1. Spectrochimica Acta Part A 67 (2007) 976–979
Spectrophotometric determination of anilines based on
charge-transfer reaction
Hao Wu, Li Ming Du∗
Analytical and Testing Center, Shanxi Normal University, Shanxi Linfen 041004, PR China
Received 13 January 2006; received in revised form 30 August 2006; accepted 14 September 2006
Abstract
The molecular interactions between aniline, p-toluidines, benzidine and p-phenylenediamine as electron donors and 7,7,8,8-
tetracyanoquinodimethane (TCNQ) as acceptor have been investigated by spectrophotometric method. Different variables affecting the reaction
were studies and optimized. At the optimum reaction conditions Beer’s law was obeyed in a concentration limit of 0.6–3.0, 0.3–3.0, 0.3–3.0 and
0.3–2.7 ␮g ml−1
for aniline, p-toluidines, benzidine and p-phenylenediamine. The developed methods were applied successfully for the determi-
nation of the studied compounds in waste water and relative standard deviation of the methods were 0.8–3.0%. Percentage recoveries ranged from
97.22% to 102.78%.
© 2007 Published by Elsevier B.V.
Keywords: Aniline; p-Toluidine; Benzidine; p-Phenylenediamine; 7,7,8,8-Tetracyanoquinodimethane; Charge-transfer reaction; Spectrophotometry
1. Introduction
Aromatic amines such as aniline and its derivatives are an
important class of environmental water pollutants. Anilines are
used in the manufacturing of rubbers and plastics, dyes, agro-
chemicals and pharmaceuticals [1]. The anilines can be released
into the environment directly as industrial effluent from, e.g., the
chemical, textile or leather industry, or indirectly as breakdown
productsofherbicidesandpesticides.Duetotheirhighsolubility
in water, anilines can easily permeate through soil and contam-
inate ground water, and therefore they can be present at trace
levels in drinking water. Several aromatic amines are strongly
toxic and suspected carcinogens [2]. Moreover, aniline com-
pounds may be converted into carcinogens in the environment
or in the body.
Several methods have been developed for the determina-
tion of anilines in environmental samples. Gas chromatography
(GC) is a classical method [3–8], and presently GC–mass spec-
trometry (MS) is often used [9,10]. For non-volatile aniline
compounds high-performance liquid chromatography (HPLC)
may be used with fluorimetric detection [11,12], amperometric
detection[13,14]andspectrophotometricdetection[15].GCand
∗ Corresponding author. Tel.: +86 357 2051158; fax: +86 357 2051158.
E-mail address: lmd@dns.sxnu.edu.cn (L.M. Du).
HPLC method generally requires complicated equipment, pro-
vision for use and disposal of solvents, labor-intensive sample
preparationprocedure,andpersonnelskilledinchromatographic
techniques.
TCNQ is a strong electron acceptor and has been used for
the determination of electron donors such as norfloxacin [16],
cephalosporins [17], ␤-adrenergic blocking agents [18], etc.
In the present study, TCNQ is used for determination of ani-
line, p-toluidine, benzidine and p-phenylenediamine. However,
no spectrophotometric method for determination of anilines
through charge-transfer complexation with TCNQ has been
reported.
2. Experimental
2.1. Apparatus
A Shimadzu Model UV-2201, Ultraviolet–visible spec-
trophotometer (Tokyo, Japan) was used for recording absorption
spectra, using 10 mm path-length quartz cells. The pH was mea-
sured on a Model PHS-3 precise acidometer (Shanghai Tianda
Apparatus Ltd.).
2.2. Reagents
All chemicals and solvents used were of analytical reagent
grade. TCNQ (Fluka Chemical Co., USA) was prepared as
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2006.09.016

2. H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007) 976–979 977
300 ␮g ml−1 in acetonitrile, solution was found be stable for at
least 1 week at 4 ◦C. About 10 mg four compounds were placed
in a 100 ml volumetric flask and 10 ml methanol was added and
the solution was diluted to volume with distilled water. Working
standard of 10 ␮g ml−1 was prepared by dilution of stock stan-
dard solution with distilled water and the pH of solutions was
adjusted use NaOH 0.01 mol l−1.
2.3. Procedure
2.3.1. General procedure
About 1.0 ml work solution was transferred into a 10 ml
volumetric flask, 1.0 ml of TCNQ solution was added, and
the solution was diluted to volume with methanol and mixed
thoroughly. The solution was thermostated at 55 ± 0.5 ◦C for
40 min. After cooling, the absorbance of CT complexes of
aniline, p-toluidine, benzidine and p-phenylenediamine were
measured at 462, 463, 487 and 492 nm against a blank solution,
respectively. The calibration graph was constructed in the same
way with studied anilines solutions of known concentrations.
The amount of anilines was computed from their calibration
graphs.
2.3.2. Procedure for water sample of aniline
About 100 ml water sample were pipetted into retort, distilled
at alkalescence. Fraction was collected into 100 ml volumet-
ric flask. A suitable amount of fraction was tested as described
above.
2.3.3. Procedure for water sample of p-toluidine, benzidine
and p-phenylenediamine
About 1-l water sample were condensed by heating in a water
bath at 50 ◦C (the compounds in water was destroyed least), the
condensed solution was filtered and tested as Section 2.3.1.
3. Results and discussion
3.1. Absorption spectra
The absorption spectra of the reaction product between
TCNQ and anilines are shown in Fig. 1. Aniline, p-toluidine,
benzidine and p-phenylenediamine which does not have a
chromophore that absorbs above 300 nm, can be determined
colorimetrically by the formation of complex with TCNQ.
The formation of charge-transfer complex is based on n–␲*
interaction between anilines as donating compound to TCNQ
as acceptor and produces a bathochromic shift of 228,
228, 205 and 249 nm for aniline, p-toluidine, benzidine and
p-phenylenediamine,respectively,theabsorbanceincreasessub-
stantially. The absorbance of the complex is then measured at
its maximum wavelength (462, 463, 487 and 492 nm for aniline,
p-toluidine, benzidine and p-phenylenediamine, respectively).
Investigations were carried out to establish the most favorable
conditions for the charge-transfer formation. The influence of
some variables on the reaction has been tested as follows.
Fig. 1. Absorption spectra of aniline and its ramification: 1, aniline; 2,
p-toluidine; 3, benzidine; 4, p-phenylenediamine; 1 , aniline–TCNQ; 2 ,
p-toluidine–TCNQ; 3 , benzidine–TCNQ; 4 , p-phenylenediamine–TCNQ;
c(anilines) = 2.4 ␮g ml−1; c(TCNQ) = 30 ␮g ml−1.
3.2. Effect of solvent
The solvents studied were water, methanol, ethanol, ace-
tonitrile, acetone, chloroform and dichloromethane. Experiment
indicated that a mixed solvent of water–acetonitrile–methanol
gave the maximum and stable absorbance for studied com-
pounds, the ratio of water:acetonitrile:methanol is 1:1:8 (v/v/v).
3.3. Effect of reaction temperature
The effect of temperature on the formed CT complexes was
studied in the range of 20–60 ◦C. The suitable temperature and
time for obtaining maximum and stable absorbance were carried
out at 55 ◦C and 40 min. The stable time of CT complex at room
temperature is at least 12 h.
3.4. Effect of pH of working solution
The absorption spectra of the color product–CT complex in
working solution of varying pH values (3.0–12.0) were recorded
in order to select the optimum pH (Fig. 2). This also gives us an
Fig. 2. Effect of the amount of pH: 1, benzidine; 2, aniline; 3, p-toluidine; 4,
p-phenylenediamine; c(anilines) = 1.2 ␮g ml−1.

3. 978 H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007) 976–979
idea about the possible species that can exist in such media. The
spectral measurements in the visible region show an increase in
the absorbance with the increase of pH of working solution at
the specific wavelength till pH 10.0 (maximum absorbance). At
pH more than 10.0, has a stable value (Fig. 2) by increasing of
pH. So the optimum pH value of working solution is taken at
11.0, because little lower pH may cause high error.
3.5. Effect of TCNQ concentration
The influence of CT reagent concentration was studied in the
range 10–100 ␮g ml−1. Experiment indicated that 30 ␮g ml−1
of TCNQ concentration is enough for each compound.
3.6. Mechanism of reaction
TCNQ is an ␲-acceptor, can formed n–␲* or ␲–␲* charge-
transfer complex have been reported for determination of many
compounds [16]. Anilines has two electron rich group, benzene
ring and amido, may form ␲–␲* and n–␲* charge-transfer com-
plex with TCNQ at the same time. Under the specific condition,
benzene, toluene, dimethylbenzene, naphthalene and ethylene-
diamine reaction with TCNQ has been studied. Though the
benzene ring in molecule of benzene, toluene, dimethylbenzene
and naphthalene is the most electron rich group, no ␲–␲* CT
complexes are formed. Addition of TCNQ to ethylenediamine
solution causes an immediate change in the absorption spec-
trum with a new characteristic band at 395 nm. As the result,
the n–␲* CT complexes have been formed between anilines
and TCNQ. According to the order of give electron ability of
anilines, the absorption spectrum of CT complexes varied regu-
larity. The order of increasing maximum absorption wavelength
is p-phenylenediamine > benzidine > p-toluidine > aniline.
The compositions of all the CT complexes were found to be
1:1 (Table 1) by molar ratio and Job’s methods. This indicates
that only one nitrogen is responsible for the formation of the
Table 1
Structures of anilines charge-transfer complexes with TCNQ
Compounds R
Aniline H
p-Toluidine CH3
Benzidine
p-Phenylenediamine NH2
complex although p-phenylenediamine and benzidine have two
nitrogen atoms. This can be explained on the basis that a uni-
valent, partially positively charged droperidol species may be
formed initially during the CT process, which may not be easily
engaged in additional complex formation.
3.7. Analytical parameters
Under the experimental conditions described, standard cal-
ibration curves of CT complexes for aniline, p-toluidine,
benzidine and p-phenylenediamine with TCNQ were con-
structed by plotting absorbance versus concentration, the linear
regression equation for each method are listed in Table 2. The
correlation coefficients ranged from 0.996 to 0.998, indicating
good linearity.
3.8. Association constant and free energy change
The association constant for the interaction of each com-
pound with TCNQ was calculated using the Benesi–Hildebrand
Table 2
Quantitative parameters for anilines CT complexes
Parameters Aniline–TCNQ p-Toluidine–TCNQ Benzidine–TCNQ p-Phenylenediamine–TCNQ
λmax (nm) 462 463 487 492
Beer’s law limits (␮g ml−1) 0.6–3.0 0.3–3.0 0.3–3.0 0.3–2.7
Limit of detection (␮g ml−1) 0.54 0.14 0.13 0.09
Slope 0.3053 0.2300 0.2310 0.2635
Intercept −0.1000 0.0341 0.0346 0.0411
Molar absorptivity (l mol−1 cm−1) 22,157 27,828 48,270 35,262
Correlation coefficient 0.9968 0.9960 0.9973 0.9970
Sandell sensitivity (␮g cm−2) 0.0042 0.0039 0.0038 0.0031
Table 3
Association constants and Gibbs free energy
Parameters Aniline–TCNQ p-Toluidine–TCNQ Benzidine–TCNQ p-Phenylenediamine–TCNQ
Association constant 6.542 × 103 2.820 × 103 5.470 × 103 4.926 × 103
Free energy (kcal mol−1) −5.212 −4.713 −5.106 −5.043
Correlation coefficienta (r) 0.998 0.996 0.999 0.995
a Average of five determinations.

4. H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007) 976–979 979
Table 4
Determination results of anilines in environmental samples
Sample Present method Reference method
Found (␮g ml−1) Added (␮g) Found (␮g) Recovery (%) R.S.D. (n = 5) (%) Found (␮g) Recovery (%)
Aniline
I 0 1.8 1.78 98.89 0.8 0 96.53
II 1.20 1.2 1.18 98.33 3.0 1.21 101.28
p-Toluidine
I 0 1.8 1.77 98.04 1.2 0 99.23
II 1.81 1.8 1.81 100.56 2.5 1.79 98.48
Benzidine
I 0 1.8 1.75 97.22 2.1 0 101.31
II 1.51 1.5 1.49 99.33 2.7 1.53 97.78
p-Phenylenediamine
I 0 1.8 1.85 102.78 1.5 0 99.02
II 1.39 1.4 1.42 101.43 2.6 1.41 102.56
I: tap water and II: waste water of laboratory.
equation [19]:
[A0]
AAD
=
1
εAD
+
1
KAD
c εAD
×
1
[D0]
where [A0] and [D0] are the concentrations of the acceptor and
donor, respectively, AAD is the absorbance of the complex, εAD
themolarabsorptivityofthecomplex,andKAD
c istheassociation
constant of the complex (l mol−1 mol).
From the previous equation, on plotting the values of
[A0]/AAD versus 1/[D0], straight lines were obtained, from
which the association constants and correlation coefficients were
obtained (Table 3). The standard free energy changes of com-
plexation ( G◦) were calculated from the association constants
by the following equation [20]:
G◦
= −2.303RT log Kc
where G◦ is the free energy change of the complex (kJ mol−1),
R the gas constant (1.987 cal mol−1 deg−1), T the temperature
in Kelvin (273 + ◦C), and Kc is the association constant of
compound–acceptor complexes (l mol−1).
3.9. Analytical application
The proposed method was applied to assay some water
sample. The results are shown in Table 4. Five replicate deter-
minations were made, and satisfactory results were obtained.
Moreover, to check the validity of the proposed methods, the
standard addition method was applied by adding aniline, p-
toluidine, benzidine and p-phenylenediamine to the previously
analyzed water sample. Compared the result obtained by the
proposed method with those obtained by official method [21],
the accuracy is satisfying.
4. Conclusion
The results obtained from the present study indicate that
n–␲* CT complex’s formation between the anilines and TCNQ
was applied in the spectrophotometric assay of aniline, p-
toluidine, benzidine and p-phenylenediamine in some water
sample.Indicatetheadvantagesofeasyoperation,highrecovery,
less time-expense, and less use of organic solvent. The investi-
gation of real samples revealed the potential of the method in
environmental analysis.
Acknowledgement
This research was supported by the Natural Science Founda-
tion of Shanxi.
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