www.scielo.br/eq Volume 34, número 2, 2009 7 Direct and simultaneous spectrophotometric determination of Ni (II) and Co (II) using diethanoldithiocarbamate as complexing agent. P. A. Antunes1,a, G. Bannach1,b, G. O. Chierice1, E. T. G. Cavalheiro1*. 1Instituto de Química de São Carlos – USP CEP 13560-970 São Carlos, SP, Brazil. *cavalheiro@iqsc.usp.br a Present address: Universidade do Oeste Paulista. 19050-900 - Presidente Prudente, SP – Brasil b Present address: Faculdade de Ciências de Bauru – UNESP CEP 17033-360 Bauru, SP, Brazil. Abstract: A direct spectrophotometric method for simultaneous determination of Co(II) and Ni(II), with diethanoldithiocarbamate (DEDC) as complexing agent, is proposed using the maximum absorption at 360 and 638 nm (Co(II)/DEDC) and 390 nm (Ni/DEDC). Adjusting the best metal/ligand ratio, supporting eletrolite, pH, and time of analysis, linear analytical curves from 1.0 10-6-4.0 10-4 for Co(II) in the presence of Ni 1.0 10-6-1.0 10-4 mol L-1 were observed. No further treatment or calculation processes have been necessary. Recoveries in different mi- xing ratios were of 99%. Interference of Fe(III), Cu(II), Zn(II) and Cd(II), and anions as NO3 -, Cl-, ClO4 -, citrate and phosphate has been evaluated. The method was applied to natural waters spiked with the cations. Keywords: Simultaneous determination, Cobalt, Nickel, Dithiocarbamate, Spectrophotometry. Introduction Several methods have been proposed for the simultaneous determination of Co(II) and Ni(II). Eletroanalytical techniques, optical, chro- matographic and flow injection methods are wide- ly described with this purpose [1-8]. The spectrophotometric methods are the most usually used and mathematical handling of the data is frequently required [9-13]. The present work is based on the differen- ces in the color of solutions containing Co(II) or Ni(II) and dithiocarbamates (DTC). The best ad- vantage of this procedure is that the results can be obtained by direct measurements and any further mathematical treatment is necessary. The choice of bis-2-hydroxyethyldithiocarbamate (DEDC), was made on the basis of the higher solubility of its complexes in relation to other DTC’s, due to the presence of two -OH groups in its the struc- ture: HO CH2 CH2 N C S S- HO CH2 CH2 DEDC The DTC, which are the products of reac- tions between an amine and carbon dissulphide, has been applied in the medicine, as coadjuvants in AIDS, cancer, tuberculosis treatment and heavy metals removal from leaving organisms [14]; in the agriculture, as anti-fungicidal and anti-bac- terial agents [15]; in industry as rubber vulcani- zations agents, lubrificants, anti-oxidizing agents Ecl. Quím., São Paulo, 34(2): 7 - 13, 2009 Ecl. Quím., São Paulo, 34(2): 7 - 13, 20098 and catalysts [16]; in chemistry as synthesis in- termediates, determination of metallic species and thermoanalytical studies [17, 18]. Experimental Apparatus Spectrophotometric experiments were made with an HP-8451A diode array spectrophotometer and 10 mm quartz cells. The pH was measured with a Corning IA-250 pHmeter and a Metrohm EA 121 glass electrode. Atomic absorption was measured on a Hi- tachi Z-8100 atomic absorption spectrophotome- ter, using air-acetylene flame. Reagents and Solutions All the chemicals and solvents used were of analytical grade, PA, and used without any fur- ther purification. The pH was adjusted with McIlvaine buffer solutions, and the ionic strength controlled to 0.5 mol L-1 with KCl [19]. The metal ions stock solutions were pre- pared from NiCl2.6H2O and CoCl2.6H2O salts (Merck), and standardized by flame atomic ab- sorption. DEDC was synthesized by the reaction of ethanolamine and carbon dissulphide in presence of ammonium hydroxide into an ice bath, and re- crystallized from ethanol/water mixtures, as des- cribed elsewhere [20]. Procedures Maximum Wavelength Absorption and ε Determination for the Complexes Spectra were obtained from solutions con- taining the ligand and each metal both 1.0 10-4 mol L-1 and using the ligand, one metal or mixtures, in different proportions. The attribution of the peaks of the complexes was performed on the basis of the increase in its absorbance when the metallic concentration was increased using a metallic solu- tion with the same concentration as blank. The molar absorptivity coefficients were determined with each metal concentration ranging from 1.0 10-5-5 10-4 and the ligand 1.0 10-4-5.0 10-3 mol L-1. CL/CM Ratio optimization In these experiments the concentration of the metal was fixed in 1.0.10-4 mol L-1, and the ligand was varied in the range 1.0 10-5-8.0 10-3 mol L-1, with each cation separately in phosphate buffer (pH=7). The absorbances were measured at the maximum wavelengths determined as above using the phosphate buffer as reference. Mixtures of 3.0 10-5 mol L-1 of each metal and the ligand varying from 3.0 10-5 - 3.0 10-2 mol L-1 in phosphate buffer were used to determine the best CL/CM ratio. Ionic Strength Controller Salt Effect The controller salt influence was investiga- ted using KCl, NaCl, KNO3, NaNO3, NaClO4, and McIlvaine buffer system at fixed ionic strength of 0.50 mol L-1. The ratio CL/CM was fixed according to a previous study and the absorbance measured at the λmax of each complex. Using McIlvaine buffers the pH influence on the system was also investigated for 5.5, 6.5, 7.5 and 8.3. This pH range was used to prevent the ligand decomposition [21], and metallic hydroly- sis. Influence of Time and Temperature Fixing the parameters already studied, the complex stability was evaluated by measuring the absorbance as a function of the time (0-130 min), for each complex individually. The influence of the temperature on the absorbance was also inves- tigated from 15-41°C, using a thermostatic bath (± 1ºC). Ecl. Quím., São Paulo, 34(2): 7 - 13, 2009 9 Evaluation of the effect of interferents The influence of the presence of ca- tions generating colored (Fe(III) and Cu(II)) and colorless complexes (Zn(II) and Cd(II)) with DEDC was investigated in the 1.0 10-6-1.0 10-4 range and fixing the Ni and Co concentrations at 5.0 10-5 and the DEDC at 5.0 10-3 mol L-1. Evaluation of the Method in Synthetic Sam- ples The method was evaluated by measuring the metals intentionally added to mineral (drinking) water samples. In this study the concentration of one ca- tion was fixed while varying the concentration of the second one, at a fixed ligand concentration. In the first series of experiments Ni(II) was fixed at 1.0 10-4, 5.0 10-5 and 1.0 10-5, with Co(II) varying from 10-3-10-5 mol L-1. In a second series Co(II) was fixed at 1.0 10-4, 5.0 10-5 and 1.0 10-5, and Ni(II) varied in the 10-3-10-5 mol L-1 range. In both cases the ligand concentration was maintained 100 times higher than the fixed metal, using all the parameters defined in the previous experiments. Results and Discussion The Co(II)/DEDC solutions showed green color with maximum absorption at 360 and 638 nm, while Ni(II)/DEDC are yellowish-green with λmax at 390 nm in agreement with Yoshida et al [22]. The spectra are presented in Figure 1. The strong band at 330 nm was assigned to the ligand [23]. 15 Figure 1. The absorption spectra of complexes. a) Ni/DEDC λmáx - 390nm. b) Co/DEDC λmáx - 360 and 638 nm. Figure 1. The absorption spectra of complexes. a) Ni/DEDC λmáx - 390nm. b) Co/DEDC λmáx - 360 and 638 nm. The peaks were attributed to the complexes on the basis of their presence in the spectra obtained using either metallic solutions or ligand as blank. Ecl. Quím., São Paulo, 34(2): 7 - 13, 200910 The molar absorptivity coefficients, ε, were determined by plotting absorbance vs. the complex concentration (CM ≈ Ccpx; in ligand excess). The values obtained are presented in Table 1, at the maxi- mum absorption wavelengths. Table 1. Molar Absorptivity of the complexes Complex λmáx (nm) ε(cm-1mol-1L).103 Co2+/DEDC 360 45.4 638 1.71 Ni2+/DEDC 390 4.57 Since the ε638 < ε360, more concentrated Co(II) solutions can be determined at 638 nm, while less concentrated ones can be determined at 360 nm, extending the detection range for this cation. The limiting ligand/metal concentration ratio is different for solutions containing the individual cations or mixtures of them. For solutions containing only Co(II) or Ni(II), the ratio is about CL/CM = 10. For the mixtures of the two cations the limiting ratio CL/CNi+Co = 100. It must be addressed that the limiting value should not be attributed to precipitation. If precipi- tation had occurred the absorbance value should decrease while increasing the ligand concentration. Therefore the limiting value was most probably due to the presence of enough ligand to reach the maxi- mum coordinating capacity of the system at such conditions. When they are mixed the differences in the affinity of the cations for the ligand make necessary much more ligand to reach saturation. Examples of curves of absorbance vs. the CL/CM ratio are presented in the Figure 2. According to the results presented in Table 2 different ionic strength controlling salts and the pH ranging from 5.5 to 8.3, had no significant change in the measured absorbance. In addition the tempe- rature range of 5-40°C and a measuring time of 0-130 min (results not presented) showed no influence in the measurements. According to these results, McIlvaine Buffer pH=7.0, µ = 0.50 mol L-1/KCl and temperature of 25.0°C, were adopted to perform the determinations. 16 Figure 2. Determination of the limiting ligand concentration. 0 20 40 60 80 0.0 0.5 1.0 1.5 2.0 CM = 1.00·10-4 mol L-1 CL = 1.00·10-5 mol L-1 a A b s o rb a n c e CL / CM 0 100 200 300 400 0.0 0.1 0.2 0.3 0.4 0.5 0.6 c CNi2+ = CCo2+ = 3.50·10-5 mol L-1 CL = 1.00·10-5 - 8.00·10-3 mol L-1 A b s o rb a n c e CL / CNi + Co 0 20 40 60 80 0.0 0.1 0.2 0.3 0.4 0.5 0.6 CM = 1.00·10-4 mol L-1 CL = 1.00·10-5 mol L-1 b A b s o rb a n c e CL / CM Figure 2. Determination of the limiting ligand concentration. Ecl. Quím., São Paulo, 34(2): 7 - 13, 2009 11 Table 2. Controlling Ionic Strength Salts and pH influence Absorbance Co2+/DEDC Ni2+/DEDC 360nm 638nm 390nm KCl 1.558 0.06316 0.4760 Salt NaCl 1.558 0.06090 0.4977 (0.50 mol L -1) KNO3 1.634 0.06284 0.4953 NaNO3 1.601 0.06000 0.4936 NaClO4 1.600 0.06221 0.4630 phosphate/citrate 1.624 0.05994 0.5134 5.49 1.609 0.06413 0.5166 5.58 1.610 0.08569 0.5025 pH 6.58 1.653 0.07044 0.5155 7.45 1.770 0.06499 0.4853 8.32 1.655 0.06744 0.5318 The precision of the method was checked using synthetic samples and analytical curves. Exam- ples for Ni and Co determinations are presented in Figure 3. In the Tables 3 and 4 the results for the de- termination of cations, fixing one and varying the concentration of the other, in different concentration ranges and mixing ratios are presented. The mean recovery was 99.0% for both cations. 17 Figure 3. Example of analytical curves for Co2+ and Ni2+ determination and the absorbance of samples 1 an 2. 0 2 4 6 8 10 0.0 0.2 0.4 0.6 0.8 b CNi2+ = 1.00.10-4mol L-1 CCo2+ = 1.00.10-3 - 1.00.10-5 mol L-1 CDEDC = 1.00.10-2 mol L-1 λ = 638 nm < sample 2 < sample 1 A bs or ba nc e CCo2+ / mol L-1 0 2 4 6 8 10 0.8 1.0 1.2 1.4 1.6 1.8 c < sample 1 CCo2+ = 1.00.10-4 mol L-1 CNi2+ = 1.00.10-3 - 1.00.10-5 mol L-1 CDEDC = 1.00.10-2 mol L-1 λ = 390 nm A bs or ba nc e CNi2+ / 1·10-5 mol L-1 0 2 4 6 8 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 a < sample 2 CNi2+ = 1.00.10-4mol L-1 CCo2+ = 1.00.10-3 - 1.00.10-5 mol L-1 CDEDC = 1.00.10-2 mol L-1 λ = 360 nm < sample 1 A bs or ba nc e CCo2+ / 1·10-4 mol L-1 Figure 3. Example of analytical curves for Co2+ and Ni2+ determination and the absorbance of samples 1 an 2. Table 3. Results of Method Application in Synthetic Samples, with Different Ni2+ Concentrations. CNi 2+, mol L-1 CDEDC, mol L-1 CCo 2+, mol L-1 calc. λ, nm CCo 2+, mol L-1 found Corr. Coef. Recovery % a) 1.0 10-4 1.0 10-2 1.49 10-5 360 1.60 10-5 0.9998 107 638 1.37 10-5 0.9998 92.0 5.0 10-5 5.0 10-3 2.61 10-4 360 - - - 638 3.05 10-4 0.9995 116 1.0 10-5 1.0 10-3 7.46 10-6 360 6.91 10-6 0.9995 92.0 638 6.53 10-6 0.9994 87.6 a) mean of two determinations Ecl. Quím., São Paulo, 34(2): 7 - 13, 200912 The effect of the presence of Fe(III), Cu(II), Zn(II) and Cd(II), are summarized in Table 5. All these cations influenced in the Ni(II) determination except Fe3+ when the interfering cation concentra- tion was ten times smaller. Satisfactory results were obtained for the Co(II) determination in all the cases using λ=360 nm. Precipitation occurred in the presence of Fe(III) and Cu(II) at 10-4 mol L-1. It has been observed that alkali metals and anions as NO3 -, Cl-, ClO4 -, citrate and phosphate had no significant influence in the determination. Table 4. Results of Method Application in Synthetic Samples, with Different Co2+ Concentrations. CCo 2+, mol L-1 CDEDC, mol L-1 CNi 2+, mol L-1 calc. λ, nm CNi 2+, mol L-1 found Corr. Coef. Recovery % a) 1.0 10-5 1.0 10-3 5.00 10-5 390 5.08 10-5 0.9928 101 1.0 10-4 1.0 10-2 2.00 10-5 390 2.20 10-5 0.9985 110 1.0 10-4 1.0 10-2 9.00 10-5 390 7.93 10-5 0.9985 88.1 % mean recovery 99.70 a) mean of two determinations Table 5. Studies of Some Interfering Cations for Ni2+ and Co2+. Interferant Conc. Ni/DEDC Co/DEDC (complex colour) Mol L-1 Found / mol L-1 rec % Found / mol L-1 rec% Zn2+ 1.0 10-4 3.9 10-5 130 5.31 10-5 85.0 (colourless) 1.0 10-5 4.4 10-5 150 4.25 10-5 106 1.0 10-6 4.5 10-5 147 5.31 10-5 106 Cd2+ 1.0 10-4 4.5 10-5 150 4.56 10-5 91.0 (colourless) 1.0 10-5 4.4 10-5 147 5.61 10-5 112 1.0 10-6 4.1 10-5 137 4.56 10-5 91.0 Fe2+ 1.0 10-4 precipitation precipitation (brown) 1.0 10-5 6.0 10-5 200 5.07 10-5 101 1.0 10-6 3.0 10-5 100 5.07 10-5 101 Cu2+ 1.0 10-4 precipitation precipitation (blue) 1.0 10-5 5.5 10-5 183 5.08 10-5 102 1.0 10-6 4.2 10-5 140 5.08 10-5 102 Conclusions The proposed spectrophotometric method is simple, rapid and inexpensive. It provides gain in sensitivity without need of additional step as extraction or heating. The method does not invol- ve stringent reaction conditions and gives precise and accurate results. Its usefulness to Co(II) and Ni(II), in presence of diethanoldithiocarbamate complexing agent, suggests its use as an attractive alternative to many other previously reported me- thods for analysis related in literature. Acknowledgements Authors are indebted to Capes, CNPq and FAPESP foundations (Brazil) for financial sup- port. Ecl. Quím., São Paulo, 34(2): 7 - 13, 2009 13 Determinação espectrofotométrica simultânea direta de Ni(II) e Co(II) usando dietanolditiocar- bamato como agente complexante. Received November 03 2008 Accepted 14 2008 Resumo: Um método espectrofotométrico simultâneo para a determinação de Co(II) e Ni(II), na presença do ligante dietanolditiocarbamato (DEDC), é proposto usando-se os comprimen- tos de onda dos máximos de absorção em 360 e 638 nm (Co(II)/DEDC) e 390 nm (Ni/DEDC). Após otimizar a melhor razão metal/ligante, eletrólito suporte, pH e tempo de análise, curva analíticas lineares foram obtidas nos intervalos de concentração 1,0 10-6-4,0 10-4 para Co(II) na presença de Ni 1,0 10-6-1,0 10-4 mol L-1. Nenhum tratamento de amostra ou processo de cálculo foi necessário. Coeficientes de recuperação da ordem de 99% foram obtidos e a interferência de Fe(III), Cu(II), Zn(II) e Cd(II), além de alguns ânions foi avaliada. O método foi aplicado, com sucesso, em amostras de água mineral nas quais os cátions foram intencionalmente adi- cionados. Palavras chave: Determinação simultânea, cobalto, níquel, ditiocarbamato, Espectrofotometria. References [1] P. A. Antunes, E. T. G. Cavalheiro, S. T. Breviglieri and G. O. Chierice, Quím. Nova, 21 (1998) 289. [2] V. Kaur, A. K. Malik, Talanta, 73 (2007) 425. [3] T. Khayamian, A. A. Ensafi, B. Hemmateenejad, Talanta, 49 (1999) 587. [4] L. Ow, H. Sy, Anal. Chim. Acta, 280 (1993) 269. [5] V. T. Yilmaz , T. K. Yazicilar , H. 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