Heterogeneous Chemical Equilibrium: Evaluation Of Equilibrium Constants Of Ion Exchange Processes Between Hydrogen Ions And Trivalent Chelated Metal Counter Ions In Some Coordination Biopolymers Of Metal- Alginate Complexes

Hassan RM

Published on: 2019-11-05

Abstract

Complexometric and titrimetric techniques have been applied for studying the heterogeneous chemical equilibrium for cation-exchange processes between some chelated such as Al3+, Cr3+, Fe3+, La3+ and Ce3+ counter ions in its respective coordination biopolymer metal-alginate complexes of granule nature and H+ ions of HClO4 acid electrolyte at a constant ionic strength of 0.1 mol dm-3. The factors that affected the ion exchange processes such as ionic radius, nature of metal ion, coordination geometry of chelation in the complex, the strength of chelation between the chelated metal ion, the functional groups within the macromolecular chains of alginate and the temperature have been examined. The thermodynamic parameters of ion exchange processes in such heterogeneous equilibria have been evaluated and discussed in terms of the coordination geometry and complexes stability. The values of equilibrium constants were found to increase in the order Al < Fe < Cr < Ce < La-alginate complexes which are indicative of decreasing the stability in the same order.

Keywords

Heterogeneous equilibria; Ion exchange; Coordination biopolymers; Equilibrium constants; Thermodynamic parameters

Introduction

Alginate is water-soluble anionic polyelectrolyte characterized by biocompatibility, biodegradability and non-toxicity natural polymer. It consists of D-mannuronic (M block) and L-guluronic (G block) acid units linked through- positions of linear block copolymer structure as shown below [1-4].The configuration structure is Illustrated in the following diagram.

 

Alginate sol has a high affinity for chelation with polyvalent metal ions forming its corresponding coordination biopolymer metal – alginate gel complexes in either colloidal hydrogel or solid granule gel forms [5-7]. This sol-gel transformation can take place by the replacement of the Na+ counter ions of alginate macromolecule by either silver (I) or polyvalent metal ions of its electrolyte solutions. The interdiffused metal ion chelates two or more carboxylate and one or more pairs of hydroxyl functional groups of alginate macromolecule depending on the valence of the interdiffused metal ion and its coordination number. This chelation occurs through formation of partially ionic and partially coordinate bonds, respectively in linear blocks of egg-carton like structures [5-10]. In case of formed colloidal hydrogels, the coordination biopolymer of metal-alginate complexes are characterized by elastic modulus, clarity and fine network structure and can be prepared in different shapes such as spheres (pellets), membranes (discs), fibers and columns depending on the types of apparatus used for preparation of the hydrogels. The formed hydrogel has anisotropic property and, hence, it is called inotropic hydrogel where the solvent molecules and macromolecular chains are oriented toward the chelated metal ion. For the membrane shapes, porous and capillary structure may be formed or not depending on the direction of the interdiffused metal ion into the alginate sol upward or downward during the replacement of the leaving Na+ counter ions of alginate macromolecule. When the metal ions are diffused downward (descending), the diffusion leads to formation of identical, parallel and symmetrical trapping capillaries in a longitudinal section or pores of the same diameter in case in a traverse section in such formed capillaries. On the other hand, when the diffusion of metal ions takes place in the upward direction (ascending), absence of such formed morphological structure is observed [5-10].

Experimental

Materials

Sodium alginate (Fluka) was used without further purification. The degree of substitution for alginate used was found to be 3.84 mol /g (0.7 mol/mol). Again, the inherent and reduced viscosities measured by using Ubbelhode viscometer was found to be 2.78 and 9.87 dl/g, respectively, for a 4 % alginate sol in doubly-distilled water (w/w) at 25 oC. All other materials used were of Analar (BDH) grade. Doubly distilled conductivity water was used in all preparations.

Preparation of Metal Alginate Complexes

Coordination biopolymers of solid granule shapes of trivalent metal-alginate gel complexes were prepared by the replacement of Na+ counter ions of alginate macromolecule by the corresponding trivalent metal ion. The process was performed by stepwise addition of sodium alginate powder to the electrolyte solution of trivalent metal ions (0.5-1.0 mol dm-3) whilst Mixtures containing stoichiometric ratios of both the metal alginate complex and the aqueous HClO4 of known concentrations were thermally equilibrated in a constant temperature water-bath at the desired temperature within ± 0.1C with continuous stirring using a magnetic stirrer. After equilibrium had been attained (24h), clear solutions containing both metal and hydrogen ions were pipetted out and the concentrations of the metal ion and the hydrogen ion were determined complexometrically and titrimetrically, respectively [22]. All cited data were an average of five experimental runs. The ionic strength of the mixture was maintained constant at 0.1 mol dm-3 by adding NaClO4 as a non-complexing agent. Some experimental runs were performed using absorption spectra, conductometric and pH-metric methods to check the reproducibility of the data obtained from the complexometric and titrimetric methods, respectively. The results obtained were found to be in good agreement with each other within negligible experimental errors (± 4%). This result indicates the reproducibility of the data obtained by the techniques used. In order to examine the dependence of the equilibrium constants on temperature and to evaluate the thermodynamic parameters, some experimental runs were performed at various temperatures with keeping all other reagents concentration constant.

Results And Discussion

The ion exchange process in macromolecular complexes is usually depending on the nature of the chemical composition of the complex [23,24]. The stoichiometry of ion exchange requires that the concentrations of the two exchanging counter ion be equal in magnitude, even the mobilities of two counter ions may be quite different, i.e. the ion exchange is inherently a stoichiometric process [25]. The metal-alginate complexes can be prepared in either granules or gel forms depending on the method of preparation [26-29] This, in turn, will affect the electrical, optical and mechanical properties of the complexes formed [30]. The stoichiometry of the ion exchange processes can be represented by the following equation.

solid aqueous where (Na-Alg)n denotes the sodium alginate, (M-Alg)n is the corresponding coordination biopolymer complex, M denotes the metal ion and z stands for its valence. The trivalent metal alginate complexes obtained in granule forms showed accepted mechanical properties for practical handling rather than that of the gel forms. The geometrical configuration of the formed coordination biopolymer complexes in the case of chelated trivalent metal ions is illustrated by Scheme (I). Hydrogen ions were used for exchange purposes in the present study owing to the exchange simplicity and in order to avoid the complications of the suggested equations in particularly in calculations. The exchange equilibrium between trivalent metal ions and hydrogen ions conforms to the following equation.

Solid aqueous where M3+ denotes the trivalent metal ion. Applying the law of mass-action for such heterogeneous systems, one concludes that

where a`s are the activities of the constituents and Ka is the true thermodynamic equilibrium constant, respectively. It was postulated [31,32] an approximate equation to calculate the thermodynamic equilibrium constant, Ka, which varies with the composition of the solid phase. Using the Gibbs-Duhem relationship for ions in the solid phase, and considering that the activity of water in the solid phase remains constant, the following equation is obtained.

Where C is the concentration of the counter ion in appropriate units. Plotting ln K against CH+ and extrapolated to CH+ = 0, gives ln Ka. Since our experimental observations showed no appreciable variations in equilibrium constant values within the variation of hydrogen ion concentration, no practical advantage on using such treatment [34,35] Therefore, assuming that the activities of the solid phases are unity [31,32], Eq.(3) may be rewritten as 

 

Where Ka is the true thermodynamic equilibrium constant and varies with the composition of the solid phase, g `s are the activity coefficients and R is the matrix of the ion exchanger. Using the Gibbs-Duhem relationship for the ions in the solid phase, and assuming that the activity of the solid phase remains constant, then the thermodynamic equilibrium constant (K`) can be evaluated from the following relationship

Where C is the concentration of the counter ion A+ in appropriate units. When log K is plotted against CA+ and extrapolated to CA+ = 0, gives log K`. [23] However, it appears that no particular advantage in that alternative treatment of [21,22,32] defined by Eq. (4) since our experimental observations showed no appreciable variation in Ka value within variation of H+ ion concentration used. Consequently, assuming that the mass action constant is not varied with the concentration of the interacting ions in order to avoid the complexity in selection of suitable procedure for evaluating the thermodynamic equilibrium constant, and

Table 1: FTIR spectra of sodium alginate and some coordination biopolymers of trivalent metal-alginate gel complexes of granule nature.

Complex

υsOCO

υasOCO

υOH

υM-O

Formula

Na(I)-alginate

1400

1600

3500

850

C6H7O6Na

Al(III)-alginate

1425

1648

3490

830

C18H21O17 Al.4H2O

Cr(III)-alginate

1420

1637

3463

810

C18H21O17 Cr.4H2O

Fe(III)-alginate

1422

1630

3448

820

C18H21O17 Fe.4H2O

La(III)-alginate

1419

1698

3426

815

C18H21O17 La.4H2O

Ce(III)-alginate

1421

1618

3441

815

C18H21O17 Ce.4H2O

considering that the activities of the solid phases are unity [24], Eq. (2) can be rewritten in the form,

Since the activity coefficients were not available at various temperatures, the thermodynamic chemical equilibria, Ka, could not be evaluated. Then, assuming that the activity coefficients under our experimental conditions of lower electrolyte concentrations of counter ions to be unity [32,33] and, hence, the values of Kc will be approximately equal to that of Ka. . The values of Kc were calculated using the least-squares method and are summarized in Table 1. The thermodynamic parameters of Kc values were evaluated from the temperature–dependence of the equilibrium constants and are listed in Table 2.

Table 2:  The thermodynamic parameters for exchange equilibria of some trivalent metal-alginate ionotropic gel complexes at I = 0.1 mol dm-3.

Metal-alginate

10 -3 K298

dm3 mol-1

- DG°298

kJmol-1

- DS°

J K-1 mol-1

- DH°

kJmol-1

Al 3+

0.10 (0.74)*

11.52(75.97*

25.27(44.27)*

19.0 (44.2)*

Cr 3+

0.20

13.07

25.07

20.54

Fe3+

0.13(0.46)*

12.09(76.95)*

21.40(97.59)*

18.47(47.87)*

La 3+

0.71

16.28

15.27

20.83

Ce 3+

0.33

14.35

28.88

22.63

Values of K between brackets were measured kinetically for the metal-alginate complexes in its hydrogels forms using conducoimetric technique at 20C  (*Ref 20).).

Experimental errors ( ± 4 %)

The noticed differences between the Kc values (Table 1) which were calculated for metal alginate complexes in the gel forms than that in the granule forms [35] (for Al and Fe –alginate complexes) may be attributed to the presence of capillary channels in the former but not in the later. These channels will enhance the exchange process between the exchanging counter ions and, hence, should lead to the increase of the magnitude of Kc values as was experimentally observed. This means that the formation of alginic acid is more easiness in case of using metal-alginate gels rather than that of the granule forms in such exchange process. Again, the geometrical configuration for chelation between the metal ion and the functional hydroxyl and carboxylate groups of alginate macromolecule play an important role in the magnitude of equilibrium constant of exchange. The more stable complex is the smaller tendency for exchanging its counter ions and, hence, will be the smaller Kc values. Therefore, two models for the geometrical structures were suggested for chelation [36-39]. The first geometry corresponds to an intramolecular association in which the functional groups involved in chelation belong to the same chain (planar geometry). The second type of geometry corresponds to an intermolecular association in which the functional groups are related to different chains (non-planar geometry). The two configurations are shown in Schemes I.

Intermolecular Association

Scheme I:  Geometrical configuration for chelation of trivalent metal ions in coordination biopolymer metal-alginate complexes.

Divalent metal ions may chelate the functional groups of alginate macromolecule via both two geometrical structures whereas the trivalent and tetravalent metal cations are restricted to chelate the functional groups of alginate macromolecule only through the intermolecular association geometrical structure owing to energy and stability reasons. Physicochemical studies on such metal alginate complexes indicated that the stability increased in the order trivalent- < divalent- < tetravalent-metal alginate complexes [8-11], [15-17], [35], [37] consequently, the equilibrium constants of exchange were found to decrease in the same order [22,23,35]. The values of equilibrium constants are increased in the order Al < Fe < Cr < Ce < La-alginate complexes. This magnitude may reflect the stability of these studied coordination biopolymeric metal-alginate complexes which decreases in the same order in good agreement with results reported elsewhere [8].the values of DG° observed in Table 2 may support this suggestion. Again, the negative values observed for DH° indicates that the exchange process is endothermic in nature. Despite the variation of the chelated metal ions, the values of DG seemed to be of the same magnitude indicating the similarity of the exchange processes in these metal-alginate complexes. Furthermore, there are several factors which play very important role in the exchange process and, hence, may affect the chemical equilibrium such as the ionic radii of the metal ions, the orientation of the macromolecular chains and the coordination water molecules toward the chelated metal ions and the strength of chelation between the chelated metal and the functional groups of alginate macromolecules. It reported that the mobility of the metal ions increases with decreasing the ionic radius [8,40].

Figure 1: A typical plot of equilibrium constant values against ionic radii of chelated metal ions for some trivalent metal-alginate complexes at 25 C.

 

The rate constants of exchange were found to increase with decreasing the ionic radius (r), i.e. in the order of Cr < Fe < Al –alginates [8,35,40]. A plot of ln K vs. r is shown in Figure 1. This result may be considered as indirect evidence to support the suggested order of stability of these studied complexes. Again, lanthanum has the lowest polarizability [41] and, hence, the lower stability compared to aluminum of higher polarizability and the largest stability in their alginate complexes. Also, the orientation of the macromolecular chains and the coordination water molecules toward the chelated metal ions play a role in the stability of the complexes [8]. Aluminum alginate complex is known to be well-oriented [29] and, hence, is the more stable complex as was experimentally observed. Moreover, as the strength of chelation between the chelated metal ion and the hydroxyl and carboxylate groups of alginate increases (M-O bond energies), the complexes stability should be increased. The bond energies of oxygen-metal bonds in these metal complexes were found to agree with the observed order of stability [8,42].

Conclusion

Adding sodium alginate powder reagent to an electrolyte solution of polyvalent metal ions in stepwise addition with vigorous and contentious stirring will lead to the formation of coordination biopolymer metal-alginate complexes of granule shapes. The FTIR of sodium alginate and its corresponding trivalent metal-alginate coordination biopolymer complexes were measured. The chelated trivalent metal counter ions in these formed complexes were replaced by hydrogen ions of an acid electrolyte solution. The factors affected the physicochemical properties of the ion exchange processes such as the nature of the complexes' geometrical configuration, the ionic radii of chelated metal ions, the bonding strength between the metal ions and the functional groups of alginate macromolecule and the temperature have been examined. The equilibrium constants were determined from the well-known chemical thermodynamic equilibrium equations. The experimental results indicated that values of the equilibrium constants of ion exchange were increased in the order Al < Fe < Cr < Ce < La-alginate complexes which are indicative of decreasing the stability in the same order.

Acknowledgments

This work supported by the Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt. The authors would like to thank Dr. Samia M. Ibrahim, Assistant Professor, Chemistry Department, Faculty of Science, New Valley University, El-Kharga 72511, New Valley, Egypt for her valuable helpful throughout performing this research.

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