The Use of 7,7′,8,8′-Tetracyanoquinodimethane for the Spectrophotometric Determination of Some Primary Amines Application to Real Water Samples

A sensitive, simple, and accurate spectrophotometric method was developed for the quantitative determination of some primary aliphatic and aromatic amines, that is, ethylamine, 1,2-diaminopropane, aniline, p-aminophenol, and benzidine. The method is based on the interaction of these amines in aqueous medium with 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) reagent in the presence of a buffer solution and surfactant (in the case of aromatic amines) to form charge-transfer complexes measurable at maximum wavelengths ranging between 323 and 511 nm. Beer's law is obeyed over the concentration ranges of 0.025 and 3.0 μg/mL and the molar absorptivity is ranged between 8.977 × 103 and 5.8034 × 104  L·mol−1 ·cm−1 for these amines. The method was applied for the determination of benzidine in the river, sea, and tap waters. The TCNQ complexes with the previously mentioned amines were formed in the ratio of 1 : 1 amine : TCNQ, and their stability constants ranged between 8.78 × 104 and 1.844 × 105 L·mol−1.


Introduction
A great number of aromatic amines are of considerable importance in industrial, toxicological, and pharmaceutical aspects [1]. Short-chain aliphatic amines are presented widely in the aquatic environment due to their widespread use in several industrial, chemical, and manufacturing applications [2,3]. Also these amines are common components of biological systems as degradation products of organic materials such as amino acids and proteins. In addition to hygienic problems due to stinging smell, these compounds may be hazardous to human health as they are sensitizers and irritants to skin, eyes, mucus membranes, and respiratory tract. Also they can react with certain nitrogen-containing compounds to form nitrosamines, which are potentially carcinogenic substances [4].
The present research aims chiefly to develop a sensitive, selective, simple, and quick spectrophotometric method for the determination of samples of primary aliphatic and aromatic amines, namely, ethylamine, 1,2-diaminopropane, aniline, p-aminophenol, and benzidine, in aqueous solution with TCNQ reagent by measuring the absorbance at max 2 Journal of Analytical Methods in Chemistry of new charge-transfer absorption band and without any derivatization or catalysis. In addition to applying the method for determination of benzidine in various water samples.

Determination of Primary Aliphatic Amines.
Aliquots of standard primary amine solutions of ethylamine and 1,2diaminopropane were transferred separately into a series of 10 mL calibrated flasks. To each of these were added 0.5 and 1.0 mL of 1 × 10 −3 M TCNQ and the solutions were heated at 40 and 50 ∘ C for 20 and 30 min for ethylamine and 1,2diaminopropane, respectively; then the solutions were cooled to room temperature and diluted to the mark with distilled water. The absorbances of the complexes were measured at 464 and 470 nm for the previous amines, respectively, against corresponding reagent blank.

Determination of Primary Aromatic Amines.
Aliquots of standard primary amine solutions of aniline, p-aminophenol, and benzidine were transferred separately into a series of 10 mL calibrated flasks. To each of these were added the optimum amounts of TCNQ, phosphate buffer solution, and CPC    (or CTAB in the case of benzidine) according to the order of addition as listed in Table 3. The solutions were diluted to the mark with distilled water and the absorbances of the complexes were measured at room temperature immediately (in the case of p-aminophenol the solutions were heated at 40 ∘ C for 5 min and cooled to room temperature then diluted to the mark with distilled water) at 323, 511, and 500 nm for aniline, p-aminophenol, and benzidine, respectively.

Absorption Spectrum of the CT Complexes.
Primary aromatic and aliphatic amines react with TCNQ in the presence of phosphate buffer solution (in the case of aromatic amines) to give a red coloured complexes with maximum absorption spectra at 511, 500, 464, and 470 nm for p-aminophenol, benzidine, ethylamine, and 1,2-diaminopropane, respectively, whereas aniline gave a yellow coloured complex at 323 nm ( Figures 1 and 2), and their reagent blanks gave maximum absorption at 320 nm for p-aminophenol and benzidine, whereas they gave it at 395 nm for aniline and 832 nm for ethylamine and 1,2-diaminopropane under their optimum conditions.

Effect of Solvent.
Different solvents such as acetonitrile, acetone, ethanol, methanol, dioxane, and water were tested as reaction media for interaction between amines and TCNQ -acceptor. It was found that TCNQ reagent reacted with primary, secondary, and tertiary aliphatic and aromatic amines in the medium of previous solvents and produced different colours, but using water as a solvent for amines and acetonitrile or acetone for TCNQ and dilution with water, selective reactions between TCNQ and primary aliphatic and aromatic amines occurred and forming coloured -chargetransfer complexes with maximum absorption at wavelengths ranged between 323 and 511 nm for given amines, whereas other amines were either unreacted or gave low absorption response. Therefore, this system of solvents is recommended in our method.

Effect of pH and Buffer Solutions.
The effect of pH on the absorption of the complexes produced by the reaction of TCNQ with primary amines was studied using different pHs ranging from 2 to 12. It was found that these complexes are formed in the final pH of 9.86, 6.02, and 10.65 for paminophenol, aniline, and benzidine, respectively, by addition of NaOH solution and decreases in absorbances were found through addition of HCl, which may be attributed to the liberation of hydrogen cyanide, whereas it was found that the absorbances of aliphatic amines were decreased through addition of NaOH or HCl and the final pHs of their reaction solutions were 7.20 and 6.62 for ethylamine and 1,2-diaminopropane, respectively. Different buffers of the same pHvalues, as mentioned before, were prepared by using carbonate, borate, phosphate, ammonia, and citrate buffers to investigate the sensitivity of the amine-TCNQ complexes. It was found that phosphate buffer solution (KH 2 PO 4 + KOH) increased the sensitivity for the aromatic amine-TCNQ complexes (Table 1) and caused a bathochromic shift for paminophenol (460 → 475 nm), whereas the absorbances of aliphatic amine-TCNQ complexes were decreased. However, the amounts of phosphate buffer solution at fixed pH values as cited before for aromatic amines were studied and found to be 0.5, 1.0, and 0.8 mL which are the optimum amounts for aniline, p-aminophenol, and benzidine, respectively, and which are recommended in the subsequent experiments.

Effect of Reaction Time and
Temperature. The reaction time was determined by following the color development at room temperature (28 ∘ C) and in thermostatically controlled water bath at different temperatures up to 50 ± 1 ∘ C. The absorbance was measured at 5-and 10-minute intervals against reagent blank treated similarly. It was observed that the absorbance reached maximum after addition of the reagent solution immediately at room temperature for aniline and benzidine, after 5 min at 40 ∘ C for p-aminophenol, and after 20 and 30 min at 40 and 50 ∘ C for ethylamine and 1,2-diaminopropane, respectively, and the stability of their absorbances was achieved after cooling to the room temperature (Table 3). These temperatures and reaction times were chosen for colour development.

Effect of TCNQ Concentration.
The effect of changing the TCNQ concentration (0.2-2.0 mL of 1 × 10 −3 M) on the absorbance of solution containing a fixed amount of each primary amine (1.0 g mL −1 ) was studied. It is evident that the absorbance increases with increasing TCNQ concentration and reached maximum on using 0.5 mL of 1 × 10 −3 M TCNQ for aniline, p-aminophenol, benzidine, and 1,2diaminopropane, and 1.0 mL for ethylamine (Table 3). Therefore, these concentrations were used in all subsequent work.

Effect of Surfactant.
Effect of various surfactants including CTAB, CPC, tween-80, and triton x-100 of 0.1% concentration was tested. It was found that the cationic surfactants CPC and CATB caused a bathochromic shift with increasing in the sensitivity for aromatic amines (Table 2), while it decreased the sensitivity of aliphatic amines. However, 0.5-3.0 mL of 0.1% CPC were tested on the absorption of 2.5 and 1.5 g mL −1 aniline and p-aminophenol, respectively, and 0.2-2.0 mL of 0.1% CTAB on the absorption of 1.0 g mL −1 benzidine, in the presence of phosphate buffer solution. It was found that 0.5 mL of CPC is the optimum amount for aniline and p-aminophenol and 1 mL of CTAB for benzidine which is recommended in subsequent experiments.

Effect of Order of Addition.
In order to obtain the high colour intensity, the order of addition of reagents for aromatic amines should be followed as given in the Table 3; otherwise a loss in colour intensity was observed.
However, the optimum reaction conditions for developing the colour intensity of primary amine-TCNQ complexes are summarized in Table 3.

Analytical Parameters
Under the experimental conditions described in Table 3, standard calibration curves of CT complexes for aliphatic and aromatic amines with TCNQ were constructed by plotting absorbance versus concentration. The correlation coefficients ranged from 0.9961 to 0.9993, indicating good linearity. Beer's law is obeyed in the ranges cited in Table 4, and the molar absorptivity values indicate the high sensitivity of the method.

Precision and Accuracy
Six replicate measurements are performed at three different concentrations of each amine. The relative standard deviation and recovery % results indicated the high precision and accuracy of the proposed method (Table 5).

Interferences
The interference from various organic nitrogen compounds including secondary, tertiary amines and amides in addition to sodium chloride, n-hexane, and glucose on the determination of 1 g/mL of p-aminophenol (as an example for primary aromatic amines) and 0.5 g/mL of ethylamine (as an example for primary aliphatic amines) was examined. It was found that these compounds did not affect the recovery % in the range from 5 g/mL of triethylamine to 2500 g/mL of acrylamide for ethylamine and from 5 g/mL of diethylamine to 100 g/mL of acetanilide for p-aminophenol. The results are summarized in Table 6.

Application to Real Water Samples
The described method was applied to the analysis of benzidine in tap, river, and sea waters. Three different concentrations 2.5, 5.0, and 10 g/mL of benzidine were added to the various filtered volumes of tap, Dijlla river, and synthetic sea waters [20] in final volume of 10 mL and treated as described in the previous general procedure. The results in Table 7 indicated that benzidine could be determined in the presence of small volumes of tap and river waters, but an interference was observed in the presence of small volumes  of sea water; this may be attributed to the formation of metalligand complexes.

Stoichiometry
The stoichiometry of the reaction of primary amines with TCNQ was studied by Job's method [21], using solutions of equimolar (1 × 10 −3 M) of each primary aliphatic and aromatic amines and TCNQ reagent (1 × 10 −4 M in the case of benzidine). The results obtained in Figure 3 show that 1 : 1 amine to reagent was formed. This indicates that only one amino group is responsible for the formation of the products.

Stability Constant of Amine-TCNQ Complexes
The apparent stability constant was estimated by comparing the absorbance of a solution containing stoichiometric amounts of the primary amine and TCNQ (As) to one containing an excessive (optimum) amount of TCNQ reagent (Am). The average conditional stability constants of the complexes are calculated by the following equation: where Kc is the association constant (L⋅mol −1 ), the dissociation degree, and the concentration of the complex which is equal to the concentration of primary amine. The results shown in Table 8 indicate that the complexes are relatively stable.

Reaction Mechanism
The nature of the reaction between primary amines in aqueous solution and TCNQ reagent is not clearly understood.
Most of the spectrophotometric methods with TCNQ [22][23][24][25][26] are based on the charge-transfer interaction of radical anion TCNQ .− with the radical cation donors (D .+ ) formed in acetonitrile medium leading to enhancement of the absorption bands of TCNQ reagent in acetonitrile solvent at 840, 825, 762, and 742 nm. However, in the present work it was observed that the complexes are formed in aqueous medium in the ratio of 1 : 1 amine : TCNQ with the appearance of a new absorption bands at 464, 470, 323, 511, and 500 nm for ethylamine, 1,2-diaminopropane, aniline, p-aminophenol, and benzidine, respectively, which is not shown by either of the components present in solution which may be attributed to the complete transfer of the unshared pair of electrons on the nitrogen atom to TCNQ reagent and assigned as a charge-transfer complex absorption bands. On this basis, a tentative reaction mechanism has been proposed and given in Scheme 1.

Comparison with Other Reported Spectrophotometric Methods
A comparison of some parameters, linearity range, and sensitivity of the current method with those of some other reported spectrophotometric methods, using differentacceptors, was described (Table 9). It is quite clear that current method is more sensitive than the reported methods and all the charge-transfer complexes of TCNQ reagent having maximum absorption at visible regions except of TCNQ-aniline complex appears at UV region.

Conclusion
The proposed method is simple, rapid, sensitive, and economical compared to already reported methods and does   not require any pretreatment of the primary amines or extraction procedure and has a good accuracy and precision. On the other hand, in terms of simplicity and expense, the method could be considered superior in comparison with the previously reported methods, especially with those based on nonaqueous medium.