A highly sensitive spectrophotometric method for the determination of Cr(VI) concentration

Article abstract of DOI:10.1246/cl.2005.816

A highly sensitive spectrophotometric method, which is based on the oxidation of leuco methylene blue (colorless) to methylene blue (colored) by Cr(VI), was developed for the determination of the dissolved hexavalent chromium concentrations in water. Copyright

Full text of DOI:10.1246/cl.2005.816

816
Chemistry Letters Vol.34, No.6 (2005)
A Highly Sensitive Spectrophotometric Method for the Determination
of Cr(VI) Concentration
Soo-Keun Lee and Wonyong Choiꢀ
School of Environmental Science and Engineering, Pohang University of Science and Technology,
Pohang 790-784, Korea
(Received March 7, 2005; CL-050311)
A highly sensitive spectrophotometric method, which is
in acidic solutions (pH ꢃ2). It has been recently reported that
based on the oxidation of leuco methylene blue (colorless) to
methylene blue (colored) by Cr(VI), was developed for the de-
termination of the dissolved hexavalent chromium concentra-
tions in water.
LMB solution (prepared by the above method) is stable over
20 min of period in ambient atmospheric condition at pH 2.11
Therefore, LMB can be used as a colorimetric reagent at least
for 20 min. Standard reduction potentials of Cr(VI)/Cr(III)12
and MBþ/LMB13 couples are 1.33 and 0.532 V (vs NHE) at
pH 0, respectively. Thus, LMB can react with Cr(VI) and reduce
it to Cr(III). The overall reaction can be summarized as:
Chromium ions exist mostly as Cr(III) or Cr(VI) in natural
water.1 Cr(III) ion is relatively nontoxic and an essential trace
nutrient for the human.2 However, Cr(VI) ion is known to be tox-
ic for animals and humans, highly mobile in aquatic environ-
ments, and not easily removed by simple adsorption.3 Therefore,
the quantitative analysis and removal of Cr(VI) ion in the drink-
ing water are very important.
3LMB þ 2Cr(VI) ! 3MBþ þ 2Cr(III)
ð4Þ
K2Cr2O7 (Aldrich) and Cr(NO3)3 (Aldrich) were used for
the preparation of Cr(VI) and Cr(III) standard solutions, respec-
tively. LMB solution was prepared by the reduction of MBþ
(Aldrich) by adding Zn amalgam as a reducing agent at pH 2.
Detailed preparation method is described elsewhere.14 A double
beam UV–vis spectrophotometer (Shimadzu, UV2401PC) was
used for absorbance measurements. The pH of the solution
was adjusted with a standard HCl solution and measured with
a pH meter (Orion).
An aliquot of the 10 mM LMB solution was transferred into a
quartz cell (1 cm pathlength, 4 mL volume), which was located
within the spectrophotometer, and then Cr(VI) solutions of var-
ious concentrations were added. The subsequent change in ab-
sorbance was monitored at 664 nm as a function of time.
Throughout the measurement, the solution was stirred magneti-
cally. Cr(VI) quantitatively oxidized LMB into its blue-colored
MBþ in water solution at pH 2, and the resulting colored dye
showed a maximum absorbance at 664 nm. The reagent blank
had negligible absorbance at this wavelength.
The spectrophotometric determination of Cr(VI) concentra-
tion is possible without an additional indicator because Cr(VI)
has yellow color itself. This method is very precise, but not very
sensitive because Cr(VI) has a low molar absorption coefficient
(ꢀmax ¼ 373 nm; " ¼ 1:4 ꢁ 103 Mꢂ1 cmꢂ1). Diphenylcarbazide
(ꢀmax ¼ 545 nm; " ¼ 4:3 ꢁ 104 Mꢂ1 cmꢂ1) is the most popular
reagent for the colorimetric determination of Cr(VI).4,5 Howev-
er, it has some disadvantages such as interferences from Fe(III),
Mo(VI), Cu(II), and Hg(II),6 and the formed complex is stable
for only a limited time in phosphate buffer.7 Therefore, the de-
velopment of the more stable, sensitive, reliable, and inexpen-
sive reagent for the determination of Cr(VI) is needed.
Methylene blue (MBþ) is a member of thiazine dye group
and it has been used as a redox indicator in several chemical
analyses. For example, commercial oxygen indicator, Ageless
Eyeꢀ, employed MBþ as a key indicator for the detection of
oxygen in modified food packaging.8 MBþ is also used in the
quantitative analysis of reducing agents, such as ascorbic acid
and glucose. The principle behind such redox indicating systems
is that the reduced (leuco) form of MB is colorless but the
oxidized form of MB is bright blue (ꢀmax ¼ 664 nm; " ¼ 1 ꢁ
105 Mꢂ1 cmꢂ1).
Figure 1 shows the measured absorbance change (at 664 nm)
vs time profiles for LMB solution with the addition of Cr(VI) in
0.6
0.5
0.5
0.4
0.4
0.3
0.2
0.1
0
0.3
0.2
0.1
0
Leuco methylene blue (LMB) solution can be easily pre-
pared from a MBþ solution either by the addition of reducing
agent such as sodium dithionite or Zn amalgam (reaction 1) or
by visible light illumination in the presence of ascorbic acid
(AA) (reaction 2).9 LMB is very easily reoxidized back to
MBþ by atmospheric oxygen (reaction 3).
0
2
4
Cr(VI) / µM
0
500
1000
MBþ þ Hþ þ 2eꢂ ! LMB
MBþ ꢂꢂꢂ! MBþꢀ þ AA ! LMB þ AAox
2LMB þ O2 ! 2MBþ þ 2OHꢂ
ð1Þ
t / s
hꢁ
ð2Þ
ð3Þ
Figure 1. Measured absorbance (at 664 nm) vs time profiles for
a LMB solution (10 mM, pH 2) added with Cr(VI) solutions of
various concentrations (bottom to top: 0, 0.25, 0.5, 1, 2, and
4 mM). The inset is a standard calibration curve in this condition
(data obtained at 300 s).
However, this reaction is very pH sensitive, with the rate of
reaction 3 increasing with pH.10 Thus, LMB is moderately stable
Copyright ꢁ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.6 (2005)
817
Table 1. Sensitivity of three different Cr(VI) determination
methods
sult of reaction 4. After the first analysis, LMB was regenerated
(i.e., bleached) under visible light irradiation and then the second
spike of Cr(VI) was followed with the subsequent rise of the ab-
sorbance. This process could be repeated over and over.
LMB
0.1139b
DPC
Cr(VI) itself
sensitivitya
0.0415
0.0015
Possible interferences from other aquatic species should be
considered. For example, the effect of ferric ions, which can ox-
idize LMB like Cr(VI) and hence interfere in the determination
of [Cr(VI)], was investigated. Their presence did influence the
LMB analysis of Cr(VI): 10 mM Fe(III) in a sample water con-
taining 1 mM Cr(VI) induced about 30% error. In the case of
DPC method, 2% error has been reported at the above condi-
tions.15 Therefore, the problem of interferences should be re-
solved for the real sample applications. The interference of
Fe(III) might be masked by the addition of complexing reagents
such as EDTA. Other redox and adsorbing species found in var-
ious water samples may also interfere. Their potential effects on
this new analytical method and the method for eliminating or
correcting the interference errors are being investigated.
The reproducibility of the LMB method was established by
the analysis of standard solutions of 1.25, 2.5, and 5 mM of
Cr(VI). Three replicate determination of each concentration
gave the standard deviation of 0.05, 0.15, and 0.33%, respective-
ly. To check out how much unknown impurities in tap water in-
fluence the LMB analysis of Cr(VI), the known concentration of
Cr(VI) samples which were prepared using ultrapure water and
tap water were measured and they were compared. The calibra-
tion curve obtained using tap water was essentially identical to
the one obtained using ultrapure water, which indicates that im-
purities in tap water little influence the LMB determination of
Cr(VI). Therefore, this simple and highly sensitive spectrophoto-
metric determination can be proposed as a new analytical meth-
od for Cr(VI) concentration in drinking water and can be further
applied to wastewater and natural water samples when any inter-
fering components are removed or masked prior to the analysis.
b
aSlope of the calibration curve (unit: abs./mM). taken from
Figure 1.
various concentrations. The blue coloration of the mixed solu-
tion rapidly proceeded in the initial stage and then reached a pla-
teau region. The absorbance changes (ꢀA) at either the initial or
saturated period can be used for plotting the calibration curve
(i.e., ꢀA vs [Cr(VI)]) and either of the two methods showed a
good linear relationship as shown in the inset of Figure 1. The
slow rise of absorbance observed in the absence of Cr(VI) is
due to the slow oxidation of LMB by atmospheric oxygen (reac-
tion 3). For the test of Cr(III) reactivity, an aliquot of Cr(III) was
added into LMB solution. As we expected, there was no marked
change in the overall absorbance of the sample solution. Thus,
this analytical system utilizing LMB showed good selectivity
to Cr(VI) in the mixture of Cr(VI) and Cr(III) solution.
For the comparison of the sensitivity of different analytical
methods, Cr(VI) solutions were also analyzed by the direct ab-
sorbance measurement of Cr(VI) itself and the diphenylcar-
bazide (DPC) complexation method, respectively. As shown in
Table 1, LMB method showed the highest sensitivity among
the three different determination methods. LMB method has al-
most three times higher sensitivity compared to the conventional
DPC method.
Another advantage of the LMB method is that the reagent
can be repeatedly used in successive analyses when LMB is pre-
pared from the ascorbic acid (AA)–visible light system (reac-
tion 2). Whenever we need LMB, it can be regenerated from
MB simply by irradiating visible light (500-W tungsten lamp)
in the presence of AA. Five successive measurements of Cr(VI)
could be achieved in the same spectrophotometric cell without
depleting initially added MB as shown in Figure 2.
We gratefully acknowledge financial supports from KOSEF
through the Center for Integrated Molecular Systems (CIMS)
and the Brain Korea 21 project.
Upon spiking Cr(VI) to the initial LMB solution prepared by
reaction 2, the absorbance at 664 nm rapidly increased to reach a
saturation within a few minutes as Cr(VI) was depleted as a re-
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Figure 2. Successively measured absorbance (at 664 nm) vs
time profile for a LMB solution (10 mM, pH 2) added (") with
various concentrations of Cr(VI) (left to right; 1, 1.96, 3.77,
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level when the light was off.
Published on the web (Advance View) May 14, 2005; DOI 10.1246/cl.2005.816