assumption of the PIE is as follows:
1) Micelles act as a separate phase from water, all re-
actants are distributed quickly between water and micel-
lar phase, and the reaction rate can be considered as the
sum of that in two phases.
2) The reaction in the micellar pseudo phase occurs
mainly at micelle surface.
3) The reactant ions and the inert ions compete at the
charged micellar surface.
The data reveal that SDS and TX-100 accelerate the
rate where as CPC decreases the rate. The rate accelera-
tion is higher in the case of SDS than TX-100. This can
be explained by Schemes 2 and 3.
kWProduct
kMProduct
[Neutral ester]+ [H+]
[Neutral ester]+ [H+]
K
KS
ww
M
M
Scheme 2. Partitioning of the reactive species between the
aqueous and micellar phases.
Scheme 3. Structural representation of anionic & cationic
surfactants.
Ever since J. W. Mc Bain proposed the presence of
molecular aggregates in soap solution on the basis of the
unusual changes in electrical conductivity observed with
changing soap concentration [19], the structure of micel-
lar aggregates has been a matter of discussion. G. S.
Hartley proposed that micelles are spherical with charged
groups situated at the micellar surface [20], whereas Mc
Bain suggested that lamellar and spherical forms coexist
[21]. X-ray studies by Harkins et al., [22,23] then sug-
gested the sandwich or lamellar model. Later, P. Debye
and E. W. Anacker proposed that micelles are rod-shaped
rather than spherical or disk like [24]. The cross section
of such a rod would be circular, with the polar heads of
the detergent lying on the periphery and the hydrocarbon
tails filling the interior. The ends of the rod would almost
certainly have to be rounded and polar. In 1956, Hart-
ley’s spherical micelle model was established by I. Reich
[25] from the view point of entropy, and the spherical
form is now generally accepted as approximating the
actual structure. The formation of micelles by ionic sur-
factants is ascribed to a balance between hydrocarbon
chain attraction and ionic repulsion. The net charge of
micelles is less than the degree of micellar aggregates,
indicating that a large fraction of counter ions remains
associated with the micelle; these counter ions form the
Stern layer at the micellar surface. For nonionic surfac-
tants, however, the hydrocarbon chain attraction is op-
posed by the requirements of hydrophilic groups for hy-
dration and space. Therefore, the micellar structure is
determined by equilibrium between the repulsive forces
among hydrophilic groups and the short-range attractive
forces among hydrophobic groups. For bimolecular reac-
tions inhibition arises from incorporation of one reactant
into the micellar pseudo phase and exclusion of the other
from it. Catalysis is apparently caused, for the most part,
by concentration of the two reactants into a small volume
in the micellar Stern layer [26].
The substrate is partitioned in the Stern layer of the

A. BASU ET AL.29
micellar phase. SDS being an anionic surfactant, owing
to the electrostatic attraction between the positively
charged [H+] species and negatively charged micellar
head groups. [H+] easily attaches to the Stern layer of the
micelle. The reaction takes place in both the micellar and
aqueous media. The observed rate acceleration is due to
the favored reaction in the micellar phase, where both H+
and the neutral ester are preferably accumulated. In the
case of TX-100, H+ also attaches to the Stern layer of the
micelle, but the amount is less compared to SDS because
TX-100 is a neutral surfactant, so no electrostatic attrac-
tion takes place. CPC is a cationic surfactant and con-
sequently due to the electrostatic repulsion between the
positively charged [H+]species and positively charged
micellar head group, [H+] does not attaches to the Stern
layer of micelle through the substrate. The reaction takes
place only in aqueous media, which is depleted in the
substrate concentration.
3.4.6. Effect of Added Electrolyte
Experimental evidence has shown that electrolyte inhibi-
tion of micellar catalysis is a general phenomenon [27-29]
with one apparent exception [30]. Here we report a sys-
tem which connects inhibition and enhancement. The
proposed study has taken into consideration for better
under standing of Cr (VI) carcinogenity. The added elec-
trolytes are common to biological systems. Electrolyte
inhibition is rationalized by assuming that a counter ion
competes with an ionic reagent (e.g., OH-, H3O+, and X-)
for a site on the ionic micelle [31]. Enhancement of mi-
cellar catalysis by added salt is caused by their changing
the shape or reducing the charge density of the micelle.
Salts decrease the cmc (critical micelle concentration)
and increase the aggregation number of ionic micelles
[31-33] probably because increased screening by the
counter ions decreases the effective charge density of the
micelle. Added salts are very common sodium chloride
(NaCl) and ammonium chloride (NH4Cl). In SDS me-
dium the rate is retarded up to 0.7 mol·L-1 solution of
NH4Cl and 0.8 mol·L-1 solution of NaCl. After that the
rate is increased (Figure 7). Up to 0.7 mol·L-1 of NH4Cl
counter ion competition is important and after that mi-
cellar shape is important. The reason is same for NaCl up
to 0.8 mol·L-1.
4. Conclusions
Kinetics and mechanism of chromic acid reduction by
butan-1-ol in aqueous acid media have been studied un-
der the conditions [butan-1-ol]T >> [Cr(VI)]T. Under the
kinetic conditions, the monomeric species of Cr(VI) has
been found kinetically active. Cr(VI)-substrate ester ex-
periences a redox decomposition through 2 electron
transfer at the rate determining step. The reaction shows
both 1st order dependency on [butan-1-ol]T and [Cr(VI)]T
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
0.00002
0.00004
0.00006
0.00008
0.00010
0.00012
Fig 7
B
A
NaCl
NH4Cl
104kobs(s-1)
[Electrolyte](x) (mol dm-3)
Figure 7. Dependence of kobs on [electrolyte(x)] for the Cr(VI)
oxidation of 1-butanol at 30℃ in SDS medium. [Cr(VI)] = 5
× 10-4 mol·L-1, [1-butanol] = 1500 × 10-4 mol·L-1, [H2SO4] =
0.50 mol·L-1. A: x= [NH4CL]. B: x = [NaCl].
and second order dependency on [H+] ion. In the pres-
ence of non-functional surfactant, the order of the reac-
tion remains unaltered. CPC has been found to retard the
rate while SDS and TX-100 shows rate acceleration ef-
fect. The effect of added electrolyte is inhibition fol-
lowed by enhancement in the rate.
5. Acknowledgements
Financial support from UGC, New Delhi is thankfully
acknowledged. Co-operation from Dr. Bholanath Mandal
of this department is thankfully acknowledged.
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