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Spectrophotometric study of complexation of cobalt (II) with HEDP in aqueous solutions
Journal of Molecular Liquids 286 (2019) 110909
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Spectrophotometric study of complexation of cobalt (II) with HEDP in aqueous solutions N.V. Scheglova a, T.V. Popova b,⁎, A.V. Druzhinina a, T.V. Smotrina a a b
Mari State University, 424001, Lenin square, 1, Yoshkar-Ola, Russia State University of Humanities and Technologies, 142611, Zelyonaya St., 22, Orekhovo-Zuevo, Russia
a r t i c l e
i n f o
Article history: Received 12 November 2018 Received in revised form 28 April 2019 Accepted 2 May 2019 Available online 03 May 2019 Keywords: Complexation Cobalt HEDP Spectrophotometry Speciation
a b s t r a c t The complexation of cobalt(II) with HEDP (1-hydroxyethylidene-1,1-diphosphonic acid) in aqueous solutions was studied by spectrophotometry over a wide range of pH values, including strongly acidic and strongly alkaline solutions. The formation of 2:1, 1:1, 1:2 and 1:3 Co(II)–HEDP complexes was proved by decomposing the electronic absorption spectra recorded in strongly alkaline solutions into individual bands using Gaussian functions and plotting changes in absorbance against the metal:ligand mole ratio at increasing values of the ligand concentrations. Species composition dynamics in the 1:4 Co(II)–HEDP system in alkaline solutions was determined by the change in the metal:ligand mole ratio with time. A decrease in the amount of bis- and the predominant formation of tris-phosphonate species with preservation of the amount of the 2:1 and 1:1 complexes was observed after three months. The modeling of complexation in the Co(II)–HEDP system and the calculation of stability constants (β) were carried out on the basis of experimental dependences of the absorbance of the solutions on pH. High thermodynamic stability was found for the binuclear chelates [Co2HHEDP], [Co2HEDP]−, [Co2(OH) − 2− , log β are equal 18.90, 7.11, 23.85 and 31.45, respectively. Speciation diagrams 2H2HEDP] , [Co2(OH)2HHEDP] of complexes in the 1:1 and 1:4 Co(II)–HEDP systems were calculated for freshly prepared solutions. © 2019 Elsevier B.V. All rights reserved.
1. Introduction HEDP is a polydentate phosphonate ligand and widely used as a chelating agent in industry and technology. On the one hand HEDP have two geminal phosphonate groups and therefore is capable of complexing in a highly acidic environment. On the other hand it has the hydroxy group of the oxyethyl moiety and the oxygen atom is capable of coordinating metal ions in acidic and neutral environments without deprotonation, and in alkaline environment with deprotonation. Thus, the structure of the HEDP causes complexation ability in a wide range of pH values and the unique properties of metal–HEDP complexes [1]. The stability of such complexes was established for many metals. Effects of stabilization and substoichiometric interaction were explained [2–9] and the mechanism of crystallization and the structure of coordination polyhedra were studied [10–14]. Information about the anticorrosive properties of HEDP appeared in the patent literature in the 1960s and after mastering the industrial production of HEDP systematic studies of its inhibitory action due to the formation of surface adsorption complexes were carried out [15,16]. The ability of HEDP to bind metal cations into stable water-soluble complexes allow its using as scale inhibitors in detergents to remove ⁎ Corresponding author. E-mail address: [email protected] (T.V. Popova).
https://doi.org/10.1016/j.molliq.2019.110909 0167-7322/© 2019 Elsevier B.V. All rights reserved.
iron oxide and carbonate magnesium‑calcium deposits in the heat supply systems of the fuel and energy industry [17,18]. The chemical removal of scale and corrosion products using HEDP chelates of 3d metals is ensured by the formation of crystal defects due to transfer processes at the solid phase boundary. The catalytic activity of metal–HEDP complexes was also established, and the mechanism of the decomposition of hydrogen peroxide in the presence of water-soluble chelates was studied [14]. The biological properties of metal–HEDP complexes are determined by the presence of a trace element and phosphorus necessary for the normal development of plants, which has a positive effect on seed germination, growth and increase in yield of agricultural plants. Physicochemical and agrochemical studies showed that the effect of HEDP on the behavior of biometals in plants was consistent with the constants of the stability of their complexes in solutions. A multiple increase in the solubility of metal–HEDP chelates in water is achieved by converting them into non-crystallizing salts by reacting aqueous suspensions of poorly soluble complexes with primary and secondary organic amines containing hydroxyl or hydroxyethylidene groups. The ability of HEDP to bind cobalt(II) cations into stable watersoluble complexes forms the basis for the preparation of micronutrients and electrolytes of electroplating and chemical cobalting [19]. Currently, methods for the synthesis of Co(II)–HEDP complexes in the solid state have been developed and their crystal structures have been
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characterized by X-ray analysis. However, information on the speciation of cobalt(II) phosphonates in multicomponent aqueous solutions is sparse. The purpose of this work is a detailed spectrophotometric study of complexation in the 1:1 and 1:4 Co(II)–HEDP systems in aqueous solutions over a wide range of pH values including strongly acidic and strongly alkaline solutions and dynamics of the coordination sphere composition in alkaline solutions.
used to determine the stability constants β of the complex ions: Y β ¼ K eq = i K ai
ð5Þ
where Kai are the stepwise dissociation constants of НEDP. 3. Results and discussion
2. Materials and methods
3.1. Electronic absorption spectra
2.1. Chemicals and equipment
In the electronic absorption spectra of the 1:1 Co2+–HEDP system, the bathochromic shift of the maxima of light absorption bands from the characteristic wavelength of [Co(H2O)6]2+ (λmax = 510 nm) was observed when the acidity of the solution decreased (Fig. 1). Hereinafter, the designation l for the optical path length is used in the captions. In addition, an increase in the intensity of the pink color of the solutions was observed, and in strongly alkaline solutions a broadening of the light absorption band was recorded with the color of the solution turning from pink to violet. The observed optical effects indicated the presence of exchange processes in the coordination sphere of hexaquacobalt (II) ions. Spectrophotometry has already been used successfully to study such processes in the Co(II) complexes with some other O-donor ligands [21,22]. Phosphonate ligands, in particular HEDP, are able to stabilize even unstable oxidation states of 3d elements due to the absence of a redox conflict with the central atom [5,23]. The oxidation state of cobalt in the Co(II)–HEDP complexes is quite stable in the air. Therefore, the experiment did not require the creation of an inert atmosphere. This has also been indicated in experiments where solid complexes of cobalt (II) with HEDP were obtained by slow evaporation of aqueous solutions [24]. In the 1:4 Co2+–HEDP system a fourfold excess of the ligand ensured maximum saturation of the coordination sphere. In the electronic absorption spectrum of strongly acidic solutions (pH = 2.5), a single maximum was observed at a wavelength of 525 nm. A decrease in the acidity of the solution (pH = 6.5) led to the bathochromic shift of the maximum to 535 nm with a simultaneous hyperchromic effect. In strongly alkaline solutions, a splitting of the absorption band and the formation of a triplet electronic absorption spectrum was observed, and the hyperchromic change in the absorbance of the solutions became even more significant (Fig. 2). To determine the absorption maxima wavelengths in the triplet spectra of the 1:4 Co2+–HEDP system in strongly alkaline solutions, the spectrum was decomposed into individual bands using Gaussian functions. Spectrum deconvolution was performed by solving a system
We used standard laboratory grade reagents. Initial aqueous solutions of CoCl2 and H5HEDP with concentrations 0.1 mol/L were prepared by dissolving precise weights of chemical reagents in distilled water. The spectrophotometric studies of complex formation were carried out for the 1:1 and 1:4 Co(II)–HEDP systems. The solutions were prepared by mixing the initial solutions in 1:1 and 1:4 ratios, followed by dilution with distilled water. The required acidity of the solutions was created with solutions of hydrochloric acid and sodium hydroxide. Electronic absorption spectra were recorded on an SF-2000 spectrophotometer (OKB SPEKTR, St. Petersburg, Russia). The acidity of the solutions was monitored on an Anion 4100 pH meter equipped with an ESC-10601/7 combined electrode (Infraspak-Analit, Novosibirsk, Russia). The absolute error of measurement of absorbance and pH was ±0.005 and ±0.01, respectively. The constancy of the ionic strength was maintained with 0.1 M KCl solution. All measurements were performed at a temperature of 20 ± 2 °C. 2.2. Stability constants calculations The stability constants of Co(II)–HEDP complexes were calculated by the Rossotti-Rossotti method [20] on the basis of experimental dependences of the absorbance of solutions on pH. The modeling of cobalt (II)–HEDP complex formation in solutions was carried out on the basis of the reaction equation: 2a–bðxþyÞ aCo2þ þ bH5–x HEDPx– ↔ Coa H5–x–y HEDP b þ byHþ
ð1Þ
The equilibrium concentrations of substances involved in the complexation process were expressed in terms of the corresponding absorbance A and pH values of the solutions: pA −ðp−bÞAmin −bAx φC Co2þ H5−x HEDPx− ¼ max Amax −Amin h
2a−bðxþyÞ
Coa ðH5 ‐x‐yHEDPÞb
i
¼
Ax −Amin 2þ C Co Amax −Amin
ð2Þ
ð3Þ
where Amax, Amin and Ах are the maximum, the minimum and an intermediate values of the absorbance of solutions in the pH range of complexation; p is the multiplicity of excess HEDP in the solution (pC(Co2 + ) = C(H5–xHEDPx–)); φ is the average mole fraction of the ligand ionization form that dominates in the pH range of complexation, C(Co2+) is the molar concentration of cobalt(II) cations in the initial solution (mol/ L). The equilibrium constants of the complexation reactions were calculated using the following relationship: K eq ¼
ðAx −Amin ÞðAmax −Amin Þaþb−1 ðaH Þby aþb−1 b ðAmax −Ax Þa ðpAmax −ðp−bÞAmin −bAx Þ C Co2þ φb
ð4Þ
where aH is the hydrogen ion activity that was calculated by using the measured pH values of the solutions. The equilibrium constants were
Fig. 1. Electronic absorption spectra of solutions of CoCl2 (1) and complexes in the 1:1 Co2 –HEDP system (2,3). C(Co2+) = 0.01 mol/L; pH = 2.5 (2), 8.0 (3); l = 30 mm.
+
N.V. Scheglova et al. / Journal of Molecular Liquids 286 (2019) 110909
Fig. 2. Electronic absorption spectra of solutions of CoCl2 (1) and complexes in the 1:4 Co2+– HEDP system (2–4). C(Co2+) = 0.01 mol/L; pH = 2.5 (2), 6.5 (3) and 11.3 (4); l = 20 mm.
of equations including individual absorption bands in the form
Aðλi Þ ¼ Amax
" # ðλi −λmax Þ2 exp w2i
ð6Þ
where A(λi) is the absorbance of the solution at wavelength λi of an individual absorption band, Аmax is the absorbance of the individual band at the absorption maximum (λmax), wi is the half-width of an individual spectral band. The characteristic maxima of the absorption bands at the wavelengths of 535, 580 and 625 nm were established by solving the system of equations. The recorded optical effects were an unambiguous confirmation of the presence of processes of competing complexation in the 1:4 Co2+–HEDP system.
3
Fig. 3. Absorbance (A) vs. pH plot for the 1:1 Co2+–HEDP (l = 30 mm) and 1:4 Co2+–HEDP (l = 20 mm) systems. C(Cо2+) = 0.01 mol/L, λ = 580 nm.
3.3. Molar ratio influence The method of saturation of the complex forming ion with ligand was used to establish the Co(II):HEDP mole ratio in the coordination sphere of chelates. The concentration of cobalt(II) chloride in the solution remained constant and the amount of the ligand was varied while maintaining a certain acidity of the solution. The dependences of absorbance on 1:x molar ratio of Co(II)–HEDP systems at different pH values of the solutions is shown in Fig. 4. On the basis of experimental data it was established that mononuclear 1:1 and 1:3 and binuclear 2:1 Co(II):HEDP complexes were formed in the acidic solution (pH = 3.0). In the neutral and weakly alkaline solution (pH = 8.0) HEDP was coordinated by Cо2+ cations with the formation of binuclear cobalt(II) chelate only. In the solutions with high alkalinity (pH = 10.5) 2:1, 1:1 and 1:2 Co(II):HEDP complexes were formed.
3.2. pH influence
3.4. Stability of cobalt(II)–HEDP complexes
The acidity of solution plays a crucial role in the formation of complex compounds involving protonated forms of a ligand. Therefore, for a detailed study of this effect on the HEDP chelation of cobalt (II) and the determination of the optimum acidity of solutions for complexation, the absorbance of the 1:1 Co2+–HEDP and 1:4 Co2+–HEDP systems at variable pH values were measured (Fig. 3). It was established that the coordination of НEDP with cobalt(II) cations in the 1:1 Co2+–HEDP system was realized even in a strongly acidic solution (pH ≥ 0.5). The corresponding two pH ranges of complexation were found to be ΔрН1 = 0.5–2.0 and ΔрН2 = 3.2–6.7. The optimal regions of the acidity of the solutions for Co(II)–HEDP complexes stability were found to be ΔрН3 = 2.0–3.2 and ΔрН4 = 6.7–10.5. The subsequent decrease in the acidity (pH N 10.5) was accompanied by a color transition from pink to intensively violet. A solid precipitate of hydrated cobalt(II) oxide was obtained after keeping the solutions for 24 h. The chelation reactions of cobalt(II) cations in the 1:4 Co2+:HEDP system also began in a strongly acidic solution (pH ≥ 1.0). The change in the optical characteristics of the solutions was observed in three pH ranges of complexation: ΔрН1 = 1.0–2.8, ΔрН2 = 4.5–7.5 and ΔрН3 = 8.0–10.3. The optimal acidity of the solutions for the existence of cobalt(II)–HEDP complexes were found to be ΔрН4 = 2.8–4.5, ΔрН5 = 7.5–8.0 and ΔрН6 = 10.3–11.4. For ΔрН6 the solution of the 1:4 Co2+: HEDP system had intense blue color. The violation of the homogeneity of the system with the formation of poorly soluble hydrated cobalt(II) oxide was registered in the solutions with pH N 11.4 within 24 h.
The thermodynamic stability of Co(II)–HEDP complexes formed in aqueous solutions was calculated using the previously described
Fig. 4. Absorbance (A) vs. 1:x molar ratio of Co(II)–HEDP systems plot at different pH values. λ = 525 nm, l = 50 mm (рН = 3.0); λ = 535 nm, l = 50 mm (рН = 8.0); λ = 535 nm, l = 20 mm (рН = 10.5). C(Cо2+) = 0.016 mol/L.
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Table 1 The stability constants of cobalt(II)–HEDP complexes. Mole ratio Co(II): HEDP
Complex composition
log β
1:1
[СоН4HEDP]+ [СоН3HEDP]0 [СоН2HEDP]− [Со(OH)Н3HEDP]− [Со(OH)Н2HEDP]2− [Со(НHEDP)2]6− [Со(Н4HEDP)3]− [Со(Н3HEDP)3]4− [Со2Н3HEDP]2+ [Со2Н2HEDP]+ [Со2НHEDP]0 [Со2HEDP]− [Со2(ОН)2Н2HEDP]− [Со2(ОН)2НHEDP]2−
1.67 ± 0.01 3.32 ± 0.02 8.52 ± 0.04 5.60 ± 0.03 10.19 ± 0.06 7.87 ± 0.04 13.90 ± 0.06 15.56 ± 0.09 6.23 ± 0.03 8.93 ± 0.04 5.55 ± 0.03 10.61 ± 0.06 18.90 ± 0.1 27.11 ± 0.1
1:2 1:3 2:1
mathematical method. Calculation formulas for estimating the stability of the chelates with different coordination sphere compositions were obtained by modeling the corresponding complexes in the pH ranges of their formation. The HEDP ionization forms dominating at the specific pH values of the solutions were used in the simulation. Their mole fractions were determined by plotting the distribution of the neutral and anionic forms of the ligand in solutions of different acidity. The known values of the HEDP dissociation constants [25,26] were used to construct the distribution diagram and to calculate the stability constants of the complexes. The logarithms of the stability constants of Co(II)– HEDP complexes are listed in Table 1. The change in thermodynamic stability of Co(II)–HEDP chelates with different compositions in aqueous solutions is consistent with the known facts about increasing the stability of complexes with an increase in amount of anions of HEDP in coordination sphere and with a decrease in its degree of protonation. The formation of mixed ligand hydroxy-HEDP chelates of cobalt(II) in alkaline solutions caused the triplet electronic absorption spectrum since coordination of hydroxy groups during complexation contributed to the decrease in the splitting parameter of the d-atomic orbitals of the central atom and, as a result, led to an increase in maximum absorption wavelength. In accordance with the literature data [11] the coordination of the HEDP molecule consisting of three interpenetrating tetrahedra by one central atom of a 3d element is due to the oxygen atoms of different phosphonate groups. Consequently the structure of mononuclear cobalt(II)–HEDP complexes implies the formation of a six-membered chelate cycle comprising a cobalt(II) cation and two oxygen atoms of the deprotonated hydroxo groups of the phosphonate moieties:
Fig. 5. Speciation curves for complexes formed in the 1:1 Cо2+:HEDP system. C(Со2+) = 0.04 mol/L.
A similar HEDP coordination scheme is implemented in cobalt(II) bis- and tris-1-hydroxyethylidene diphosphonates:
The formation of a binuclear chelate is most likely due to the formation of four-membered chelate cycles through the oxygen atoms of the phosphonate groups with both cations of cobalt (II):
N.V. Scheglova et al. / Journal of Molecular Liquids 286 (2019) 110909
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Fig. 6. Speciation curves for complexes formed in the 1:4 Cо2+:HEDP system. C(Со2+) = 0.04 mol/L. Fig. 8. Changes in absorbance of three absorption bands (535, 580 and 625 nm) in the 1:4 Co(II)–HEDP system with time. C(Cо2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
Speciation diagrams for the 1:1 Co2+:HEDP and 1:4 Co2+:HEDP systems are shown in Figs. 5 and 6. The diagrams were obtained by mathematical calculations of the equilibrium composition of solutions containing Co2+ and HEDP in a wide range of pH values. When calculating the distribution diagrams it was considered that bis and trisphosphonate species could not be formed in the 1:1 Co2+:HEDP system and there could be no binuclear species in the 1:4 Co2+:HEDP system.
3.5. Dynamics of species composition in alkaline solutions The changes in the electronic absorption spectra with time were recorded to study the kinetic stability of cobalt(II)–HEDP complexes in aqueous solutions. It was established that the greatest changes in the
Fig. 7. Changes in the electronic absorption spectra of the 1:4 Co(II):HEDP system with time. C(Cо2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
spectral characteristics were observed in alkaline solutions with an excess of the ligand. In the 1:4 Co(II):HEDP system the color of the solution with pH = 11.3 turned from an intensely blue to a violet after keeping the solutions for the period of three months due to a change in the species distribution. The change in the electronic absorption spectra of the solution with time is shown in Fig. 7. The storage of the solution led to a decrease in absorbance of all three characteristic maxima. The electronic absorption spectra were decomposed into individual bands approximated by Gaussian functions (Eq. (6)). It was found that after keeping the solutions for four months the absorption bands with maxima at 580 and 625 nm in the long-wavelength region of the spectrum were absent and the spectrum became singlet with λmax = 535 nm (Fig. 8). The singlet character of the electronic absorption spectrum and the absorbance at the maximum in alkaline solutions of the 1:4 Co(II): HEDP system were maintained when storing the solutions for more than four months. The observed changes in the spectrum led to the conclusion that the composition of the equilibrium mixture of Co(II)–HEDP species in alkaline multicomponent solutions changed over time. The dynamics of the species composition was registered by the change in Co(II):HEDP mole ratio with time. The changes in the curves of the saturation method are shown Fig. 9.
Fig. 9. The change in Со2+:HEDP mole ratio in the 1:4 Co(II):HEDP system with time. C (Со2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
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Both 2:1 and 1:1 Co(II):HEDP complexes were found to form in the 1:4 Co(II):HEDP system after storing the solution for three months. At the same time a decrease in the amount of cobalt(II) bisphosphonates and the predominant formation of cobalt(II) trisphosphonates were observed. 4. Conclusions The formation of Со2+–HEDP complexes of different composition in a wide range of pH values was studied spectrophotometrically. The composition of cobalt(II)–HEDP species in polycomponent solutions was established by decomposing the electronic absorption spectra into individual bands and by using the saturation series method. The stability of cobalt(II)–HEDP complexes increased with an increase in the amount of anions of the ligand in the coordination sphere and with a decrease in the degree of its protonation. The change in the species distribution with the predominant formation of cobalt(II) trisphosphonates with time was observed in alkaline solutions in the presence of an excess of HEDP. References [1] M.I. Kabachnik, T.Ya. Medved, N.M. Dyatlova, M.V. Rudomino, Organophosphorus Complexones, Russ. Chem. Rev. 43 (1974) 733https://doi.org/10.1070/ RC1974v043n09ABEH001851. [2] H. Wada, Q. Fernando, Interaction of methanehydroxyphosphonic acid and ethane1-hydroxy-1,1-diphosphonic acid with alkali and alkaline earth metal ions, Anal. Chem. 44 (1972) 1640https://doi.org/10.1021/ac60317a003. [3] E.N. Rizkalla, М.Т.М. Zaki, M.I. Ismail, Metal chelates of phosphonate-containing ligands. V. Stability of some 1-hydroxyethane-1,1-diphosphonic acid metal chelates, Talanta. 27 (1980) 715. https://doi.org/10.1016/0039-9140(80)80164-0. [4] D.M. Puri, Organophosphonic acids as complexones. Part VI. Reactions of hydroxyethylidenediphosphonic acid with bivalent metal ions, J. Indian Chem. Soc. 61 (1984) 899. [5] T.V. Popova, T.V. Smotrina, O.N. Denisova, N.V. Aksenova, Chromium(II) сomplexes with hydroxyethylidenediphosphonic acid, Russ. J. Coord. Chem. 27 (2001) 38https://doi.org/10.1023/A:1009536824427. [6] T.M. Domrachevа, T.V. Popovа, Oxyetilidendiphosphonates and nitriletrimethylenephosphonates of Cr3+, Russ. J. Coord. Chem., 25 (1999) 198. (in Russian). [7] S. Lacour, V. Deluchat, J.-C. Bollinger, B. Serpaud, Complexation of trivalent cations (Al(III), Cr(III), Fe(III)) with two phosphonic acids in the pН range of fresh waters, Talanta. 46 (1998) 999. https://doi.org/10.1016/S0039-9140(97)00369-X. [8] K. Popov, H. Rоnkkоmäki, L.H.J. Lajunen, Critical evaluation of stability constants of phosphonic acids (IUPAC technical report), Pure Appl. Chem. 73 (2001) 1641https://doi.org/10.1351/pac200173101641. [9] V.I. Kornev, T.N. Kropacheva, U.V. Sorokina, Coordination compounds of oxovanadium(IV) with organophosphonic complexones in aqueous solutions, Russ. J. Inorg. Chem. 60 (2015) 403https://doi.org/10.1134/S0036023615030110.
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Spectrophotometric study of complexation of cobalt (II) with HEDP in aqueous solutions N.V. Scheglova a, T.V. Popova b,⁎, A.V. Druzhinina a, T.V. Smotrina a a b
Mari State University, 424001, Lenin square, 1, Yoshkar-Ola, Russia State University of Humanities and Technologies, 142611, Zelyonaya St., 22, Orekhovo-Zuevo, Russia
a r t i c l e
i n f o
Article history: Received 12 November 2018 Received in revised form 28 April 2019 Accepted 2 May 2019 Available online 03 May 2019 Keywords: Complexation Cobalt HEDP Spectrophotometry Speciation
a b s t r a c t The complexation of cobalt(II) with HEDP (1-hydroxyethylidene-1,1-diphosphonic acid) in aqueous solutions was studied by spectrophotometry over a wide range of pH values, including strongly acidic and strongly alkaline solutions. The formation of 2:1, 1:1, 1:2 and 1:3 Co(II)–HEDP complexes was proved by decomposing the electronic absorption spectra recorded in strongly alkaline solutions into individual bands using Gaussian functions and plotting changes in absorbance against the metal:ligand mole ratio at increasing values of the ligand concentrations. Species composition dynamics in the 1:4 Co(II)–HEDP system in alkaline solutions was determined by the change in the metal:ligand mole ratio with time. A decrease in the amount of bis- and the predominant formation of tris-phosphonate species with preservation of the amount of the 2:1 and 1:1 complexes was observed after three months. The modeling of complexation in the Co(II)–HEDP system and the calculation of stability constants (β) were carried out on the basis of experimental dependences of the absorbance of the solutions on pH. High thermodynamic stability was found for the binuclear chelates [Co2HHEDP], [Co2HEDP]−, [Co2(OH) − 2− , log β are equal 18.90, 7.11, 23.85 and 31.45, respectively. Speciation diagrams 2H2HEDP] , [Co2(OH)2HHEDP] of complexes in the 1:1 and 1:4 Co(II)–HEDP systems were calculated for freshly prepared solutions. © 2019 Elsevier B.V. All rights reserved.
1. Introduction HEDP is a polydentate phosphonate ligand and widely used as a chelating agent in industry and technology. On the one hand HEDP have two geminal phosphonate groups and therefore is capable of complexing in a highly acidic environment. On the other hand it has the hydroxy group of the oxyethyl moiety and the oxygen atom is capable of coordinating metal ions in acidic and neutral environments without deprotonation, and in alkaline environment with deprotonation. Thus, the structure of the HEDP causes complexation ability in a wide range of pH values and the unique properties of metal–HEDP complexes [1]. The stability of such complexes was established for many metals. Effects of stabilization and substoichiometric interaction were explained [2–9] and the mechanism of crystallization and the structure of coordination polyhedra were studied [10–14]. Information about the anticorrosive properties of HEDP appeared in the patent literature in the 1960s and after mastering the industrial production of HEDP systematic studies of its inhibitory action due to the formation of surface adsorption complexes were carried out [15,16]. The ability of HEDP to bind metal cations into stable water-soluble complexes allow its using as scale inhibitors in detergents to remove ⁎ Corresponding author. E-mail address: [email protected] (T.V. Popova).
https://doi.org/10.1016/j.molliq.2019.110909 0167-7322/© 2019 Elsevier B.V. All rights reserved.
iron oxide and carbonate magnesium‑calcium deposits in the heat supply systems of the fuel and energy industry [17,18]. The chemical removal of scale and corrosion products using HEDP chelates of 3d metals is ensured by the formation of crystal defects due to transfer processes at the solid phase boundary. The catalytic activity of metal–HEDP complexes was also established, and the mechanism of the decomposition of hydrogen peroxide in the presence of water-soluble chelates was studied [14]. The biological properties of metal–HEDP complexes are determined by the presence of a trace element and phosphorus necessary for the normal development of plants, which has a positive effect on seed germination, growth and increase in yield of agricultural plants. Physicochemical and agrochemical studies showed that the effect of HEDP on the behavior of biometals in plants was consistent with the constants of the stability of their complexes in solutions. A multiple increase in the solubility of metal–HEDP chelates in water is achieved by converting them into non-crystallizing salts by reacting aqueous suspensions of poorly soluble complexes with primary and secondary organic amines containing hydroxyl or hydroxyethylidene groups. The ability of HEDP to bind cobalt(II) cations into stable watersoluble complexes forms the basis for the preparation of micronutrients and electrolytes of electroplating and chemical cobalting [19]. Currently, methods for the synthesis of Co(II)–HEDP complexes in the solid state have been developed and their crystal structures have been
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characterized by X-ray analysis. However, information on the speciation of cobalt(II) phosphonates in multicomponent aqueous solutions is sparse. The purpose of this work is a detailed spectrophotometric study of complexation in the 1:1 and 1:4 Co(II)–HEDP systems in aqueous solutions over a wide range of pH values including strongly acidic and strongly alkaline solutions and dynamics of the coordination sphere composition in alkaline solutions.
used to determine the stability constants β of the complex ions: Y β ¼ K eq = i K ai
ð5Þ
where Kai are the stepwise dissociation constants of НEDP. 3. Results and discussion
2. Materials and methods
3.1. Electronic absorption spectra
2.1. Chemicals and equipment
In the electronic absorption spectra of the 1:1 Co2+–HEDP system, the bathochromic shift of the maxima of light absorption bands from the characteristic wavelength of [Co(H2O)6]2+ (λmax = 510 nm) was observed when the acidity of the solution decreased (Fig. 1). Hereinafter, the designation l for the optical path length is used in the captions. In addition, an increase in the intensity of the pink color of the solutions was observed, and in strongly alkaline solutions a broadening of the light absorption band was recorded with the color of the solution turning from pink to violet. The observed optical effects indicated the presence of exchange processes in the coordination sphere of hexaquacobalt (II) ions. Spectrophotometry has already been used successfully to study such processes in the Co(II) complexes with some other O-donor ligands [21,22]. Phosphonate ligands, in particular HEDP, are able to stabilize even unstable oxidation states of 3d elements due to the absence of a redox conflict with the central atom [5,23]. The oxidation state of cobalt in the Co(II)–HEDP complexes is quite stable in the air. Therefore, the experiment did not require the creation of an inert atmosphere. This has also been indicated in experiments where solid complexes of cobalt (II) with HEDP were obtained by slow evaporation of aqueous solutions [24]. In the 1:4 Co2+–HEDP system a fourfold excess of the ligand ensured maximum saturation of the coordination sphere. In the electronic absorption spectrum of strongly acidic solutions (pH = 2.5), a single maximum was observed at a wavelength of 525 nm. A decrease in the acidity of the solution (pH = 6.5) led to the bathochromic shift of the maximum to 535 nm with a simultaneous hyperchromic effect. In strongly alkaline solutions, a splitting of the absorption band and the formation of a triplet electronic absorption spectrum was observed, and the hyperchromic change in the absorbance of the solutions became even more significant (Fig. 2). To determine the absorption maxima wavelengths in the triplet spectra of the 1:4 Co2+–HEDP system in strongly alkaline solutions, the spectrum was decomposed into individual bands using Gaussian functions. Spectrum deconvolution was performed by solving a system
We used standard laboratory grade reagents. Initial aqueous solutions of CoCl2 and H5HEDP with concentrations 0.1 mol/L were prepared by dissolving precise weights of chemical reagents in distilled water. The spectrophotometric studies of complex formation were carried out for the 1:1 and 1:4 Co(II)–HEDP systems. The solutions were prepared by mixing the initial solutions in 1:1 and 1:4 ratios, followed by dilution with distilled water. The required acidity of the solutions was created with solutions of hydrochloric acid and sodium hydroxide. Electronic absorption spectra were recorded on an SF-2000 spectrophotometer (OKB SPEKTR, St. Petersburg, Russia). The acidity of the solutions was monitored on an Anion 4100 pH meter equipped with an ESC-10601/7 combined electrode (Infraspak-Analit, Novosibirsk, Russia). The absolute error of measurement of absorbance and pH was ±0.005 and ±0.01, respectively. The constancy of the ionic strength was maintained with 0.1 M KCl solution. All measurements were performed at a temperature of 20 ± 2 °C. 2.2. Stability constants calculations The stability constants of Co(II)–HEDP complexes were calculated by the Rossotti-Rossotti method [20] on the basis of experimental dependences of the absorbance of solutions on pH. The modeling of cobalt (II)–HEDP complex formation in solutions was carried out on the basis of the reaction equation: 2a–bðxþyÞ aCo2þ þ bH5–x HEDPx– ↔ Coa H5–x–y HEDP b þ byHþ
ð1Þ
The equilibrium concentrations of substances involved in the complexation process were expressed in terms of the corresponding absorbance A and pH values of the solutions: pA −ðp−bÞAmin −bAx φC Co2þ H5−x HEDPx− ¼ max Amax −Amin h
2a−bðxþyÞ
Coa ðH5 ‐x‐yHEDPÞb
i
¼
Ax −Amin 2þ C Co Amax −Amin
ð2Þ
ð3Þ
where Amax, Amin and Ах are the maximum, the minimum and an intermediate values of the absorbance of solutions in the pH range of complexation; p is the multiplicity of excess HEDP in the solution (pC(Co2 + ) = C(H5–xHEDPx–)); φ is the average mole fraction of the ligand ionization form that dominates in the pH range of complexation, C(Co2+) is the molar concentration of cobalt(II) cations in the initial solution (mol/ L). The equilibrium constants of the complexation reactions were calculated using the following relationship: K eq ¼
ðAx −Amin ÞðAmax −Amin Þaþb−1 ðaH Þby aþb−1 b ðAmax −Ax Þa ðpAmax −ðp−bÞAmin −bAx Þ C Co2þ φb
ð4Þ
where aH is the hydrogen ion activity that was calculated by using the measured pH values of the solutions. The equilibrium constants were
Fig. 1. Electronic absorption spectra of solutions of CoCl2 (1) and complexes in the 1:1 Co2 –HEDP system (2,3). C(Co2+) = 0.01 mol/L; pH = 2.5 (2), 8.0 (3); l = 30 mm.
+
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Fig. 2. Electronic absorption spectra of solutions of CoCl2 (1) and complexes in the 1:4 Co2+– HEDP system (2–4). C(Co2+) = 0.01 mol/L; pH = 2.5 (2), 6.5 (3) and 11.3 (4); l = 20 mm.
of equations including individual absorption bands in the form
Aðλi Þ ¼ Amax
" # ðλi −λmax Þ2 exp w2i
ð6Þ
where A(λi) is the absorbance of the solution at wavelength λi of an individual absorption band, Аmax is the absorbance of the individual band at the absorption maximum (λmax), wi is the half-width of an individual spectral band. The characteristic maxima of the absorption bands at the wavelengths of 535, 580 and 625 nm were established by solving the system of equations. The recorded optical effects were an unambiguous confirmation of the presence of processes of competing complexation in the 1:4 Co2+–HEDP system.
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Fig. 3. Absorbance (A) vs. pH plot for the 1:1 Co2+–HEDP (l = 30 mm) and 1:4 Co2+–HEDP (l = 20 mm) systems. C(Cо2+) = 0.01 mol/L, λ = 580 nm.
3.3. Molar ratio influence The method of saturation of the complex forming ion with ligand was used to establish the Co(II):HEDP mole ratio in the coordination sphere of chelates. The concentration of cobalt(II) chloride in the solution remained constant and the amount of the ligand was varied while maintaining a certain acidity of the solution. The dependences of absorbance on 1:x molar ratio of Co(II)–HEDP systems at different pH values of the solutions is shown in Fig. 4. On the basis of experimental data it was established that mononuclear 1:1 and 1:3 and binuclear 2:1 Co(II):HEDP complexes were formed in the acidic solution (pH = 3.0). In the neutral and weakly alkaline solution (pH = 8.0) HEDP was coordinated by Cо2+ cations with the formation of binuclear cobalt(II) chelate only. In the solutions with high alkalinity (pH = 10.5) 2:1, 1:1 and 1:2 Co(II):HEDP complexes were formed.
3.2. pH influence
3.4. Stability of cobalt(II)–HEDP complexes
The acidity of solution plays a crucial role in the formation of complex compounds involving protonated forms of a ligand. Therefore, for a detailed study of this effect on the HEDP chelation of cobalt (II) and the determination of the optimum acidity of solutions for complexation, the absorbance of the 1:1 Co2+–HEDP and 1:4 Co2+–HEDP systems at variable pH values were measured (Fig. 3). It was established that the coordination of НEDP with cobalt(II) cations in the 1:1 Co2+–HEDP system was realized even in a strongly acidic solution (pH ≥ 0.5). The corresponding two pH ranges of complexation were found to be ΔрН1 = 0.5–2.0 and ΔрН2 = 3.2–6.7. The optimal regions of the acidity of the solutions for Co(II)–HEDP complexes stability were found to be ΔрН3 = 2.0–3.2 and ΔрН4 = 6.7–10.5. The subsequent decrease in the acidity (pH N 10.5) was accompanied by a color transition from pink to intensively violet. A solid precipitate of hydrated cobalt(II) oxide was obtained after keeping the solutions for 24 h. The chelation reactions of cobalt(II) cations in the 1:4 Co2+:HEDP system also began in a strongly acidic solution (pH ≥ 1.0). The change in the optical characteristics of the solutions was observed in three pH ranges of complexation: ΔрН1 = 1.0–2.8, ΔрН2 = 4.5–7.5 and ΔрН3 = 8.0–10.3. The optimal acidity of the solutions for the existence of cobalt(II)–HEDP complexes were found to be ΔрН4 = 2.8–4.5, ΔрН5 = 7.5–8.0 and ΔрН6 = 10.3–11.4. For ΔрН6 the solution of the 1:4 Co2+: HEDP system had intense blue color. The violation of the homogeneity of the system with the formation of poorly soluble hydrated cobalt(II) oxide was registered in the solutions with pH N 11.4 within 24 h.
The thermodynamic stability of Co(II)–HEDP complexes formed in aqueous solutions was calculated using the previously described
Fig. 4. Absorbance (A) vs. 1:x molar ratio of Co(II)–HEDP systems plot at different pH values. λ = 525 nm, l = 50 mm (рН = 3.0); λ = 535 nm, l = 50 mm (рН = 8.0); λ = 535 nm, l = 20 mm (рН = 10.5). C(Cо2+) = 0.016 mol/L.
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Table 1 The stability constants of cobalt(II)–HEDP complexes. Mole ratio Co(II): HEDP
Complex composition
log β
1:1
[СоН4HEDP]+ [СоН3HEDP]0 [СоН2HEDP]− [Со(OH)Н3HEDP]− [Со(OH)Н2HEDP]2− [Со(НHEDP)2]6− [Со(Н4HEDP)3]− [Со(Н3HEDP)3]4− [Со2Н3HEDP]2+ [Со2Н2HEDP]+ [Со2НHEDP]0 [Со2HEDP]− [Со2(ОН)2Н2HEDP]− [Со2(ОН)2НHEDP]2−
1.67 ± 0.01 3.32 ± 0.02 8.52 ± 0.04 5.60 ± 0.03 10.19 ± 0.06 7.87 ± 0.04 13.90 ± 0.06 15.56 ± 0.09 6.23 ± 0.03 8.93 ± 0.04 5.55 ± 0.03 10.61 ± 0.06 18.90 ± 0.1 27.11 ± 0.1
1:2 1:3 2:1
mathematical method. Calculation formulas for estimating the stability of the chelates with different coordination sphere compositions were obtained by modeling the corresponding complexes in the pH ranges of their formation. The HEDP ionization forms dominating at the specific pH values of the solutions were used in the simulation. Their mole fractions were determined by plotting the distribution of the neutral and anionic forms of the ligand in solutions of different acidity. The known values of the HEDP dissociation constants [25,26] were used to construct the distribution diagram and to calculate the stability constants of the complexes. The logarithms of the stability constants of Co(II)– HEDP complexes are listed in Table 1. The change in thermodynamic stability of Co(II)–HEDP chelates with different compositions in aqueous solutions is consistent with the known facts about increasing the stability of complexes with an increase in amount of anions of HEDP in coordination sphere and with a decrease in its degree of protonation. The formation of mixed ligand hydroxy-HEDP chelates of cobalt(II) in alkaline solutions caused the triplet electronic absorption spectrum since coordination of hydroxy groups during complexation contributed to the decrease in the splitting parameter of the d-atomic orbitals of the central atom and, as a result, led to an increase in maximum absorption wavelength. In accordance with the literature data [11] the coordination of the HEDP molecule consisting of three interpenetrating tetrahedra by one central atom of a 3d element is due to the oxygen atoms of different phosphonate groups. Consequently the structure of mononuclear cobalt(II)–HEDP complexes implies the formation of a six-membered chelate cycle comprising a cobalt(II) cation and two oxygen atoms of the deprotonated hydroxo groups of the phosphonate moieties:
Fig. 5. Speciation curves for complexes formed in the 1:1 Cо2+:HEDP system. C(Со2+) = 0.04 mol/L.
A similar HEDP coordination scheme is implemented in cobalt(II) bis- and tris-1-hydroxyethylidene diphosphonates:
The formation of a binuclear chelate is most likely due to the formation of four-membered chelate cycles through the oxygen atoms of the phosphonate groups with both cations of cobalt (II):
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Fig. 6. Speciation curves for complexes formed in the 1:4 Cо2+:HEDP system. C(Со2+) = 0.04 mol/L. Fig. 8. Changes in absorbance of three absorption bands (535, 580 and 625 nm) in the 1:4 Co(II)–HEDP system with time. C(Cо2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
Speciation diagrams for the 1:1 Co2+:HEDP and 1:4 Co2+:HEDP systems are shown in Figs. 5 and 6. The diagrams were obtained by mathematical calculations of the equilibrium composition of solutions containing Co2+ and HEDP in a wide range of pH values. When calculating the distribution diagrams it was considered that bis and trisphosphonate species could not be formed in the 1:1 Co2+:HEDP system and there could be no binuclear species in the 1:4 Co2+:HEDP system.
3.5. Dynamics of species composition in alkaline solutions The changes in the electronic absorption spectra with time were recorded to study the kinetic stability of cobalt(II)–HEDP complexes in aqueous solutions. It was established that the greatest changes in the
Fig. 7. Changes in the electronic absorption spectra of the 1:4 Co(II):HEDP system with time. C(Cо2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
spectral characteristics were observed in alkaline solutions with an excess of the ligand. In the 1:4 Co(II):HEDP system the color of the solution with pH = 11.3 turned from an intensely blue to a violet after keeping the solutions for the period of three months due to a change in the species distribution. The change in the electronic absorption spectra of the solution with time is shown in Fig. 7. The storage of the solution led to a decrease in absorbance of all three characteristic maxima. The electronic absorption spectra were decomposed into individual bands approximated by Gaussian functions (Eq. (6)). It was found that after keeping the solutions for four months the absorption bands with maxima at 580 and 625 nm in the long-wavelength region of the spectrum were absent and the spectrum became singlet with λmax = 535 nm (Fig. 8). The singlet character of the electronic absorption spectrum and the absorbance at the maximum in alkaline solutions of the 1:4 Co(II): HEDP system were maintained when storing the solutions for more than four months. The observed changes in the spectrum led to the conclusion that the composition of the equilibrium mixture of Co(II)–HEDP species in alkaline multicomponent solutions changed over time. The dynamics of the species composition was registered by the change in Co(II):HEDP mole ratio with time. The changes in the curves of the saturation method are shown Fig. 9.
Fig. 9. The change in Со2+:HEDP mole ratio in the 1:4 Co(II):HEDP system with time. C (Со2+) = 0.02 mol/L; pH = 11.3; l = 10 mm.
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Both 2:1 and 1:1 Co(II):HEDP complexes were found to form in the 1:4 Co(II):HEDP system after storing the solution for three months. At the same time a decrease in the amount of cobalt(II) bisphosphonates and the predominant formation of cobalt(II) trisphosphonates were observed. 4. Conclusions The formation of Со2+–HEDP complexes of different composition in a wide range of pH values was studied spectrophotometrically. The composition of cobalt(II)–HEDP species in polycomponent solutions was established by decomposing the electronic absorption spectra into individual bands and by using the saturation series method. The stability of cobalt(II)–HEDP complexes increased with an increase in the amount of anions of the ligand in the coordination sphere and with a decrease in the degree of its protonation. The change in the species distribution with the predominant formation of cobalt(II) trisphosphonates with time was observed in alkaline solutions in the presence of an excess of HEDP. References [1] M.I. Kabachnik, T.Ya. Medved, N.M. Dyatlova, M.V. Rudomino, Organophosphorus Complexones, Russ. Chem. Rev. 43 (1974) 733https://doi.org/10.1070/ RC1974v043n09ABEH001851. [2] H. Wada, Q. Fernando, Interaction of methanehydroxyphosphonic acid and ethane1-hydroxy-1,1-diphosphonic acid with alkali and alkaline earth metal ions, Anal. Chem. 44 (1972) 1640https://doi.org/10.1021/ac60317a003. [3] E.N. Rizkalla, М.Т.М. Zaki, M.I. Ismail, Metal chelates of phosphonate-containing ligands. V. Stability of some 1-hydroxyethane-1,1-diphosphonic acid metal chelates, Talanta. 27 (1980) 715. https://doi.org/10.1016/0039-9140(80)80164-0. [4] D.M. Puri, Organophosphonic acids as complexones. Part VI. Reactions of hydroxyethylidenediphosphonic acid with bivalent metal ions, J. Indian Chem. Soc. 61 (1984) 899. [5] T.V. Popova, T.V. Smotrina, O.N. Denisova, N.V. Aksenova, Chromium(II) сomplexes with hydroxyethylidenediphosphonic acid, Russ. J. Coord. Chem. 27 (2001) 38https://doi.org/10.1023/A:1009536824427. [6] T.M. Domrachevа, T.V. Popovа, Oxyetilidendiphosphonates and nitriletrimethylenephosphonates of Cr3+, Russ. J. Coord. Chem., 25 (1999) 198. (in Russian). [7] S. Lacour, V. Deluchat, J.-C. Bollinger, B. Serpaud, Complexation of trivalent cations (Al(III), Cr(III), Fe(III)) with two phosphonic acids in the pН range of fresh waters, Talanta. 46 (1998) 999. https://doi.org/10.1016/S0039-9140(97)00369-X. [8] K. Popov, H. Rоnkkоmäki, L.H.J. Lajunen, Critical evaluation of stability constants of phosphonic acids (IUPAC technical report), Pure Appl. Chem. 73 (2001) 1641https://doi.org/10.1351/pac200173101641. [9] V.I. Kornev, T.N. Kropacheva, U.V. Sorokina, Coordination compounds of oxovanadium(IV) with organophosphonic complexones in aqueous solutions, Russ. J. Inorg. Chem. 60 (2015) 403https://doi.org/10.1134/S0036023615030110.
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