Journal of Materials

Journal of Materials / 2013 / Article

Research Article | Open Access

Volume 2013 |Article ID 897343 |

Petya Vassileva Racheva, Kiril Blazhev Gavazov, Vanya Dimitrova Lekova, Atanas Nikolov Dimitrov, "Complex Formation in a Liquid-Liquid Extraction System Containing Cobalt(II), 4-(2-Pyridylazo)resorcinol, and Nitron", Journal of Materials, vol. 2013, Article ID 897343, 7 pages, 2013.

Complex Formation in a Liquid-Liquid Extraction System Containing Cobalt(II), 4-(2-Pyridylazo)resorcinol, and Nitron

Academic Editor: Concepción López
Received12 Dec 2012
Revised23 Feb 2013
Accepted24 Feb 2013
Published21 Mar 2013


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.

1. Introduction

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 [1]. 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 [47].

4-(2-Pyridylazo)resorcinol (PAR) has been proved to be one of the most important reagents for cobalt separation, preconcentration and determination [626]. PAR forms with Co(II) intensively colored anionic chelates, or , which can readily react with bulky organic compounds [2036] 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 [37]. 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 [38].

2. Experimental

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.

Optimum conditionsAnalytical characteristics

Wavelength: 520 nmMolar absorptivity:  L mol−1 cm−1
pH: 5.3 (acetate buffer)Beer’s law range:
up to 1.7 μg mL−1
:  mol L−1Limit of detection: 0.06 μg mL−1
:  mol L−1Limit of quantification: 0.20 μg mL−1
Shaking time: 15–20 secSandell’s sensitivity: 0.99 ng cm−2

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 [40] (Figure 5), molar ratio method [43] (see Figure 4), and the method of Asmus [44] (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 [4547] and Nt [48] 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 [41], Harvey-Manning method [42], and mobile equilibrium method [40] (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%.

Equilibrium Equilibrium constantValue

(1) Log
(2) Log
(3) Log

aCalculated by the mobile equilibrium method [40].
bCalculated by the Holme-Langmihr method [41].
cCalculated by the Harvey-Manning method [42].
dCalculated by the formula , where is determined by the Holme-Langmihr method.
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.

Additional reagent(s)Organic solventMolar absorptivity,
L mol−1 cm−1
, nmRef.

Xylomethazoline hydrochloride Chloroform 535[24]
Diphenylguanidine Chloroform 520–530[27]
Triphenyltetrazolium chlorideChloroform 515[36]
Iodonitrotetrazolium chlorideChloroform 515[36]
ZephiramineChloroform 520[28]
Tetradecyl-dimethylbenzyl-ammonium chloride + EDTAChloroform 520[35]
Dicyclohexyl-18-crown-6 1,2-dichloroethane 515[26]
Tetraphenylarsonium chloride Chloroform 520[29]
Tetraphenylphosphonium chloride Chloroform 520[29]
NitronChloroform 520This work

The molar absorptivity of the binary cobalt-PAR complex in aqueous medium is  L mol−1 cm−1 [39].

4. Conclusions

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.


  1. 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
  2. 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
  3. D. G. Barceloux and D. Barceloux, “Cobalt,” Clinical Toxicology, vol. 37, no. 2, pp. 201–216, 1999. View at: Publisher Site | Google Scholar
  4. R. A. Meyers, Ed., Encyclopedia of Analytical Chemistry: Applications, Theory, and Instrumentation, Wiley, Chichester, UK, 2000.
  5. M. Jakubowski and M. Trzcinka-Ochocka, “Biological monitoring of exposure: trends and key developments,” Journal of Occupational Health, vol. 47, no. 1, pp. 22–48, 2005. View at: Publisher Site | Google Scholar
  6. 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
  7. R. E. Taljaard and J. F. V. Staden, “Simultaneous determination of cobalt(II) and Ni(II) in water and soil samples with sequential injection analysis,” Analytica Chimica Acta, vol. 366, no. 1–3, pp. 177–186, 1998. View at: Publisher Site | Google Scholar
  8. 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
  9. 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
  10. 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
  11. 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
  12. 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
  13. 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
  14. Z. T. Jiang, J. C. Yu, and H. Y. Liu, “Simultaneous determination of cobalt, copper and zinc by energy dispersive X-ray fluorescence spectrometry after preconcentration on PAR-loaded ion-exchange resin,” Analytical Sciences, vol. 21, no. 7, pp. 851–854, 2005. View at: Publisher Site | Google Scholar
  15. C. C. Nascentes and M. A. Z. Arruda, “Cloud point formation based on mixed micelles in the presence of electrolytes for cobalt extraction and preconcentration,” Talanta, vol. 61, no. 6, pp. 759–768, 2003. View at: Publisher Site | Google Scholar
  16. I. V. Vyshcherevich and I. E. Kalinichenko, “Photometric determination in drinking water of cobalt and nickel with 4-(2-pyridylazo)-resorcinol,” Journal of Water Chemistry and Technology, vol. 32, no. 1, pp. 33–38, 2010. View at: Publisher Site | Google Scholar
  17. M. Ince, G. Kaya, and M. Yaman, “Solid phase extraction and preconcentration of cobalt in mineral waters with PAR-loaded Amberlite XAD-7 and flame atomic absorption spectrometry,” Environmental Chemistry Letters, vol. 8, no. 3, pp. 283–288, 2010. View at: Publisher Site | Google Scholar
  18. 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
  19. 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
  20. C. E. Säbel, J. L. Shepherd, and S. Siemann, “A direct spectrophotometric method for the simultaneous determination of zinc and cobalt in metalloproteins using 4-(2-pyridylazo)resorcinol,” Analytical Biochemistry, vol. 391, no. 1, pp. 74–76, 2009. View at: Publisher Site | Google Scholar
  21. B. F. Liu, L. B. Liu, and J. K. Cheng, “Analysis of metal complexes in the presence of mixed ion pairing additives in capillary electrophoresis,” Journal of Chromatography A, vol. 848, no. 1-2, pp. 473–484, 1999. View at: Publisher Site | Google Scholar
  22. 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
  23. 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
  24. 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
  25. 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
  26. 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
  27. 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
  28. 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
  29. M. Široki, L. Marić, Z. Štefanac, and M. J. Herak, “Characterization of complexes involved in the spectrophotometric determination of cobalt with 4-(2-pyridylazo)resorcinol,” Analytica Chimica Acta, vol. 75, no. 1, pp. 101–109, 1975. View at: Publisher Site | Google Scholar
  30. 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
  31. 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
  32. 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
  33. 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
  34. 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
  35. 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
  36. 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
  37. 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
  38. 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
  39. 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
  40. 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
  41. 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
  42. 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
  43. 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
  44. E. Asmus, “Eine neue methode zur ermittlung der zusammensetzung schwacher komplexe,” Fresenius' Zeitschrift für Analytische Chemie, vol. 178, no. 2, pp. 104–116, 1960 (German). View at: Publisher Site | Google Scholar
  45. 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
  46. F. I. Lobanov, G. K. Nurtaeva, and E. E. Ergozhin, Extraction of Metal Complexes With Hydroxyazo Compounds of Pyridine, Alma-Ata: Nauka, 1983.
  47. L. Marić and M. Široki, “Extraction of 4-(2-pyridylazo) resorcinol and 4-(2-thiazolylazo) resorcinol with chloroform and tetraphenylarsonium and phosphonium chlorides,” Analytica Chimica Acta, vol. 318, no. 3, pp. 345–355, 1996. View at: Publisher Site | Google Scholar
  48. 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

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