Complex Formation in a Liquid-Liquid Extraction System Containing Cobalt(II), 4-(2-Pyridylazo)resorcinol, and Nitron
Complex formation and liquid-liquid extraction were studied in a system containing cobalt(II), 4-(2-pyridylazo)resorcinol (PAR), 1,4-diphenyl-3-(phenylamino)-1H-1,2,4-triazole (Nitron, Nt), water, and chloroform. The effect of some experimental parameters (pH, shaking time, concentration of PAR, and concentration of Nt) was systematically investigated, and the optimum conditions for cobalt extraction as an ion-association complex, (NtH+)[Co3+(PAR)2], were found. The following key equilibrium constants were calculated: constant of association , constant of distribution , and constant of extraction . Beer’s law was obeyed for Co concentrations up to 1.7 μg mL−1 with a molar absorptivity of L mol−1 cm−1 at nm. Some additional characteristics, such as limit of detection, limit of quantification, and Sandell’s sensitivity, were estimated as well.
Cobalt is a transition metal which plays an essential role in industry and all living organisms. Its main applications are in the production of special steels and alloys, permanent magnets, cutting tools, batteries, catalysts, pigments for enamels and glass, and dryers for oil, paints, and varnishes. In biological systems cobalt acts as an active nutrient and an active center of coenzymes called cobalamines. The most important representative of this class of compounds is vitamin B-12: a key substance, which is normally involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and neurologic function . Cobalt deficiency (and hence vitamin B-12 deficiency) can lead to a wide spectrum of hematologic, neuropsychiatric, and cardiovascular disorders. On the other hand, cobalt can be toxic when consumed in excessive quantities [2, 3]. That is why its content in various samples is monitored, despite the fact that the existing methods for cobalt determination are not enough sensitive or cost effective [4–7].
4-(2-Pyridylazo)resorcinol (PAR) has been proved to be one of the most important reagents for cobalt separation, preconcentration and determination [6–26]. PAR forms with Co(II) intensively colored anionic chelates, or , which can readily react with bulky organic compounds [20–36] to give ternary complexes with good extraction behavior and analytical potential. In the present paper, we investigated the complex formation in a liquid-liquid extraction system containing Co(II), PAR, and Nitron (Nt). Nt (Figure 1) is a low-cost and low-toxic analytical reagent that has been commercially available for more than a century; however, some novel aspects of its chemical nature have been recently disclosed . It should be said that extraction systems containing both PAR and Nt have been weakly studied. To the best of our knowledge, the only reported investigations in this field concern vanadium(V) extraction and spectrophotometric determination .
2.1. Reagents and Apparatus
(i) (ReagentPlus, ≥99%, Sigma-Aldrich), 1000mg stock aqueous solution. Working solutions () were prepared by dilution.(ii)PAR (96%, Sigma-Aldrich) dissolved in slightly alkalized distilled water, mol .(iii)Nitron (≥97%, Fluka), mol chloroform solution freshly prepared each day. (iv)Acetate buffer solution, prepared by mixing of 2 mol aqueous solutions of CH3COOH and NH4OH. The resulting pH was checked by HI 83140 pH meter (Italy).(v)Chloroform (additionally distilled).(vi)Ultrospec 3300 pro UV/visible spectrophotometer (Amersham Biosciences), equipped with 10 mm path-length cells.
2.2. Procedure for Establishing the Optimum Operating Conditions
Aliquots of Co(II) solution, PAR solution (up to 1.4 mL), and buffer solution (5 mL; pH ranging from 3.0 to 6.2) were introduced into 250 mL separatory funnels. The resulting solutions were diluted with distilled water to a total volume of 10 mL. Appropriate amounts of Nt solution and chloroform were added in a total volume of 10 mL. Then the funnels were shaken for a fixed time (up to 5.0 min). A portion of the organic extract was filtered through a filter paper (to prevent the opportunity of water droplets transfer) into a cell and the absorbance read against a blank. The blank extraction was performed at the same manner, but in the absence of Co.
2.3. Procedure for Determination of the Distribution Constant
The distribution constant was found from the ratio , where and are the absorbances (measured against blanks) obtained after a single and double extraction, respectively. The single extraction and the first stage of the double extraction were performed under the optimum extraction-spectrophotometric conditions (Table 1). The organic layers were transferred into 25 mL calibrated flasks and the flask for the single extraction was brought to volume with Nt solution. The second stage of the double extraction was performed by adding another 10 mL portion of the Nt solution to the aqueous phase, which remained after the first stage. After shaking, the organic layer was added to the one obtained after the first stage and the volume was brought to the mark with Nt solution. Before the spectrophotometric measurement, the calibrated flasks were shaken for homogenization.
3. Results and Discussion
3.1. Absorption Spectra
Spectra of the extracted ternary Co-PAR-Nt complex and the blank are shown in Figure 2. A maximum is recorded at 520 nm, where the blank absorbs insignificantly. It is shifted to 10 nm as compared to the maximum of the binary Co-PAR chelate existing in aqueous medium (in the pH interval from 3.5 to 10): 510 nm [9, 27, 29, 31, 39]. The observed bathochromic effect is small and gives us grounds to suggest the formation of a ternary compound of the ion-association type.
3.2. Effect of pH
Results showed that the optimal pH for the extraction of Co with PAR and Nt is 5.2–5.4 (Figure 3). A buffer solution with a concentration of 2 mol L−1 was applied to control pH. The use of 0.25–5 mL of the buffer solution per 10 mL (final aqueous solution) was found to give a constant absorbance. All further experiments were carried out with 5 mL buffer solution with pH = 5.2–5.3.
3.3. Effect of Reagents’ Concentrations
The effect of PAR and Nt concentrations on the absorbance is shown in Figure 4. For up to 1.7 μg of Co, the use of about 0.5 mL of PAR and 8.5 mL of Nt was found to be sufficient for a complete cobalt extraction.
3.4. Effect of Shaking Time
The extraction equilibrium is reached for a short shaking time (about 5 seconds). It was found that a shaking time longer than 1 min can bring about to a slight decrease (5-6%) of the absorbance values. To avoid this disadvantage and to guarantee complete transfer of the complex into organic phase, even under nonoptimum conditions, the authors extracted in their experiments for 15–20 seconds.
3.5. Composition of the Complex and Suggested Formula
The molar PAR-to-Co(II) and Nt-to-Co(II) ratios were determined by the mobile equilibrium method  (Figure 5), molar ratio method  (see Figure 4), and the method of Asmus  (Figure 6). The results showed that the ternary complex has a composition of 1 : 2 : 1 (Co : PAR : Nt). Having in mind the obtained molar ratios and several reports, which convincingly demonstrate that the labile Co(II)-PAR complex can be easily oxidized to an inert Co(III)-PAR complex, , by the atmospheric oxygen [13, 26, 29, 32, 33, 36, 45], we suggest the following formula of the extracted ternary species: . In this formula, PAR is in deprotonated form (), while Nt is in protonated form (). The mentioned formula and the known properties of PAR [45–47] and Nt  fit well to the observed pH curve of the ternary complex presented in Figure 3: at pH values lower than pHopt PAR is hardly possible to be in its form; at pH values higher than pHopt Nt is hardly possible to be in its form. The right part of the pH curve has a steeper slope, because the limitation 2 is more strictly.
3.6. Equilibrium Constants and Recovery
Several equilibrium processes should be taken into account for the system of , , water, and chloroform.(i)Formation of ion-association complex in the aqueous phase: (ii)Distribution of the complex between the aqueous and the organic phase: (iii)Extraction from water into chloroform:
The equilibrium constants describing these equations and the obtained values are shown in Table 2. The association constant was determined by several independent methods: Holme-Langmihr method , Harvey-Manning method , and mobile equilibrium method  (Figure 5, straight line 2). The distribution constant was calculated from the absorption values obtained after single and double extraction as described above. The extraction constant was calculated by the formula . The recovery factor was estimated by the dependence / and the following value was obtained . All experiments were performed at room temperature of ~22°C and the calculations were carried out at a probability of 95%.
3.7. Beer’s Law, Molar Absorptivity, and other Analytical Characteristics
The range of adherence to Beer’s law was studied at the optimum conditions (Table 1). The linearity is observed up to of Co with a correlation coefficient of 0.9995. The obtained straight line equation is . The molar absorptivity was calculated to be . This value could compete successfully with the ones obtained for similar PAR-containing complexes (Table 3). The limit of detection (LOD) and limit of quantification (LOQ) were estimated at 3 times and 10 times standard deviation of the intercept divided by the slope. Sandell’s sensitivity was calculated as well. The values of the above-mentioned characteristics are included in Table 1.
Cobalt(II) forms well chloroform-extractable ternary complex with 4-(2-pyridylazo)resorcinol and Nitron. The complex could be regarded as an ion associate between an intensively colored anion,, in which cobalt is in oxidation state, and a bulky hydrophobic cation (protonated Nitron, ). The following equilibrium constants and analytical characteristics were determined: constant of extraction, constant of association, constant of distribution, recovery factor, molar absorptivity, Sandell’s sensitivity, limit of detection, and limit of quantification. The obtained values show that the studied extraction system in the present work could compete successfully with many similar systems used for cobalt determination.
The authors would like to thank the Research Fund of the Plovdiv University for its long-time support.
R. C. Oh and D. L. Brown, “Vitamin B12 deficiency,” American Family Physician, vol. 67, no. 5, pp. 979–986, 2003.View at: Google Scholar
B. B. Tewari, “Complex formation of some divalent metal ions with oxygen donor ligands,” Revista Boliviana de Química, vol. 26, no. 1, pp. 30–36, 2009.View at: Google Scholar
R. A. Meyers, Ed., Encyclopedia of Analytical Chemistry: Applications, Theory, and Instrumentation, Wiley, Chichester, UK, 2000.
A. Tsuyoshi, H. Hoshino, and T. Yotsuyanagi, “Retention selectivity between 4-(2-pyridylazo)resorcinol and its cobalt chelate in the solid phase extraction systems and its application to the on-line preconcentration for reversed phase HPLC,” Chemistry Letters, vol. 30, no. 4, pp. 302–303, 2001.View at: Google Scholar
A. Hol, U. Divrikli, and L. Elci, “Determination of cobalt, nickel and iron at trace level in natural water samples by in-column chelation-reversed phase high-performance liquid chromatography,” Environmental Monitoring and Assessment, vol. 184, no. 6, pp. 3469–3479, 2012.View at: Publisher Site | Google Scholar
V. M. Ivanov, N. I. Ershova, V. N. Figurovskaya, and A. V. Ivanov, “Optical and chromaticity characteristics of cobalt and palladium 4-(2-pyridylazo)resorcinates,” Journal of Analytical Chemistry, vol. 56, no. 2, pp. 143–148, 2001.View at: Google Scholar
G. Ram, R. S. Chauhan, A. K. Goswami, and D. N. Purohit, “Review of spectrophotometric methods for determination of cobalt(II),” Reviews in Analytical Chemistry, vol. 22, no. 4, pp. 255–317, 2003.View at: Google Scholar
H. Ciftci, “Solid phase extraction method for the determination of cobalt in water samples on duolite XAD-761 resin using 4-(2-Pyridylazo) resorcinol by FAAS,” Current Analytical Chemistry, vol. 6, no. 2, pp. 154–160, 2010.View at: Google Scholar
S. Tokalioǧlu and S. Kartal, “Preconcentration of iron(III), lead(II), cobalt(II) and chromium(III) on amberlite XAD-1180 resin loaded with 4-(2-pyridylazo)-resorcinol (PAR) and their determination by FAAS,” Bulletin of the Korean Chemical Society, vol. 27, no. 9, pp. 1293–1296, 2006.View at: Google Scholar
V. Cucinotta, R. Caruso, A. Giuffrida, M. Messina, G. Maccarrone, and A. Torrisi, “Separation and quantitation of metal ions by 4-(2-pyridylazo)resorcinol complexation in capillary electrophoresis-electrospray ionisation mass spectrometry,” Journal of Chromatography A, vol. 1179, no. 1, pp. 17–23, 2008.View at: Publisher Site | Google Scholar
L. L. Kolomiets, L. A. Pilipenko, I. M. Zhmud', and I. P. Panfilova, “Application of derivative spectrophotometry to the selective determination of nickel, cobalt, copper, and iron(III) with 4-(2-pyridylazo)resorcinol in binary mixtures,” Zhurnal Analiticheskoi Khimii, vol. 54, no. 1, pp. 34–36, 1999.View at: Google Scholar
H. Cıftcı, A. Olcucu, A. Ozkaya, and T. Cıftcı, “Optimization of analytical parameters for determination of iron, nickel and cobalt in plants with RP-HPLC,” Asian Journal of Chemistry, vol. 21, no. 4, pp. 2643–2652, 2009.View at: Google Scholar
K. Sato and T. Goto, “Determination of nickel(II) and cobalt(II) in an aqueous solution using 4-(2-Pyridylazo)-resorcinol/Capriquat-loaded silica gel,” Bunseki Kagaku, vol. 47, no. 10, pp. 735–738, 1998.View at: Google Scholar
H. R. Pouretedal, P. Sononi, M. H. Keshavarz, and A. Semnani, “Simultaneous determination of cobalt and iron using first-derivative spectrophotometric and h-point standard addition methods in micellar media,” Chemistry, vol. 18, no. 3, pp. 22–35, 2009.View at: Google Scholar
S. N. Bhadani, M. Tewari, A. Agrawal, and C. Sekhar, “Extractive-photometric determination of cobalt(II) in steels using 4-(2-pyridylazo)resorcinol and xylometazoline hydrochloride,” Journal of the Indian Chemical Society, vol. 75, no. 3, pp. 176–177, 1998.View at: Google Scholar
P. Berton and R. G. Wuilloud, “An online ionic liquid-based microextraction system coupled to electrothermal atomic absorption spectrometry for cobalt determination in environmental samples and pharmaceutical formulations,” Analytical Methods, vol. 3, no. 3, pp. 664–672, 2011.View at: Publisher Site | Google Scholar
A. G. Gaikwad, H. Noguchi, and M. Yoshio, “Solvent extraction studies of metal-4-(2-pyridyl-azo)-resorcinol complexes with potassium-dicyclohexyl-18-crown-6 complex,” Analytical Letters, vol. 24, no. 9, pp. 1625–1641, 1991.View at: Google Scholar
S. G. Mamuliya, I. V. Pyatnitskii, L. L. Kolomiets, and K. I. Grigalashvili, “Solvent extraction of complexes of cobalt, nickel, copper, zinc and cadmium with 4-(2-pyridylazo)-resorcinol and diphenylguanidine,” Zhurnal Analiticheskoi Khimii, vol. 35, no. 7, pp. 1306–1309, 1980 (Russian).View at: Google Scholar
R. Yamashita, T. Yotsuyanagi, and K. Aomura, “The extraction-spectrophotometric determination of traces of iron and cobalt with 4-(2-pyridylazo)-resorcinol,” The Japan Society for Analytical Chemistry, vol. 20, pp. 1282–1288, 1971.View at: Google Scholar
J. Dolezal and L. Sommer, “Reverse-phase high performance liquid chromatography of metal chelates of 4-(2-pyridylazo)resorcinol and 4-(2-thiazolylazo)resorcinol. Simultaneous determination of low concentrations of Co, Ni and Fe,” Collection of Czechoslovak Chemical Communications, vol. 59, pp. 2209–2226, 1994.View at: Google Scholar
N. T. Sizonenko and L. V. Gudzenko, “Determination of additions of cobalt in single crystals of cesium iodine, activated by thallium,” Zavodskaya Laboratoriya, vol. 51, no. 2, pp. 109–111, 1985 (Russian).View at: Google Scholar
T. Okutani, A. Sakuragawa, and M. Murakami, “Determination of iron, cobalt ad nickel by reverse phase high performance liquid chromatography following ion pair extraction of metal -4-(2-pyridylazo)resorcinol complexes,” Analytical Sciences, vol. 7, no. 1, pp. 109–112, 1991.View at: Google Scholar
J. B. Noffsinger and N. D. Danielson, “Retention characteristics of Co+3, Fe+3, and Cu+2 4-(2-Pyridylazo)resorcinol (PAR) complexes on C-18 and amino silica packings,” Journal of Liquid Chromatography, vol. 9, no. 10, pp. 2165–2183, 1986.View at: Google Scholar
H. Okochi, “Spectrophotometric determination of microamounts of cobalt in iron and steel by solvent extraction of cobalt-4-(2-pyridylazo) resorcinol complex with quaternary ammonium chloride,” Bunseki Kagaku, vol. 21, no. 1, pp. 51–56, 1972 (Japanese).View at: Google Scholar
T. Yotsuyanagi, R. Yamashita, and K. Aomura, “Spectrophotometric determination of traces of metals by solvent extraction with 4-(2-pyridylazo)-resorcin-quaternary ammonium salt-polyaminocarboxylic acid system,” The Japan Society For Analytical Chemistry, vol. 19, no. 7, pp. 981–982, 1970 (Japanese).View at: Google Scholar
V. V. Divarova, K. B. Gavazov, V. D. Lekova, and A. N. Dimitrov, “Spectrophotometric investigations on liquid-liquid extraction systems containing cobalt, 4-(2-pyridylazo)-resorcinol and tetrazolium salts,” Chemija. In press.View at: Google Scholar
C. Farber, M. Leibold, C. Bruhn, M. Maurer, and U. Siemeling, “Nitron: a stable N-heterocyclic carbene that has been commercially available for more than a century,” Chemical Communications, vol. 48, no. 2, pp. 227–229, 2012.View at: Google Scholar
R. M. Pogranichnaya, B. E. Reznik, V. V. Nerubashchenko, A. G. Zezyanova, and A. V. Tsevina, “Solvent extraction of mixed-ligand complexes of vanadium with 4-(2-pyridylazo) resorcinol and nitron,” Zhurnal Analiticheskoi Khimii, vol. 30, p. 180, 1975 (Russian).View at: Google Scholar
A. I. Busev and V. M. Ivanov, “1-(2-Pyridylazo)-resorcinol as a reagent for the photometric determination of cobalt,” Zhurnal Analiticheskoi Khimii, vol. 18, no. 2, pp. 208–215, 1963 (Russian).View at: Google Scholar
Z. Zhiming, M. Dongsen, and Y. Cunxiao, “Mobile equilibrium method for determining composition and stability constant of coordination compounds of the form MmRn,” Journal of Rare Earths, vol. 15, no. 3, pp. 218–219, 1997.View at: Google Scholar
A. Holme and F. J. Langmyhr, “A modified and a new straight-line method for determining the composition of weak complexes of the form AmBn,” Analytica Chimica Acta, vol. 36, pp. 383–391, 1966.View at: Google Scholar
A. E. Harvey and D. L. Manning, “Spectrophotometric methods of establishing empirical formulas of colored complexes in solution,” Journal of the American Chemical Society, vol. 72, no. 10, pp. 4488–4493, 1950.View at: Google Scholar
J. H. Yoe and A. L. Jones, “Colorimetric determination of iron with disodium-1,2-dihydroxybenzene-3,5-disulfonate,” Industrial and Engineering Chemistry, vol. 16, no. 2, pp. 111–115, 1944.View at: Google Scholar
A. Corsini, Q. Fernando, and H. Freiser, “The effect of metal ion chelation on the acid dissociation of the ligand 4-(2-pyridylazo)-resorcinol,” Inorganic Chemistry, vol. 2, no. 1, pp. 224–226, 1963.View at: Google Scholar
F. I. Lobanov, G. K. Nurtaeva, and E. E. Ergozhin, Extraction of Metal Complexes With Hydroxyazo Compounds of Pyridine, Alma-Ata: Nauka, 1983.
A. S. Babenko, V. N. Tolmachev, and A. N. Dzizin, “Investigation of sulforic acid salts of nitron,” Ukrainskii Khimicheskii Zhurnal, vol. 29, no. 7, pp. 702–708, 1963 (Russian).View at: Google Scholar