Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(9), 5-9, September (2013) Res. J. Chem. Sci. International Science Congress Association 5 Differential Pulse Anodic Stripping Voltammetric Study of Zinc-Ethylenediamine ComplexNurun Nahar Department of Chemistry, Jahangirnagar University, Savar Dhaka-1342, BANGLADESH Available online at: www.isca.in Received 9th July 2013, revised 27th August 2013, accepted 10th September 2013Abstract Differential pulse anodic stripping voltammetric study of zinc-ethylenediamine complex has been investigated using thin mercury film coated glassy carbon electrode at 30°C. The overall work was carried out at constant ionic strength (I = 0.20 mol dm-3) and pH (9.10±0.10). The electrode processes were found to be reversible and diffusion controlled. The results revealed that zinc forms three (1:1, 1:2 and 1:3- metal: ligand) complexes with ethylenediamine (en). The values of stability constant of zinc complexes were found to be 106.03, 1010.16 and 1013.98 for ZnL, ZnL and ZnL, respectively (the overall charges were omitted for simplicity). The percentage of all possible zinc species were calculated using the stability constant of zinc complexes and hydrolysis constant of zinc under present experimental conditions. Keywords: DPASV, ethylenediamine, zinc, stability constant. Introduction Zinc is the most important essential trace element. It is known that there are about 300 enzymes in which zinc present in their active sites and plays an important role as structural ions. Zinc presents in all body tissues and fluids. It is necessary to maintain normal physiological and biochemical functions of cell. It is present in metalloenzyme superoxide dismutase, alkaline phosphatase, alcohol dehydrogenase, and DNA and RNA polymerases. Zinc ion strongly interacts with nitrogen and oxygen and forms complexes with appropriate ligands. Some of these complexes exhibit antifungal activity. Some zinc containing macrocyclic compounds are very important as lipophilic carriers for anti-HIV substances. Free zinc ion in solution is highly toxic to plants, invertebrates and even vertebrate fish. Although zinc is essential for human body, but excessive amount of zinc is harmful because it can suppress copper and iron absorption. Thus removal of zinc is also an interesting subject of research. Zinc ion can easily interact with nitrogen containing ligand, such as ethylenediamine. In this article, we have demonstrated the formation of various complexes of zinc-ethylenediamine by differential pulse anodic stripping voltammetry (DPASV) due to its reproducibility and reliability at very low concentration4-6. Other methods are also available for the evaluation of complexing properties of metal ions in aqueous solution. For example, spectrophotometry, potentiometry, ion-selective electrodes9-10, etc. However, these methods are not suitable for low concentration of metal ions (ppb or sub ppb level) in aqueous solution. Recently differential pulse anodic stripping voltammetry has been extensively used in our laboratory to determine the stability constant of the complex between metal ion at ppb level and organic ligands in aqueous solution6,11,12. Material and Methods Equipment and Reagents: The differential pulse anodic stripping voltammetic measurements were performed by HQ-2040 electrochemical analyzer, Advanced Analytics, USA. The analyzer was coupled with computer controlled magnetic stirrer and electrochemical cell consisting of three electrodes: 3mm glassy carbon electrode, Ag/AgCl.KCl reference electrode and Pt counter electrode. The stock solution of mercury(II) chloride (HgCl) was prepared by dissolving required amounts of HgClinto 0.1mol dm-3 HCl solution. This solution was used to prepare the mercury film on glassy carbon electrode. A 1000 ppm (1000 g/mL) stock solution of Zn(II) was prepared by dissolving required amounts of ZnO in 0.1 mol dm-3 HNOsolution. Another stock solution of 0.01 mol dm-3ethylenediamine was prepared by distilled and deionized water, which was filtered with 0.45 m membrane filter before the solution was prepared. The ionic strength of the solution was maintained constant using 0.2 mol dm-3 NaNO (99.99%) solution. The pH was kept constant at 9.100.10 by the addition of borate buffer. The required standard solutions were prepared once a week by dilution of the stock solutions. All stock solutions were stored in the refrigerator at 4C. Preparation of working electrode: In the present investigation, the thin mercury film coated glassy carbon electrode was used as working electrode. The thin mercury film was prepared on properly polished and washed 3 mm glassy carbon electrode in 10 mL of 1.010-3 mol dm-3 mercury (II) solution. The solution was purged with nitrogen for 10 minutes to remove dissolved oxygen. Then the electrodes were connected to the analyzer. Any bubbles adhering to the electrodes were removed by tapping them off. The mercury film deposition was done at -400 mV for 6 min by applying differential pulse voltammetry program as described earlier6,11,12. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 5-9, September (2013) Res. J. Chem. Sci. International Science Congress Association 6 Complexation of Zn(II) with ethylenediamine (en): To measure the complexation of Zn(II) with ethylenediamine, at first the electrochemical cell was assembled with 5 mL of borate buffer having pH 9.100.10, 1 mL of 2 mol dm-3 NaNO and 4 mL of distilled and deionized water. Then, the solution was purged with pure nitrogen for 10 minutes. The background voltammogram was obtained using the following run conditions for differential pulse anodic stripping voltammetry (DPASV): Mode: Stripping; initial potential: 1250 mV; final potential: 500 mV; gain (1-20): 10; deposition time: 120s; quite time delay: 30s. The voltammograms of Zn (II) were obtained after successive addition of 20 ppb (20 g/L) of Zn (II) in the cell under the above experimental conditions. A linear calibration was obtained. To study the complexation capability of Zn (II) with ethylenedimaine, in aliquot of 10 mL solution of 300 ppb (4.59 M) Zn(II) was prepared in borate buffer having pH 9.100.10 and ionic strength, = 0.2 mol dm-3 (NaNO). The solution was purged with nitrogen gas for 10 minutes to remove dissolved oxygen. The voltammogram of free Zn(II) was recorded using the same run conditions for DPASV as described above. The process was repeated for 3 times to check the reproducibility. An aliquot of 10 L of 0.1 mol dm-3 ethylenediamine was added to the cell and the solution was stirred for 60 seconds. The DPASV run was performed again under the same experimental conditions. The peak potential shifted to more negative value and peak height reduced due to the addition of ethylenediamine. The process was continued after successive addition of ethylenediamine in it, until constant peak height was obtained. Theory for data treatment: In aqueous solution, the formation of a complex between metal ion, Mn+ and organic ligand, L give the metal complex ML (the overall charge of the complex is omitted for simplicity). The reaction occurs in several steps as follows: M ML ML .... MLn+ E°, nM(Hg)Where, i represents the number of ligand and the metal ion can be reversibly reduced on a mercury film electrode. The differential pulse anodic stripping voltametric measurement of peak current and peak potential induced by increasing ligand concentration allows the determination of overall conditional stability constant of complexes in solution. The stability constant of various species of zinc(II) with ethylenediamine can be calculated according to the model of DeFord and Hume13 on the basis of the change in peak potential with ligand concentration. According to their model, the overall conditional stability constant of individual complexes containing 1,2,….,i number of ligands are related to the formation function as follows: complexedfreeRTnFF11b (2) Where E is the change in peak portential between simple and complexed metal ions, Ifree and Icomplexed are the peak currents of simple and complexed metal ions, respectively, n = number of electrons involved in the redox reaction, F = Faraday constant, R = gas constant, [L] = free ligand concentration (not bound with proton) and i = 1, 2, …, N. The successive formation constants () are evaluated by polynomial fitting of the zero-order Leden Function F. The values of F can be calculated from the experimental change in peak potential and peak currents for each values of [L] in the voltammetric titration using equation 2. The free ligand concentrations, [L] are computed from the protonation constants at experimental pH as follows. (3)Where, C represents the total concentration of ligand and and are the protonation constants of ethylenediamine. The overall conditional stability constant of each species with respect to labile metal ion is determined using nonlinear fitting program on equation 2. The overall stability constant with respect to free metal ion is obtained by considering the inorganic side reaction coefficient of zine (zn) at pH 9.100.10. Results and Discussion The complexation of Zn(II) with ethylenediamine (EN) were studied by DPASV at constant ionic strength ( = 0.20) at pH 9.1 0.1. From the effect of pH, deposition time and potential range, the optimum parameters were selected for Zn(II) complexation. Under the optimum parameters, a calibration curve was constructed. The calibration curve i.e. the plot of diffusion current against zinc concentration was found to be linear shown in figure 1, indicating the reduction of Zn to Zn(Hg) is diffusion controlled. A typical change of voltammograms with ethylenediamine (EN) concentration is shown in figure 2. The peak of the unchelated Zn(II) appeared at – 1.08 V, under the present experimental conditions in absence of ethylenediamine. The diffusion current (I) of Zn(II) ion gradually decreased by the addition of ethylenediamine due to the formation of kinetically inert complexes shown in figure 2. The term, ‘inert complex’ indicates the metal-ligand complex in which kinetics of dissociation is very slow compared to the time of the DPASV measurements. The plot of peak potential (E) vs log (i/i-i) was found to be linear shown in the figure 3, indicating the reduction of Zn to Zn(Hg) on the mercury film electrode is a reversible process. The plot of peak potential (E) vs. log[EN]free for Zn(II) found to be a smooth curve shown in figure 4 indicating the formation of two or more complexes which are in equilibrium with each other. The zero order Leden function F was [L] = 1 + [H]K + [H(1) = exp Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 5-9, September (2013) Res. J. Chem. Sci. International Science Congress Association 7 calculated according to the DeFord and Hume method as described above. The plot of F vs. [EN]free shown in figure 5, gives the values of conditional stability constants of individual complexes. = 0.990.00.20.40.60.81.0060120180[Zn(II)], µg/L Peak Current, (µA) Figure-1 Calibration plot for the quantification of zinc(II) by DPASV using mercury thin film coated glassy carbon electrode in aqueous solution at pH 9.10 ± 0.10 and I = 0.20 mol dm-3 0.00.20.40.60.8-1300-1100-900-700E (mV) vs. Ag/AgCl Peak Current, µA [EN]=0.0000 mM [EN]=0.0999 mM [EN]=0.1996 mM [EN]=0.2991mM [EN]=0.4975 mM [EN]=0.6951mM [EN]=0.8920 mM [EN]=1.0880 mM [EN]=1.2830 mMFigure-2 Voltammograms of zinc after successive addition of varying concentration of ethylenediamine (EN); [Zn (II)] = 4.59 µM, [EN] = 0.00 to 1.28 mM, pH = 9.10 ± 0.1 and = 0.20 mol dm-3 (NaNO = 0.97-1.14-1.13-1.12-1.11-1.10-1.09-0.8-0.5-0.300.250.50.75log [i/(i-i)]Peak potential (V)Figure-3 A plot of peak potential vs. log i/(i-i) for zinc-ethylenediamine complexes when [Zn (II)] = 4.59 µM, [EN] = 0.00 to 1.28 mM, pH = 9.10 ± 0.1 and = 0.20 mol dm-3(NaNO -1.150-1.120-1.090-5.5-4.5-3.5log [EN] free Peak potential (mV) Figure-4 Plot of peak potential vs. log [EN]free ; [Zn (II)] = 4.59 µM, pH = 9.10 ± 0.1 and = 0.20 mol dm-3 (NaNO) The formation of three metal-ligand complexes, ML, ML and ML (where M represents zinc and L represents ethylenediamine) are found under the experimental conditions. In the present experiment, DPASV technique detects the labile species which consist of free hydrated form (i. e. aqua metal ion, 2+) and hydroxo complexes (MOH, M(OH) and M(OH)). So, it is necessary to consider the side reaction of Zn2+ with OHat experimental pH. The actual value of the overall stability constant is obtained by considering the inorganic side reaction coefficient of zinc (zn) at pH 9.10 and neglecting the hydrolyzed form. The value of zn can be computed by the following expression. zn = 1 +OH [OH] + OH [OH + OH [OH (4) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 5-9, September (2013) Res. J. Chem. Sci. International Science Congress Association 8 = 0.997501001502000.0E+001.0E-042.0E-04[EN]Free, mol dm-3 F Figure-5 The variation of F as a function of concentration of free ethylenediamine (EN); [Zn (II)] = 4.59 µM, pH = 9.10 ± 0.1 and = 0.20 mol dm-3 (NaNO) Where, OH, …OH are the overall hydrolysis constants of zinc. Using the hydrolysis constants of zinc, the value of the side reaction coefficient, zn was found to be 5.25 at pH 9.10. Considering the value of zn, the stability constants were calculated and listed in table 1. The large values of overall stability constants indicate that the complexes of zinc (II) with ethylenediamine in alkaline pH are highly stable. According to the hydrolysis constants of zinc, mone-, di- and tri- hydroxyl species are possible to be formed. But in the present experimental conditions, 25.09% ZnOH, 52.42% Zn(OH) and 3.46% Zn(OH) exist at pH 9.1. The remaining part exists as aqua zinc ion (Zn2+) in absence of ligand. Ethylenediamine (EN) can exist in three different forms in solution: the monoprotonated (HL), diprotonated (HL) and free form (L) depending on pH of the solution. Since the protonation constants of ethylenediamine14, = [HL][H-1[L]-1= 109.928and = [HL][HL]-1[H]-1= 106.848 only 12.87% free ethylenediamine is possible to exist at pH 9.10. Distribution of ethylenediamine as a function of pH in aqueous media is shown in figure 6. 25507510002468101214pH% of SpeciesHLHLFigure-6 Species distribution of ethylenediamine (EN) as a function of pH From the literature, it is known that zinc forms 1:1, 1:2 and 1:3 (metal: ligand) complexes with free form of ethylenediamine (L) whose stability constants have been reported14. In the present investigation, the logarithmic values of overall stability constant of zinc complexes with ethylenediamine are found to be 6.03, 10.16 and 13.98 for ZnL, ZnL and ZnL, respectively. These values are very similar to the values reported before as shown in the table 1. In this study, trace amount (300 ppb or 4.59 M) of zinc was investigated in which free and hydrolysed forms are possible to exist in absence of ligand and hydrolysed and metal-ligand complexes are possible to exist in presence of ligand at pH 9.10. The percentage of all chemical species are caluculated and shown in figure 7 for zinc-ethylenediamine complexes. The species distribution diagram shown in figure 7 is of great help in the interpretation of voltammetric results. Table-1 Stability constants of zinc complexes with ethylenediamine Ionic strength, mol dm - 3 Temperature, °°C logbb log bb log bbReferences 0.20 30 6.03 10.16 13.98 This work 0.10 25 5.70 10.60 12.60 14 0.00 25 5.66 10.60 13.90 14 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 5-9, September (2013) Res. J. Chem. Sci. International Science Congress Association 9 204060801000.0E+005.0E-051.0E-04[EN], mol dm-3% of Species ZnLZn2+Zn(OH)ZnLZnLZn(OH)Figure-7Species distribution of zinc as a function of concentration of ethylenediamine (EN) at pH 9.10 ± 0.1ConclusionZinc-ethylenediamine complexes were investigated by DPASV. It was found that trace amount (4.59 M) of zinc is able to form 1:1, 1:2 and 1:3 (metal: ligand) complexes with ethylenediamine (ligand) depending on pH as well as concentration of ligand. The formation constants of zinc complexes obtained in this study are in good agreement with those reported before14. In absence of ligand, about 52.4% zinc exists as Zn(OH) at pH 9.10, whereas, in presence of ethylenediamine it forms different complexes depending on ligand concentration. The large values of overall stability constant indicate that the complexes are highly stable. The percentage of all species of zinc was calculated. It was found that all species of zinc co-exist within ligand concentration up to 110-4 mol dm-3 under the present experimental condition. All experimental information indicates that DPASV may be a powerful tool in the study of metal organic interaction in aqueous system. Acknowledgement The author, N. Nahar gratefully acknowledges the Third World Academy of Sciences (TWAS) for financial support to purchase the major equipment through Research Grant (No. 99-107 RG/CHE/AS). References 1.Patel Ketan B, Patel Yogesh M. and Patel Raksha B., Metal complexes of 5-[benzyloxy methyl] quinoline-8-ol and 8-quinolinol mixed ligand: A new transition metal complexes with In-Vitro antifungal activity, Res. J. Recent Sci., 2 (ISC-2012), 55-60 (2013) 2.Eisler R., Zinc hazard to fish, wildlife and invertebrates: a synoptic review, Contaminant Hazard Reviews, 10 (1993) 3.Francis A.R and Masilamai D., Removal of zinc by non living biomass of Agaricus Bisporus, Res. J. 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