# Kinetic and Mechanistic Study of the Reduction of Chromium(VI) by Lactic Acid

Authors: Jinhuan Shan; Fang Wang; Changying Song; Heye Wang
Journal: Research Letters in Inorganic Chemistry
 (2008)

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## Abstract

The kinetics and mechanism of the reduction of chromium(VI) by lactic acid (Lac) in aqueous acidic medium was  studied with spectrophotometry in a temperature range of 298.15 K~313.15 K. Under the conditions of the pseudo-first order([Lac]0≫[Cr(VI)]0), the observed rate constant (kobs) increased with the increase in [Lac] and [H+]. There is no salt effect. Based on the experimental results, a probable reaction mechanism of oxidation was proposed. The rate equation derived from the mechanism could explain all the experimental phenomena. Activation parameters along with rate constant of the rate-determining step have been evaluated.


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## Body

## 1. Introduction

Lactic
acid is widely  used
in medications, foods, agriculture, industry, and so forth.
It is a fine pH regulator and has an importance physiological function.
Lactic acid can be used to prepare degradable
plastics and agroplastic
film [1], which can reduce soil
and water pollution.Chromium
exists usually in both trivalent and hexavalent
forms in aqueous systems. However, these two oxidation states
are characterized
by markedly different
physical/chemical behavior and toxicity. Cr(III) is
readily precipitated or absorbed on a variety of
inorganic and organic
surfaces at near neutral pH [2, 3].
Cr(III), as
an essential trace metal nutrient, also improves lipid profile
and glucose
tolerance by increasing the level of high density lipoprotein
cholesterol and
decreasing total serum cholesterol [4].
In contrast, the hexavalent form is of prime concern because of its
high toxicity. Cr(VI) usually occurs as the highly soluble
anion. The
main sources of chromium pollution are mining, leather tanning,
cement
industries, electro plating, production of steel and other
metal alloys,
photographic material, and corrosive paints
[5–7]. The
hexavalent form of chromium has been demonstrated to be
associated with the
toxic parameters and classified as human carcinogen and
mutagen [8]. Breathing
high levels of hexavalent chromium can damage and irritate nose,
lungs,
stomach, and intestine [9–11].
At present,
there have been some researches regarding this type of reaction
system [12, 13],
while
thorough research is essential for understanding its role in
the oxidation
of some organic compounds. The objectives of this research
were to evaluate the
kinetics of chromate reduction by lactic acid and investigate
their redox
mechanisms.


## 2. Experimental

## 2.1. Reagents and Apparatus

All chemicals used were of AR grade, and doubly distilled
water was used
throughout the work.NaClO4  and HClO4 were used to adjust
ionic strength and acidity of the reaction, respectively. The study was
performed with a TU-1900 UV-Vis spectrophotometer fitted
with a 501-thermostat (±0.1K, Shanghai).


## 2.2. Kinetic Measurements and Product Analysis

Under pseudo-first-order condition ([Lac]0≫[Cr(VI)]0),
the reaction was initiated by mixing the Cr(VI) to lactic acid solution, and the
kinetics was followed at 350 nm by monitoring decrease in absorbance due to Cr(VI)
(Figure 1).
The pseudo-first-order rate constants, kobs,
were obtained as in the previous work [14]. The way for product analysis was
similar to previous study [15], and the product was identified as
the pyruvic acid and Cr(III) [16, 17].Spectra changes during
the reduction of Cr(VI) by lactic acid at 298.15 K. [Cr(VI)]=2.5×10−4mol⋅L−1,
[Lac]=0.10mol⋅L−1, [HClO4]=0.50mol⋅L−1, [DMSO]=0.20mol⋅L−1, △t=150 seconds.
(a)(b)


## 3. Results and Discussions

## 3.1. Rate Dependence on [H+]

It was
observed that the rate constants increased by increasing [H+], and
the plots of 1/kobs versus 1/[H+] are linear with positive
intercept (Figure 2).Figure 2
Plots
of1/kobsversus1/[H+]. [Cr(VI)]=2.5×10−4mol⋅L−1,  [Lac]=0.50mol⋅L−1,  [DMSO]=0.20mol⋅L−1, I=1.0mol⋅L−1.


## 3.2. Rate Dependence on [Lac]

The influence of lactic acid on the rate
of reaction was studied at different temperatures. The plots ofkobs versus [Lac] are straight lines which pass the grid origin (Figure 3).Figure 3
Plots
ofkobsversus [Lac].  [Cr(VI)]=2.5×10−4mol⋅L−1, [HClO4]=0.50mol⋅L−1,
[DMSO]=0.20mol⋅L−1, I=1.0mol⋅L−1.


## 3.3. Effect of Solvent and Ionic Strength (I)

The addition of solvent DMSO
accelerates the rate of reduction of Cr(VI) by lactic   acid. When no catalyst or
high concentration of acid exists, the oxidation of DMSO is slow so it can be ignored [18–20]. The
increase of ionic strength of the reaction almost has no impact on the rate of reaction, indicating
that there is no salt effect (Table 1).Table 1
Effects of [DMSO] and ionic strength onkobs at 298.15 K.
[DMSO]/mol⋅L−10.0500.100.200.300.400.50103kobs/s−14.9085.9276.4667.3817.8948.328I/mol⋅L−10.600.801.01.11.2—103kobs/s−15.6165.7895.7905.6286.020—[Cr(VI)]=2.5×10−4mol⋅L−1,
[Lac]=0.50mol⋅L−1,
and   [HClO4]=0.50mol⋅L−1.


## 3.4. Mechanism

The  chromic acid also participates in  acid-base equilibrium
as follows [21]:(1)H2CrO4⇌HCrO4−+H+,Ka1=1.8×10−1,HCrO4−⇌CrO42−+H+,Ka2=3.2×10−7,2HCrO4−⇌Cr2O72−+H2O,K=33.
Hexavalent chromium exists primarily as chromic
acid (H2CrO4), hydrogen chromate ion (HCrO4−), and chromate ion (CrO42−), depending on the values of pH. In acidic medium of our
experiment, dichromate exists predominantly as monomer (HCrO4−) and chromic acid (H2CrO4), and H2CrO4 plays a role of active species. Based on the
experimental results, we can learn that lactic acid participates in the rate-determined step [22]. From the
discussion above, the reaction mechanism was proposed as follows:(2)HCrO4−+H+⇌KbH2CrO4,(3)H2CrO4+CH3CHOHCOOH→kCr(IV)+CH3COCOOH+H2O,(4)Cr(VI)+Cr(IV)→fast2Cr(V),(5)Cr(V)+CH3CHOHCOOH→fastCr(III)+CH3COCOOH+H2O. Here, reaction (3) is
the rate-determined step. Based on the mechanism above, the rate law is derived
as follow: (6)−d[Cr(VI)]Tdt=k[H2CrO4][Lac],(7)d[Cr(VI)]Tdt=kKb[H+]1+Kb[H+][Lac][Cr(VI)]T,(8)kobs=kKb[H+]1+Kb[H+][Lac],(9)Kb=1/Ka1,sokobs=k[H+]Ka1+[H+][Lac],(10)1kobs=Ka1k[Lac]⋅1[H+]+1k[Lac]. Reaction (3) shows a redox decomposition with a two-electron transfer (C–H cleavage), and the
redox product is pyruvic acid, which is
consistent with the experimental phenomena. From (8) and (10), the
plots of kobs versus [Lac]  are straight lines through the
grid origin, and the plots of 1/kobs versus 1/[H+]
are linear with positive intercept. The rate constants of the rate determining step and
activation parameters were obtained and presented in Table 2. The effect of solvent on reaction rate is an extremely complicated issue.
We hold that the formation of H2CrO4 makes its electron density
drop. So, the reaction rate increases with the decreasing of the solvent’s polarity [23].
This is consistent with the experimental phenomena.Table 2
The rate constants and
activation parameters for the reduction of
Cr(VI) by lactic acid.
T/K102k/mol−1⋅L⋅s−1Thermodynamic activationparameters (298.15 K)298.154.74Ea=30.98kJ⋅mol−1  ΔS≠=−174.6J⋅K−1⋅mol−1
ΔH≠=28.50kJ⋅mol−1303.155.93308.157.35313.158.58


## 4. Conclusion

The kinetics of the reduction of chromium(VI) by lactic
acid in the presence of DMSO has been studied. The active species of Cr(VI)
is understood to beH2CrO4. Rate constant of the slow
step and activation parameters   were computed, respectively. The product of Cr(VI) oxidation of lactic acid is pyruvic acid, which suggested that the cleavage of C–H of lactic acid. Mechanism
proposed for the reaction is in conformity with the product, mechanistic, and
kinetic studies.



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*Source: 314045-2008-12-28.xml*