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Spectrochimica Acta Part A 67 (2007) 976979

Spectrophotometric determination of anilines based oncharge-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 reactionwere studies and optimized. At the optimum reaction conditions Beers law was obeyed in a concentration limit of 0.63.0, 0.33.0, 0.33.0 and

0.32.7g ml1 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 0.83.0%. 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

products of herbicides andpesticides. Dueto their high solubility

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 [38], and presently GCmass 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] and spectrophotometric detection[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

preparation procedure,and personnelskilled in chromatographic

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, Ultravioletvisible 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

mailto:lmd@dns.sxnu.edu.cnhttp://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.saa.2006.09.016http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.saa.2006.09.016mailto:lmd@dns.sxnu.edu.cn

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H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007 ) 976979 977

300g ml1 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 10g ml1 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 sameway 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-phenylenediamineAbout 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 determinedcolorimetrically 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, the absorbance increasessub-

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, anilineTCNQ; 2,

p-toluidineTCNQ; 3, benzidineTCNQ; 4, p-phenylenediamineTCNQ;

c(anilines) = 2.4g ml1; c(TCNQ)= 30g ml1.

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 wateracetonitrilemethanol

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 2060 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 roomtemperature is at least 12 h.

3.4. Effect of pH of working solution

The absorption spectra of the color productCT complex in

working solution of varying pH values (3.012.0) 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)= 1.2g ml1

.

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978 H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007) 976979

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 10100g ml1. Experiment indicated that 30g ml1

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 wavelengthis 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 Jobs 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 BenesiHildebrand

Table 2

Quantitative parameters for anilines CT complexes

Parameters AnilineTCNQ p-ToluidineTCNQ BenzidineTCNQ p-PhenylenediamineTCNQ

max (nm) 462 463 487 492

Beers law limits (g ml1) 0.63.0 0.33.0 0.33.0 0.32.7

Limit of detection (g ml1) 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 mol1 cm1) 22,157 27,828 48,270 35,262

Correlation coefficient 0.9968 0.9960 0.9973 0.9970

Sandell sensitivity (g cm2) 0.0042 0.0039 0.0038 0.0031

Table 3

Association constants and Gibbs free energy

Parameters AnilineTCNQ p-ToluidineTCNQ BenzidineTCNQ p-PhenylenediamineTCNQ

Association constant 6.542 103 2.820103 5.470103 4.926103

Free energy (kcal mol1) 5.212 4.713 5.106 5.043

Correlation coefficienta (r) 0.998 0.996 0.999 0.995

a

Average of five determinations.

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H. Wu, L.M. Du / Spectrochimica Acta Part A 67 (2007 ) 976979 979

Table 4

Determination results of anilines in environmental samples

Sample Present method Reference method

Found (g ml1) 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

AD+

1

KADc AD

1

[D0]

where [A0] and [D0] are the concentrations of the acceptor and

donor, respectively, AAD is the absorbance of the complex, AD

themolarabsorptivityof thecomplex, andKADc is the association

constant of the complex (l mol1 mol).

From the previous equation, on plotting the values of

[A0]/AAD versus 1/[D0], straight lines were obtained, from

which the association constantsand 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 logKc

whereG is the free energy change of the complex (kJ mol1),

R the gas constant (1.987 cal mol1 deg1), T the temperature

in Kelvin (273 + C), and Kc is the association constant of

compoundacceptor complexes (l mol1).

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 complexs formation between the anilines and TCNQ

was applied in the spectrophotometric assay of aniline, p-

toluidine, benzidine and p-phenylenediamine in some water

sample. Indicate the advantages of easyoperation,high recovery,

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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