Highly Selective and Sensitive Spectrophotometric ...

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(MK); (2-(5-Bromo-2-pyridylazo)-5-dimethylaminophenol (BPADAP); 2-(3 ... (DBPADAP); Sulfochlorophenolazo thiorhodanine (SCS); Tetra-(4-.

Annali di Chimica, 97, 2007, by Società Chimica Italiana















M. GHAEDI(°)a, A. DANESHFARb, A. SHOKROLLAHI a, H. GHAEDIa, F. ARVIN PILIb a) Chemistry Department Yasouj University Yasouj Iran 75914-353. b) Chemistry Department, Ilam University, Ilam, Iran. Summary - A simple and accurate spectrophotometric method for determination of trace amounts of silver ion in tap and wastewater solution and photographic solutions has been described. The spectrophotometric determination of silver ion using 2mercaptobenzoxazole (MBO) in the presence of Triton X-100 as nonionic surfactant has been carried out. The Beer’s law is obeyed over the concentration range of 0.1-9.0 µg mL-1 of Ag+ ion with the detection limits of 1.6 ng mL-1. The influence of type and amount of surfactant, pH, complexation time and amount of ligand on sensitivity of method were optimized. Finally the repeatability, accuracy and the effect of interfering ions on the determination of silver ion were evaluated. There is a good agreement between results of proposed method and atomic absorption spectrometry. INTRODUCTION Silver is of great commercial importance due to its widespread application in photography, the electronic industry and in the field of medicine. Silver were used in production of coin, jewelry and silver ware, as well as in making solder and brazing alloys, electrical contacts and high capacity silverzinc and silver cadmium batteries. It is well known that silver inactivate sulfhydril enzymes and also combines with amine, imidazole and carboxyl group of various metabolites such as high molecular weight proteins and metallothionein in tissue of cytosol fractions. Repeated exposure of animals to silver may produce anemia, cardiac enlargement, growth retardation and degenerative changes in the liver.1 Due to their germicidal properties, silver salts are also used for water drinking. Therefore, the need for a highly sensitive and selective determination of silver ion arises from its economic value and its long-term toxicity for human and the environment. Among the various analytical techniques available, the use of spectrophotometric method is well establishing routine analytical technique.2 Specifically, strong silver ion recognition is very (°)

Corresponding Author: [email protected] Telfax: +98-741-2223048


GHAEDI and coworkers

important for silver determination probably useful for photographic industry and recovery of silver ion from wastewater in environment.3 Several Ag (I) ion complexing ligands have been investigated such as crown ethers containing sulfur atom(s) in the cyclic structure, doubled-armed4 or multi-armed macro cycles.5 The presence of sulfur atoms in crown ethers improves the selectivity towards heavy metal ions. The synthetic ionophores, when some of their oxygen atoms are replaced with sulfur atoms, exhibit high affinity toward soft metal ions such as Ag+.6-9 Thus, Ag+, is apt to interact with ًπ-electrons several crown ethers containing thiol group and the ًcoordination is quite selective and relatively weak.10-11 Appropriate assembly of ًcoordination groups can afford the coordination atmosphere for Ag+. It was pointed out that the soft coordination sites of sulfur offer great affinity toward Ag+ and especially Hg2+ as soft transition metal ions.12,13 Application of micelles in analytical chemistry involves the beneficial alteration of metal ion-ligand complex spectral properties via surfactant association. In this way, the sensitivity and the selectivity of numerous analytical reactions can be improved with the addition of certain surfactants.14 A benefit of the presence of a surfactant in the system is the capacity to solubilize an insoluble complex and/or ligands.15 By addition of surfactant to metal-chelate complexes (because of mechanisms including micellar solubilization and formation of ternary complexes containing surfactant monomers), a higher stability and an improvement in sensitivity (molar absorptivity) in coincide to red shift (approximately 50-150 nm)16,17 could be achieved. In surfactant media complexes of metal ions with complexing agent are most stable and form aggregates that cause an improvement in sensitivity and detection limits.18 Thus, development of simple, sensitive and selective analytical techniques without using expensive or complicated test equipment for the determination of trace amount of silver ion is becoming increasingly important, especially in environmental pollution.19,20 For this reason, a wide variety of spectrophotometric methods for the determination of silver have been reported,21-29 and figures of merit for some of them are presented in Table 1. Each chromogenic system has its advantages and disadvantages with respect to sensitivity, selectivity and rapidity. The coordination of 2-mercaptobenzoxazole (MBO) to transition-metal ions is of special interest because of the efficiency of this ligand as a corrosion inhibitor, an important regulator of plant growth, showing a marked bacteriostatic and fungicidal activity.30 MBO is structurally related to biological important bases, having more than one donor atom to bind transition-metal ions. Both in the solid state and in polar solvents, it exists as the thione tautomer with the protonated N-atom.31,32 Most of the electron density of the oxygen is in resonance with the aromatic ring making it a poor donor atom. Although S-N linkage isomerism in MBO complexes is possible. The evidence for this idea, as it was occurred in our previous publication,33,34 is the decrease in pH of ligand solution by addition of transition metal ions that may be attributed to complexation on S- linkage. In the present work a simple and highly selective and sensitive spectrophotometric method for determination of silver ion using 2- mercaptobenzoxazole in surfactant media (Triton X-100) for determination of silver ion was established.

973 EXPERIMENTAL SECTION Instrumentation A Jasco UV-Vis V-570 spectrophotometer was used to measure the absorbance of complex in Triton X-100 media. A 691 pH /Ion meter with a combined glass and calomel electrode has been used to adjust the pH. Reagent and solution All chemicals such as nitrate of silver and other cations were of the analytical grade and purchased from Merck company. 1.0 % (w/v) solutions of all surfactants were prepared. Surfactants are from Merck company including sodium dodecyl sulfate (SDS), Triton X-100, cetyltrimethylammonium bromide (CTAB); n-dodecytrimethylammonium bromide (DTAB) was prepared by dissolving 1.0 g of surfactant in 100 mL volumetric flask while stirring. Pretreatment of Real Samples The waste photographic samples were prepared for the measurement of their silver content as follows. To 20 ml of the sample was added 10 ml 3M nitric acid and 10 ml water. The solution was boiled for 70 min until its volume reduced to 20 mL. The resulting solution was neutralized with sodium hydroxide solution to a pH of 5.0 ± 0.1 and filtered. The filtrate and washings were diluted to 25 ml in a volumetric flask and used for spectrophotometric and atomic absorption measurements.35 Analysis of waste water and spring water sample for determination of silver ions content was done as follows: 250 mL of sample was poured in a beaker and 8 ml 3 M HNO3 and 3 ml of H2O2 of (30%) for elimination and decomposition of organic compound were added. While stirring, it was heated to reach its volume to half. After adjustment of samples pH to desired value the spectrophotometric experiment was carried out. In all of real samples amount of silver ion was found by standard addition method.1 TABLE 1. - Characteristic performance of some spectrophotometric determination of silver ion Reagent


λ max (nm)

ε(×104)(l mol-1 cm-1)

Linear range (µg ml-1)



(pH 10), SDS






(pH 5), SDS






(pH 6.8-8), SDS






(pH 5), SDS






(pH 2.8), Triton X-100






(pH 10), TritonX-100



0- 0.32


GHAEDI and coworkers

974 Dithizone

(weak acid)






(pH 11), OP



0- 0.48







TABLE 1. – (Continued) (pH 6.5), SDS

Abbreviation: 2-(5-Chloro-2-pyridylazo)-5-diethylaminoanilin (CPADAA); Thio-Michler’s ketone (MK); (2-(5-Bromo-2-pyridylazo)-5-dimethylaminophenol (BPADAP); 2-(3,5-Dibromo-2-pyridylazo)5-diethylaminophenol (DBPADAP); Sulfochlorophenolazo thiorhodanine (SCS); Tetra-(4chlorophenyl)-porphyrin (TCPP); o-Carboxyl-benzenediazo aminoazobenzene (CBDAAB), 2-(2quinolylazo)-5-diethylaminoaniline (QADEAA) Spectrophotometric Investigation Standard stock solutions of ligand (1.0×10−3 M) and the silver ions (1.0×10−3 M) were prepared by dissolving appropriate and exactly weighed (with an accuracy of 0.0001 g) pure solid compounds in precalibrated 25.0 mL volumetric flasks and diluted to the mark with methanol. Working solutions were prepared by appropriate dilution of the stock solutions. According to the spectra reported in Fig.1a, titration of the ligand solution (2.3 × 10−6 M, 2.6 mL) was carried out by the addition of microliter amounts of a concentrated standard solution of the silver ion in methanol (1.0×10−3 M) using a pre-calibrated micropipette, followed by absorbance intensity reading at 25.0 ◦C at the related λmax. Since the volume of titrant added during titration was negligible (at the most 0.05 mL) as compared with the initial volume of the ligand (2.6 mL), no volume correction was carried out.35,36 RESULTS AND DISCUSSION MBO Information 2- Mercaptobenzoxazole have sulfur, nitrogen and oxygen as donor atoms, allowing them to act as mono-dentate sulfur donors and nitrogen donors with dissociation constant of MBO being ca. 7.0.37 The existence of a donating nitrogen atom as well as –SH group in 2- mercaptobenzoxazole was expected to increase both the stability and selectivity of its silver complex over other metal ions, especially alkali and alkaline earth cations. The spectrum of MBO (L) in methanol shows absorption band at 296 nm. As it can be seen from Fig. 3a, a decrease in absorbance is observed upon addition of increasing quantities of silver ions to L Solution, whereas the absorption intensity changes as a function of the [Ag+]/[L] molar ratio (Fig. 1b). These changes could be attributed to the complexation between the ligand L and Ag+ ion. From the inflection point in the absorbance/mole ratio plot at [Ag+]/[L] values, it can be inferred that both 1:1 ([Ag(L)]) and in less extent 1:2 ([Ag(L)2]) complexes are formed in acetonitrile solution. When the ligand molecule reacts with the metal ion, it may form both 1:1 and 1:2 metal-to-ligand complexes. The resulting Log K1 and K2 values (Table 2), obtained from computer fitting of the absorbance/mole ratio data to a theoretical model including both 1:1 and 1:2 forms, were calculated. It should be noted that fitting on other possible models such as 1:1 or 1:2 lonely showed no

975 acceptable results. The good agreement between the predicted value and experimental value according to kinfit curves (Fig. 1c) is an indication for accurate determination of respective stability constant. Nowadays spectrophotometric investigation following kinfit analysis mostly has been applied for evaluating complex structure and evaluating respective stability constant.35,36 In preliminary experiments typical complexation between silver ion and MBO was examined using spectrophotometry, and the nature of respective complex was investigated and concluded that complexation is responsible for selective, sensitive and reversible spectrophotometric determination of traces of silver ion. TABLE 2. - Log β calculated by KINFIT program Log β1 Log β2

6.56 ± 0.02 8.25 ± 0.04

1.6 silver incrase




1.2 1 0.8 0.6 silver incrase

0.4 0.2 0 200





Wavelenght (nm)

1.6 1.4




1.2 1 0.8 0.6 0.4 0.2 0 0







FIGURE 1. - UV-visible spectra for titration of MBO (2.8×10 -5M) with Ag+ (1×10 -3M) in a methanol (T = 25 °C and I = 0.05 M, λmax = 300 nm) (a), and the corresponding molar ratio plot (b), obtained curve of KINFT program (c).

GHAEDI and coworkers


Thus, we decided to examine MBO capability as a suitable reagent for spectrophotometric determination of silver ion in Triton X-100 media. A sensitive decrease in absorbance of about 294 nm indicates the strong interaction between the silver and the MBO. At higher pH, formation of hydroxide inhibits complex formation. The donating nitrogen atom as well as –SH group in 2mercaptobenzoxazole were expected to increase both the stability and selectivity of its silver complex over other metal ions, especially alkali and alkaline earth cations. The effect of various parameters such as pH, type and amount of surfactant, amount of ligand and ionic strength were examined. Time dependency of complex and effect of interference of other metal ions were evaluated. Absorption spectra of Ag (MBO) in Triton X-100 media Regarding the complexation of Ag+ with MBO in methanol solvent (Fig.1), it must be concluded that the addition of surfactant is necessary to increase figures of merit of proposed method. Therefore, two similar solutions containing 5.0 µg ml-1 of Ag+, 0.09 mM MBO (in the presence and absence of 6.0 mM Triton X-100) have been prepared and spectrophotometric investigation has been conducted. Then, absorption spectra of desired complex in the presence and absence of surfactant Triton X-100 was obtained, which is shown in Fig. 2. Experiments in surfactant media have higher sensitivity without need of extraction of complex to organic phase and have no disadvantages such as time consuming, need to harmful organic solvent and labor intensive. The method is called guencho- photometric due to decrease in absorption of ligand by the increase in the concentration of silver ion. 2 A B


1.5 C



0 270





Wavelenght (nm)

FIGURE 2. - A) curve of ligand B) curve of ligand and Ag+ C) curve of ligand and Ag+ in the presence of surfactant Effect of pH on Sensitivity The pH control is one of the most critical parameter on sensitivity and selectivity of analytical method. Due to the presence of SH group in the MBO structure, it seems that pH is the most effective parameter on sensitivity. The influence of the pH of the test solution on the determination of Ag+ in Triton X-100 media with MBO was studied by measuring the absorbance of desired complex with 10 µg of Ag+ in the pH range

977 of 2.0-11.0 by introducing 0.09 mM of MBO in the 6.0 mM Triton X-100 media. Results showed that the optimal pH for the reaction of Ag (I) with MBO is 5.0 (Fig. 3). An acetate buffer solution of pH 5.0 was recommended to control pH. The MBO pKa is about 7.0; therefore, in this pH all of MBO molecules are in mild acidic form which could be reversibly exchanged with silver ion. The decrease in the pH of ligand solution after addition of silver ion increment confirms this idea. We assume that the reaction to form this complex could have competed against hydroxide precipitation at higher pH and ligand protonation at lower pH that lead to reduction in sensitivity. Therefore, pH=5.0 was chosen for the further studies. 0.4 ∆A 0.2

0 0



6 pH




FIGURE 3. - The effect of pH on Absorbance changes Effect of Surfactant on sensitivity To improve the sensitivity, the effect of type and concentration of different surfactants on the reaction were examined. To investigate the effect of types of surfactants, Triton X- 100 as nonionic, SDS as anionic surfactant and CTAB and DTAB as cationic surfactant were examined. In concentration of Ag+ ion 5.0 µg mL-1 and 0.09 mM MBO and 6.0 mM each surfactant, effect of type of surfactant on spectra and sensitivity were examined and results are displayed in Table 3. For 6.0 mM Triton X-100 media, the calibration curve with high sensitivity could be constructed and its slope was about 1.5 times more than other and in the absence of surfactant. Therefore, addition of surfactant for increasing sensitivity is necessary. In SDS media a mentionable spectra for complexes could not be obtained, which may be related to possible Ag2SO4 as precipitate. The decrease in absorption change in the presence of DTAB and CTAB may be attributed to formation of AgBr as precipitate, which decreases method sensitivity. On the other hand, since the Ag+ ion is a cation, the electrostatic attractive interaction between it and cationic surfactant is not present, and the complex-forming process is not affected. It seems that MBO combines with silver ion to form a non-polar complex, which is extracted instantaneously into the local no-polar environment of micelle of non-ionic surfactant. This suggests that above mention complex was dissolved in surfactant phase due to the hydrophobic solvation of the desired chelate.

GHAEDI and coworkers


Effect of Triton X-100 concentration Various concentrations of Triton X-100 were added to solutions at optimum and sensitivity was examined. When the concentration of Triton X-100 is less than its critical micelle concentration, the homogeneous solution is formed at a point where Ag(MBO) complex can be well dissolved. With the concentration of Triton X-100 varying from 0.02 mM to 0.01 M at pH 5.0 the absorbance of desired complexes at concentration of 5 µg mL-1 of Ag+ was investigated. The maximum absorbance change was obtained when the concentration of Triton X-100 was 6.0 mM. TABLE 3. - Effect of type of surfactant on spectra and sensitivity for 5.0 µg mL-1 of Ag+ ion, pH 5.0, .09 mM MBO, 6.0 mM each surfactant Surfactant


Maximum Wavelength (nm)

No surfactant Brij 58 DTAB SDS Triton X-100 CTAB

0.3 0.18 0.28 0.15 0.44 precipitate

294 294 294 294 294 ------

Effect of MBO concentration on sensitivity The concentration of ligand has a large effect on the absorbance change of system. The desired complex with MBO incorporated in the interior core of ad-micelle. It is known that Ag+ ion interact stoichiometrically with MBO to form a 1:1 and in less extent a 1: 2 complexes. The desired complex with MBO was incorporated in the interior core of ad-micelle. In order to investigate the influence of MBO concentration, a set of similar experiments with various amount of MBO at constant amount of Triton X-100 and Ag+ ion were conducted and results are shown in Fig. 4. The analytical sensitivity and the reproducibility in the complex spectra were good in micellar media. Results display that ligand concentration must exceed that of silver ion to reach high sensitivity. Therefore, The MBO was added more than 10 times Ag+ ion concentration to reduce fluctuation in measurement of absorbance. 0.6 0.5 ∆A

0.4 0.3 0.2 0.1 0 0



0.06 0.08 [MBO] mM




FIGURE 4. - Effect of MBO concentration on sensitivity of method Equilibration time and stability of the complex The absorbance change of the complex in surfactant media at optimum value of parameters for different time periods ranging between 5-120 s was measured and observed that a contact time of 60 s is adequate to reach equilibrium complexation of silver ion with MBO. It was observed that the extracted complex is more stable as the absorbance change remains constant for at least 100 h. The optimum value and characteristic performances of method are presented in Table 4. TABLE 4. - Figures of merit of proposed method Variable


Ligand Concentration pH Linear range (µg mL-1) Surfactant Concentration Equilibration Time Solvent Detection Limit (ng mL-1)

0.09 mM 5.0 0.1-9.0 6 mM Triton X-100 20 s Water 1.6 High repeatability, sensitivity, selectivity, wide linear range and no need to organic solvent Do not preconcentrate

Advantages Disadvantages

Calibration curve and detection limit A calibration curve was constructed at optimum conditions by conducting a set of similar experiment at various concentration of silver ion. The dynamic range of Ag+ ion was 0.1-9.0 µg mL-1. Based on the signals of ten blank solutions and the slope of calibration curve, it was found that the detection limit was 1.6 ng mL-1. Typical calibration curve are presented in Fig. 5. The complexation between the MBO ligand and Ag+ is a reversible process. Therefore, by a control pH, the MBO could be regenerated and could be used for following spectrophotometric studies. 2


1.8 1.6 1.4 1.2 1 0.8 0



6 [Ag] mg/L




GHAEDI and coworkers

FIGURE 5. - typical calibration curve (y = -0.0081x + 1.824 ,R2 = 0.9991 ) Interference Effect The capability of ligand for complex formation at optimum condition was investigated by measurement of the absorbance of complex in the presence and absence of various amounts of common interfering ions that are concomitant with silver ion in real samples. The selectivity of the proposed method was investigated by the determination of 1.0 µg mL-1 Ag+ ion in the presence of various ions within a relative error of 5.0%. Therefore various amounts of interfering ions were added to 1.0 µg mL-1 of silver and results are shown in Table 5. The Ag+ ion was quantitatively determined with high accuracy and precision in the presence of large amounts of alkaline and alkaline-earth ions and some transition metal ions. The interference of copper ion can be easily eliminated by addition of 0.01M histidine. The results of Table 5 indicate high selectivity and sensitivity of method. This may be ascribed to incorporation of aromatic ring as π donating compound and sulfur atom acting as soft acid, with its high tendency to bind silver. TABLE 5. - Effects of the matrix ions on the sensitivity (N=3)

Tolerance Limit [Ion ]/[Ag+] 1000

Na+, K+


Cu2+ a


Cd2+, Fe2+, Fe3+


NH4 +






PO4 3-


Co2+, Pb2+, Ni2+, Zn2+, Ba2+, Ca2+, Al3+, Cr3+, Hg2+

a) After addition of 0.01 m histidine Analytical application in real samples The proposed method has been successfully applied to the determination of Ag+ ion in water and waste water and in fixing and bleaching photographic solution treated according to experimental section. Spiking experiments and independent analysis checked reliability. To ensure that the method is valid and has reasonable accuracy and precision, recovery of the silver ions in the samples were determined by these proposed techniques and the results, which are shown in Table 5, have a good agreement with reference AAS method. The recovery of silver by adding known amounts of silver in

981 water sample and a standard method using atomic absorption spectrometry was also used as reference method. The results are shown in Table 6. TABLE 6. - Recovery studies of trace metal ions in spinach sample Sample Waste water a



0 50 Tap water a 0 50 River water a 0 100 Fixing solution b 0 2.0 b Bleaching solution 0 2.0 a) Value are in µg L-1 b) value are µg mL-1

67.8 119.6 79.3 130.8 75.9 180.1 6.03 8.12 6.45 8.53

Found AAS ---123 -----134.6 ----184.6 6.08 8.16 6.52 8.60


RSD % 1.4 0.9 1.7 1.0 1.2 1.0 1.0 0.7 0.8 0.6

Recovery % ---103.6 ----103.0 --104.2 ---104.5 ----104.0

CONCLUSION Most of the spectrophotometric methods for silver ion determination suffer from drawbacks including reagent cost, instability and regeneration of the reagent fro re-use, low sensitivity and selectivity. In order to cope with the difficulty, the proposed method, due to presence of sulfur atom in the ligand structure, has high tolerance limit of common ions and low detection limit. Therefore, it is a powerful tool for rapid and sensitive determination of silver ion in various media, due to the mentioned advantages such as simplicity, high selectivity, low cost, high stability and sensitivity that make the method suitable to successful determination of silver at trace level. The low RSD of real sample analysis is an indication of method versatility for real sample analysis. Received April 18th, 2007 Acknowledgement - The authors gratefully acknowledge the support of this work by the University of Yasouj Research Council. REFERENCES 1) M.K. Amini, M. Ghaedi, A. Rafi, I. Mohammad poor- Baltork, K. Niknam; Sensors and Actuators B. 2003, 96, 669-676. 2) M. Ghaedi; Spectrochimica Acta Part A, 2007, 66(2), 295-301 3) M. Oue, K. Kimura, T. Shono, Anal. Chim. Acta 1987, 194, 293. 4) H. Tsukube, K. Yamashita, T. Iwachido, M. Zenki, J.Org. Chem. 1991, 56, 268.


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