ORIGINAL PAPER Investigation of 3-amino-1,2,4

Chemical Papers 62 (6) 541–546 (2008) DOI: 10.2478/s11696-008-0071-6

ORIGINAL PAPER

Investigation of 3-amino-1,2,4-triazole azodye derivatives as reagents for determination of mercury(II) Abdalla M. Khedr* Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt Received 2 October 2007; Revised 21 March 2008; Accepted 21 March 2008

The reaction of mercury(II) with 3-(2,4-dihydroxyphen-1-ylazo)-1,2,4-triazole (HL1 ), 3-(2hydroxy-5-methylphen-1-ylazo)-1,2,4-triazole (HL2 ), 3-(2-hydroxy-5-ethoxycarbonylphen-1-ylazo)1,2,4-triazole (HL3 ), 3-(2-hydroxy-5-acetylphen-1-ylazo)-1,2,4-triazole (HL4 ), 3-(2-hydroxy-5-formylphen-1-ylazo)-1,2,4-triazole (HL5 ), and 3-(2-hydroxy-5-bromophen-1-ylazo)-1,2,4-triazole (HL6 ) was studied. A new, direct, and simple procedure was suggested for the spectrophotometric determination of mercury(II) based on its complexation reaction with HL1 –HL6 . The best reagent was found to be HL3 due to its high sensitivity and selectivity. In aqueous media of pH 9.0 containing 40 vol. % of methanol, Hg(II) reacts with HL3 to form a 1:2 (Hg(II) · HL3 ) complex having a sensitive absorption peak at 490 nm with the molar extinction coefficient of 3.31 × 104 L mol−1 cm−1 using 4 × 10−4 M of the reagent. Beer’s law is obeyed over the range from 0.00 µg mL−1 to 12.04 µg mL−1 of mercury(II). The proposed method was applied in the determination of mercury(II) in tap water, seawater and synthetic seawater samples, without the need of prior treatment, with satisfactory results. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: azo-triazole, spectrophotometric determination, mercury(II)

Introduction Mercury is a toxic element easily absorbed by humans and other organisms; thus, its occurrence in the environment is very harmful to all living organisms (O’Neil, 1995). Mercury content is to be monitored in all areas of modern life in order to prevent the environment from mercury pollution, so the determination of mercury is becoming increasingly important. Spectrophotometry is used for this purpose due to its simplicity and rapidity in analysis. The number of reagents available for the direct determination of mercury is relatively small and most of the known chromogenic reagents require extraction using organic solvents, pre-concentration (Kara & Tekin, 2005; Hosseini & Hashemi-Moghaddam, 2004), and the use of surfactants (Khan et al., 2005) to increase the sensitivity and selectivity or reactants producing toxic cyanides (Feng et al., 1999). In this paper, six deriva-

tives of 3-amino-1,2,4-triazole (Fig. 1) were prepared and their reaction with mercury(II) was studied in detail. Also, a new method for the direct spectrophotometric determination of mercury(II) using HL1 –HL6 ligands in a 40 vol. % methanol–water medium is presented. The proposed method has been applied successfully to the determination of mercury(II) ions in different natural water samples with high precession and good accuracy without the need of extraction, preconcentration or addition of any surfactants.

Experimental Unless otherwise stated, all reagents and solvents used were of analytical reagent grade. Also, bi-distilled and de-ionized water or pure methanol was used for the solution preparation. UV-VIS 240 spectrophotometer (Shimadzu, Japan) was used for absorbance measurements and elec-

*Corresponding author, e-mail: [email protected]

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A. M. Khedr/Chemical Papers 62 (6) 541–546 (2008)

OH N N N N N H

X

Ligand

X

HL1 HL2 HL3 HL4 HL5 HL6

4-OH 5-CH3 5-COOC2H5 5-COCH3 5-CHO 5-Br

1965). Universal buffer solutions of the required pH values were prepared as recommended by Britton (1952). To an aliquot of a sample solution containing no more than 25.07 µg mL−1 of Hg(II), 4 mL of 1 × 10−3 mol L−1 solution of the reagent (HL1 –HL6 ) and 4 mL of the universal buffer solution of the recommended pH value were added. The resulting solution was filled to the mark with deionized water and the absorption spectrum was measured within the wavelength range of 300–700 nm against a similarly prepared mercuryfree reagent blank of the same pH as the test solution. Using this procedure, calibration of the Hg(II)–ligand complexes absorbance vs. metal concentration yielded straight lines passing through the origin (each nine points at least).

Absorbance/a.u.

Fig. 1. Structure of the studied ligands.

Results and discussion Studies on the reaction of mercury(II) with HL1 –HL6 and optimum conditions for complex formation 250

450 Wavelength, λ/nm

650

Fig. 2. Electronic absorption spectra of HL3 (a) and Hg(II) · HL3 complex (b) in aqueous buffer solution at pH = 10 containing 40 % methanol.

tronic absorption spectra recording employing 10-mm matched quartz cells. pH of the solutions was adjusted using a digital ORION pH-meter model 201 with the sensitivity of ± 0.02 pH unit calibrated using a pH = 7.01 buffer solution ± 0.01 pH at 25 ◦C provided by HANNA instruments (Padova, Italy). The electronic spectra of mercury(II) complexes with HL1 –HL6 systems were measured against reagent blank, similarly prepared, but did not contain any mercury(II) ions. Ligands HL1 –HL6 were prepared by coupling the different substituted phenols with the diazonium salt of 3-amino-1,2,4-triazole dissolved in hydrochloric acid at 0–5 ◦C and adding an equivalent amount of icecooled sodium nitrite solution under vigorous stirring, a method described previously (Gaber et al., 1986). The precipitated azo compounds were filtered off and recrystallized from the appropriate solvent, then, the purity of each compound was checked using elemental analysis as well as considering the melting point constancy. 1 × 10−3 mol L−1 stock solutions of the reagents were prepared by dissolving the accurately weighted amount of the recrystallized reagent (HL1 –HL6 ) in the required volume of methanol. Stock solution (1 × 10−3 mol L−1 ) of mercury nitrate was prepared by dissolving the required amount of the Analar product in deionized water. The mercury(II) solution was standardized complexometrically with EDTA (Frank,

The absorption spectra of mercury(II) complexes with HL1 –HL6 were studied in solutions containing 40 vol. % of a methanol–water medium within the pH range of 2.0–12.0 in order to investigate the effect of pH on the complex formation. Fig. 2 shows the electronic absorption spectra of HL3 and the Hg(II) · HL3 complex in aqueous buffer solution of pH = 10, where the other ligands and their mercury(II) complexes show very similar behavior. The absorption maxima of the free ligands HL1 – 6 HL are located at 400 nm, 400 nm, 440 nm, 375 nm, 450 nm, and 440 nm, respectively, whereas the maxima in absorption spectra corresponding to the corresponding mercury(II) complexes could be found at 505 nm, 540 nm, 490 nm, 525 nm, 550 nm, and 495 nm, respectively. At these wavelengths, the absorbance of HL1 –HL6 reagents is negligible. Hence, these values were chosen as the most suitable wavelengths for further studies (Table 1). Absorption spectra were recorded for equimolar (ligand to mercury) solutions and solutions containing an excess of organic reagent. Fig. 3 presents only the absorbance–pH curves of mercury(II) complexes with HL1 and HL2 ligands as a very similar trend was observed also for mercury(II) complexes with the remaining reactants. The obtained curves show that the complex formation is pH-dependent with maximum yield corresponding to the absorbance maximum (Table 1), which is caused by the decreased medium acidity and the enhanced ionization of the active protons. Then, the absorbance in the strong alkaline medium decreased due to the hydrolysis of the formed complexes or the formation of hydroxy complexes with lower absorbance values. The low absorbance values in the strong acidic medium (pH range 2.0–5.0) may


543


Table 1. Optimum conditions and analytical characteristics of Hg(II) complexes with ligands HL1 –HL6 HL1

Ligand

HL2

HL3

HL4

HL5

HL6

Metal to ligand mole ratio 1:1 1:2 1:1 1:2 1:1 1:2 1:1 1:2 1:1 1:2 1:1 1:2 3.84 8.15 4.00 8.02 3.81 7.63 3.49 7.42 4.02 8.29 3.90 8.21 log β n 22.13 46.96 23.07 46.24 21.96 43.99 20.12 42.73 23.14 47.78 22.48 47.30 −∆G∗ /(kJ mol−1 ) pH 10.0 9.0 9.0 6.0 7.0 10.0 505 540 490 525 550 495 λmax /nm 2.100 1.5923 3.3056 0.6746 0.6230 1.2068 ε · 10−4 /(L mol−1 cm−1 ) Correlation coefficient 0.99998 0.99994 0.99997 0.99997 0.99997 0.99998 Standard deviation 0.0002 0.0004 0.0008 0.0006 0.0003 0.0002 0–18.05 0–25.07 0–12.04 0–20.06 0–16.05 0–16.05 Beer’s law linearity range/(µg mL−1 ) 2.01–10.03 2.51–15.44 1.20–6.02 6.02–20.06 4.81–16.05 3.21–16.05 Ringbom range/(µg mL−1 ) 1.00 1.25 0.60 1.00 0.80 0.80 LOQ · 10−5 /(mol L−1 ) 3.00 3.75 1.80 3.00 2.40 2.40 LOD · 10−6 /(mol L−1 )

0.8

Absorbance/a.u.

0.6

0.4

0.2

0

2

4

6

8

10

12

14

pH

Fig. 3. Absorbance–pH curves of Hg(II) complexes with HL1 (circles) and HL2 (squares) measured at 505 nm and 540 nm, respectively.

be attributed to the presence of HL1 –HL6 in their molecular form [HL] as prevalent species (Khedr et al., 2005). Protonation constants of the reagents under consideration (9.6, 10.0, 8.9, 5.5, 7.0, and 9.7 for HL1 –HL6 , respectively) were determined from their corresponding absorption spectra in buffer solutions of varying pH. The complexes formation takes place via hydrogen proton displacement from the free ligand and formation of [HgL]+ and [HgL2 ] complexes according to the following equilibria + + → Hg2+ + HL − ←− − − [HgL] + H

(1)

+ → [HgL]+ + HL − ←− − − [HgL2 ] + H

(2)

Addition of 3–5 mL of universal buffer was sufficient for pH adjustment. Hence, addition of 4.0 mL of buffer solution is recommended. The optimum reagent concentration for the maximum intensity of color development was determined by carrying out a set of experiments using solutions containing 10.03 µg mL−1 Hg(II). The reagent concentration was varied over a wide range, from 1 × 10−5 M

to about 1 × 10−3 M. The maximum absorbance was attained using solution with the reagent (HL1 –HL6 ) concentration of 4 × 10−4 M. The complex formation reaction was completed after 20 min under optimum conditions, and the complex was stable (absorbance was constant) for at least 18 h at temperatures below 50 ◦C, however, the absorbance dropped slowly with the increase of temperature above 50 ◦C. Among the tested solvents (acetone, methanol, ethanol, propan-2-ol, and butan-1ol), methanol was found the most suitable to dissolve Hg(II) complexes and the maximum absorbance was observed in the presence of 40 vol. % aqueous methanol. Addition in the sequence “reagent– mercury–buffer” gave the highest absorbance values for Hg(II) complexes with all tested ligands HL1 –HL6 . So, this sequence of addition was chosen as the optimum one. Composition and stability of mercury(II) complexes with HL1 –HL6 Composition ratios of Hg(II) complexes with HL1 – HL ligands (1:1 and 1:2) were obtained using the Job’s continuous variation and the mole ratio methods (Zayan et al., 1972; 1973; Issa et al., 1975). Conditional stability constants for both, 1:1 and 1:2, Hg(II) · HL complexes are usually determined using the results of mole ratio and the Job’s methods employing the following equation (Etaiw et al., 1981; Khedr & Gaber, 2005) 6

A Am n+1

βn = A 1− Am

(3) CLn n2

where β n is the stability constant, A is the absorbance at the ligand concentration CL , Am is the absorbance of the formed complex in presence of a very large amount of the ligand used, and n represents the ligand to metal stoichiometry.


544


Table 2. Effects of foreign ions on the determination of 10.03 µg mL−1 of Hg(II) using ligands HL1 , HL2 , and HL3

2.0

Absorbance/a.u.

1.6

Coexisting ion to Hg(II) mole ratio Coexisting ion

1.2

0.8

0.4

0

0

5

10 15 20 25 Content, w · 104/mass %

30

Fig. 4. Absorbance–concentration plots of Hg(II) · HL1 (circles, pH = 10, λ = 505 nm), Hg(II) · HL2 (squares, pH = 9, λ = 540 nm), and Hg(II) · HL3 (triangles, pH = 9, λ = 490 nm) complexes.

Also, the change of the Gibbs energy of the formation of mercury(II) complexes with HL1 –HL6 were calculated using the obtained values of conditional stability constants ∆G∗ = −RT ln βn

(4)

∆G∗ representing the Gibbs energy change, T the absolute temperature, and R the gas constant. The values of the conditional stability constant and the Gibbs free energy change for 1:2 (Hg(II) · HL) complexes with HL1 –HL6 are nearly double those of the 1:1 complexes for the same ligand molecule (Table 1) thus confirming the increase in the stability of the formed complexes with the increasing number of ligand molecules attached to the central metal ion. Analytical characteristics of formed complexes and the substituent effect The calibration graphs (Fig. 4) were constructed in the usual way according to the general procedure. Beer’s law was obeyed up to 18.05 µg mL−1 , 25.07 µg mL−1 , 12.04 µg mL−1 , 20.06 µg mL−1 , 16.05 µg mL−1 , and 16.05 µg mL−1 of mercury(II) at 505 nm, 540 nm, 490 nm, 525 nm, 550 nm, and 495 nm using HL1 –HL6 ligands, respectively. The simple linear regression calibration equations are: A = 21.00 CHg + (−0.0003), A = 15.92 CHg + (−0.0033), A = 33.06 CHg + 0.0007, A = 6.75 CHg + (−0.0047), A = 6.23 CHg + (−0.0009), and A = 12.68 CHg + 0.0023 (CHg is the Hg(II) concentration in mmol−1 L−1 ). An inspection of the results in Table 1 revealed that the sensitivity of the 1:2 (Hg(II) · HL) complexes with HL1 –HL6 is influenced by the inductive and/or mesomeric effects of the substituent X on the phenyl moiety. This behavior can be correlated employing the effect of different substituents on the molecular structure as follows:

NaNO2 K2 SO4 Cl− Br− I− CH3 COO− CO2− 3 SCN− Citrate Tartarate NH+ 4 Ag+ 2+ Mg Ca2+ Ba2+ Sr2+ Al3+ Sb3+ Bi3+ Cr3+ As3+ Pb2+ Cd2+ Mo4+ Mn2+ Fe2+ Co2+ Ni2+ Cu2+ Zn2+

HL1

HL2

HL3

2550 2450 205 79 60 505 770 2280 512 545 2390 205 2020 2350 1140 2210 1690 2290 2200 3460 1980 218 450 895 670 1490a 128 97 75 260

2600 2510 210 84 62 525 795 2300 523 560 2410 208 2048 2380 1190 2270 1730 2315 22230 3490 1960 220 460 920 680 1520a 132 102 79 270

2700 2600 227 90 64 580 833 2380 544 602 2500 220 2087 2500 1282 2300 1786 2400 2300 3571 1995 225 500 980 834 1600a 147 110 81 294

a) In presence of sodium potassium tartarate.

i) Higher sensitivity of HL1 , HL2 , and HL3 complexes could be attributed to the character of substitutes on the aromatic ring. The presence of the 4OH and 5-CH3 , and 5-COOEt groups increases the electron density on the chelating ring; thus enhancing chelation ability, i.e. sensitivity of these ligands. ii) Lower sensitivity of HL4 , HL5 , and HL6 complexes is caused by the presence of the electron withdrawing 5-COCH3 , 5-CHO, and 5-Br groups. This causes a negative inductive and/or mesomeric effect decreasing the ligand ability of chelation and, therefore, lower sensitivity of these reagents (Hine, 1975). iii) For comparison, the reagents sensitivity decreases in the order: 5-COOEt, 4-OH, 5-CH3 , 5-Br, 5-COCH3 , 5-CHO. Using HL1 –HL6 ligands for the mercury complexes formation, the detection limits of mercury were found to be 3.00 × 10−6 M, 3.75 × 10−6 M, 1.80 × 10−6 M, 3.00 × 10−6 M, 2.40 × 10−6 M, and 2.40 × 10−6 M, whereas the limits of quantification were found to be 1.00 × 10−5 M, 1.25 × 10−5 M, 0.60 × 10−5 M, 1.00 × 10−5 M, 0.80 × 10−5 M, and 0.80 × 10−5 M, respectively. The optimum working ranges for mercury(II) determination using the reagents under investigation


545


Table 3. Analytical data for Hg(II) determination in various natural water samples by both the present method using HL3 and the well-established method (Hosseini & Hashemi-Moghaddam, 2004) Hg contenta /µg

Error

Recovery

Sample Present method

Hosseini and Hashemi-Moghaddam (2004)

%

%

7.88 16.22 31.88

8.01 15.98 32.38

1.65 1.45 1.57

101.7 98.9 101.60

Tap water Seawahter Synthetic seawater

SDb

CVc

0.09 0.17 0.35

1.17 1.05 1.10

a) Average of six determinations;ý b) Standard deviation; c) Coefficient of variation. Table 4. Comparison of different methods for Hg(II) determination Reagent N,N -bis(2-mercaptophenyl)ethanediamide Iodide and ferroin 1,5-Diphenylthiocarbazone Ferrocyanide and 1,10-phenanthroline

Disadvantages of the method Need of solid-phase extraction, elution using acetone and pre-concentration Requires separation using heptane and preconcentration Must be solubilized in micelle Produces toxic cyanide ions

were determined from the Ringbom plots data (Table 1). Effect of interfering ions The effect of diverse ions on mercury(II) determination using HL1 , HL2 , and HL3 (selected from the above studies as the best chromogenic reagents) was studied by adding a known quantity of the desired ion to the solution containing 10.03 µg mL−1 of mercury(II), the concentration of which was determined by the given procedure. Solutions of interfering ions were prepared from the Analar products of potassium or sodium salts of the anions and from nitrates or sulfates of the tested metal cations. The tolerance criterion for a given ion was taken as a deviation of more than ± 5 % of the absorbance value from the value expected for Hg(II) alone. The results in Table 3 show that the presence of macro-amounts of foreign anions and metal ions did not interfere with the absorbance measurements of the complexes of Hg(II) with HL1 , HL2 , and HL3 . The concentrations listed in Table 3 are much higher than those expected for natural waters. Thus, no interference of foreign ions in the real sample analysis is to be expected. The interference of several cations could be decreased using a suitable masking agent. The interference effect is very small in case of the HL3 ligand and it increases employing the HL2 to HL1 ligands. Under optimum experimental conditions, it was observed that Hg(II) could not be accurately determined in the presence of EDTA or F− . Therefore, these reagents could not be used as masking agents. In a previous work (Khedr, 2006), HL1 –HL6 had been applied in the determination of nickel(II). A comparison of the results with the results obtained in this

Reference Kara and Tekin (2005) Hosseini and Hashemi-Moghaddam (2004) Khan et al. (2005) Feng et al. (1999)

work indicates that Hg(II) · HL3 is the most sensitive and selective system for Hg(II) determination whereas Ni(II) · HL1 is the most sensitive and selective system for Ni(II) determination. Also, thiourea must be added as a masking agent for mercury(II) during the determination of Ni(II) using HL1 , whereas no addition of any masking agent for Ni(II) was necessary during the determination of Hg(II) using HL3 . Applications and comparison with other known reagents In order to validate the methodology, the proposed method was applied in the determination of mercury(II) in tap water, seawater, and synthetic seawater (Fifield & Haines, 2000) samples using HL3 as the best chromogenic reagent. The proposed method was applied to each water sample after adjusting the pH to 9.0 and the absorbance was measured at 490 nm as described in the general procedure. Accuracy of the investigated method was checked by comparing the sample analysis results with those obtained by the well established standard method (Hosseini & Hashemi-Moghaddam, 2004). Mercury(II) concentrations determined by the suggested method were in a good agreement with those obtained by the standard method (Table 3). These results show a satisfactory applicability of this method for the determination of µg L−1 levels of mercury(II) in various types of water samples without the need of any pre-treatment. In comparison to the well-known extraction and pre-concentration methods which are time-consuming, use harmful solvents, or need an addition of surfactants (Table 4) (Kara & Tekin, 2005; Hosseini & Hashemi-Moghaddam, 2004; Khan et al., 2005; Feng et al., 1999), simplicity, sensitivity, and wide linearity


546


H N

X H2O AcO

N X

N

O

N N

Hg

O Hg

N N

O

N N N H

X

N N

N N

Ligand

X

HL1 HL2 HL3 HL4 HL5 HL6

4-OH 5-CH3 5-COOC2H5 5-COCH3 5-CHO 5-Br

N H

HgL

HgL2

Fig. 5. Representative structures of the Hg(II) complexes under consideration.

range are, besides accuracy and precision, the main advantages of the proposed method.

Conclusions In this work, the reaction of mercury(II) with six azodyes derivatives of 3-amino-1,2,4-triazole was studied. Based on this reaction, a new and direct method for routine determination of mercury(II) in various water samples without any prior separation or treatment was proposed. The aspects of this method are as follows: high sensitivity (ε = 3.84 for HL3 ), reaction rapidity, condition mildness, good reproducibility, and relatively little interference. When applied to determinate the Hg(II) ion in various water samples, the method produced results with high precission and good accuracy compared to the results obtained by the well-established method (Hosseini & HashemiMoghaddam, 2004). Both the data gained from the spectrophotometric studies in this work and the data from the previous research studies (Khedr & Gaber, 2005; Khedr, 2006) confirm that the metal ion is covalently chelated with the oxygen of the phenolic OH group and coordinately chelated with one nitrogen atom of the azo group. This indicates that the azodyes under investigation behave as monobasic bidentate ligands toward the Hg(II) ion as shown in Fig. 5. References Britton, H. T. S. (1952). Hydrogen ions (4th ed.). London: Chapman and Hall. Etaiw, S. H., Issa, R. M., & El-Assy, N. B. (1981). Physicochemical characters and stability constants of Sc(III), Y(III), La(III), Ce(III), Gd(III) and Yb(III) complexes. Journal of Inorganic and Nuclear Chemistry, 43, 303–309. DOI: 10.1016/0022-1902(81)90013-0. Feng, Y., Narasaki, H., Tian, L., Wu, S., & Chen, H. (1999). Flow-injection spectrophotmetric determination of mercury(II) in water by catalytic decomposition of ferrocyanide. Analytical Sciences, 15, 915–918. Fifield, F. W., & Haines, P. J. (2000). Environmental analytical chemistry (2nd ed.). London: Blackwell Science.

Frank, J. W. (1965). The analytical uses of ethylenediaminetetraacetic-acid. London: D. Van Nostrand. Gaber, M., Hassanein, M., & Ahmed, H. A. (1986). Co(II), Ni(II) and Cu(II) complexes with some 3-arylazo-1,2,4triazole dyes. Indian Journal of Textile Research, 11, 48–56. Hine, J. (1975). Structural effects of equilibria in organic chemistry. New York: John Wiley. Hosseini, M. S., & Hashemi-Moghaddam, H. (2004). Flotationspectrophotometric determination of mercury in water samples using iodide and ferroin. Analytical Sciences, 20, 1449– 1452. Issa, M. I., Issa, R. M., & Ahmed, Y. Z. (1975). The Th(IV), Ce(III) and U(VI) chelates with hydroxyanthraquinones. Egyptian Journal of Chemistry, 18, 427–433. Kara, D., & Tekin, N. (2005). Solid-phase extraction and spectrophotometric determination of trace amounts of mercury in natural samples. Microchimica Acta, 149, 193–198. DOI: 10.1007/s00604-005-0322-y. Khan, H., Ahmed, M. J., & Bhanger, M. I. (2005). A simple spectrophotometric determination of trace level mercury using 1,5-diphenylthiocarbazone solubilized in micelle. Analytical Sciences, 21, 507–512. Khedr, A. M., & Gaber, M. (2005). Spectrophotometric studies of the reaction of zinc(II) with some azo-triazole compounds and its application to the spectrophotometric determination of micro-amounts of zinc(II). Spectroscopy Letters, 38, 431– 445 (2005). DOI: 10.1081/SL-200062814. Khedr, A. M., Gaber; M., Issa, R. M., & Erten, H. (2005). Synthesis and spectral studies of 5-[3-(1,2,4-triazolyl-azo]-2,4dihydroxybenzaldehyde (TA) and its Schiff bases with 1,3diaminopropane (TAAP) and 1,6-diaminohexane (TAAH). Their analytical application for spectrophotometric microdetermination of cobalt(II). Application in some radiochemical studies. Dyes and Pigments, 67, 117–126. DOI: 10.1016/j.dyepig.2004.11.004. Khedr, A. M. (2006). Spectrophotometric determination of nickel(II) in different samples by complexation with some triazolyl-azodyes. Chemical Papers, 60, 138–142. DOI: 10. 2478/s11696-006-0025-9. O’Neil, P. (1995). Mercury in environmental chemistry (2nd ed.). London: Chapman and Hall. Zayan, S. E., Ibrahim, N. A., Issa, R. M., & Magrabi, J. Y. (1972). Physico-chemical studies of the Fe(III) dinitrosoresorcinol reaction. Egyptian Journal of Chemistry, 15, 445– 452. Zayan, S. E., Issa, R. M., Magrabi, J. Y., & El-Dessouky M. A. (1973). Spectrophotometric study on the copper(II)dinitrosoresorcinol reaction. Egyptian Journal of Chemistry, 16, 459–464.
