Nadir toprak element iyonları-fenoksiasetat komleks dengelerinin potansiyometrik yöntemle incelenmesi

Abstract

Bu çalışmada nadir toprak element iyonları ile fenoksiasetat ligandı arasında oluşan kompleks dengeleri potansiyometrik yöntemle incelendi. İncelenen tüm sistemlerde, çalışılan konsantrasyon aralığında sadece tek çekirdekli komplekslerin oluştuğu, çok çekirdekli komplekslerin ve asid komplekslerinin oluşmadığı gözlendi. Nadir toprak element iyonlarının fenoksiasetat ligandı ile oluşturduklan dengelerin, 25.0°C ve 1.0 M iyonik şiddetteki perkloratlı ortamda potansiyometik yöntemle incelenmesi ve verilerin iki farklı hesaplama yöntemiyle değerlendirilmesi sonucu, oluşan komplekslerin stabilite sabitleri saptandı. Çalışılan ligand konsantrasyonu aralığında dört metal iyonu (Samaryum, Europyum, Holmiyum ve Ytterbiyum) hariç tüm iyonlar (Lantan, Seryum, Neodyum, Gadolinyum, Terbiyum, Disprosyum, Erbiyum, Tulyum ve Lutesyum) için ancak oluşan ilk kompleksin, dört metal iyonu için ise ilk kompleks yanında oluşumu başlayan ikinci kompleksin stabilite sabitleri değerleri tespit edildi.

Quantitative measurement of complex formation in a solution is the stability constants of the formed complexes. Many research workers in chemistry, biochemistry, geo-chemistry and environmental interdisciplinary fields want to find the correct values of the stability constants from literature without doing the extensive laboratory study. In metal complexes, central ion is a metal ion or possibly a proton, the ligand is an organic or inorganic anion or neutral molecule. In the course of a complex formation reaction, the solvent molecules surrounding the central ion may be successively exchanged by ligand ions or molecules, leading finally to the complex MLn. N indicates the number of ligands in the complex. As the selectivity of the complex formation reaction depends on the stability of the complex formed, it is important to know the factors which effect the formation of the complex. Numerous researchers have been made several classifications to explain metal ligand interactions. The most widely used one of them is hard-soft classification proposed by Pearson. Pearson divided metals and ligands into two groups as soft and hard according to their polarization ability. Hard acids are small ions with a large positive charge and hence with a rigid electron shell structure, and compounds which do not contain readily excitable electrons. Hard bases similarly have rigid electronic structures and are difficult to polarize and oxidize; they are ligands containing donor atoms with high electronegativity. Soft acids (soft metal ions) are large, with a low charge and a loose electron shell structure, and they contain readily excitable outer electrons. The soft bases are similarly ligands containing donor atoms with a loose electron shell structure, that are easily polarized and oxidized, and are of low electronegativity. The properties of the hard and the soft ligands and the above stability sequences clearly show that hard metal ions form stable complexes with hard ligands, as do soft metal ions with soft ligands; hard-soft and soft-hard interactions result in relatively weak complexes. The bonding-theory background of the classification into the various types involves the tendencies to form covalent and electrostatic bonds. Hard metal ions and hard ligands primarily participate in electrostatic interactions; their reactions are accompanied by charge compensation and by the release of water molecules bound strongly to the ligand and to the metal ion. In contrast, the interactions of soft metal ions and soft ligands lead to covalent bonds. Rare earth elements (REEs) are certainly not as rare as their name implies. They collectively rank as the 22nd most abundant element and are found in over 1 80 XI different types of minerals. Present day, REEs have great economical value and their industrial use is increasing everyday. It may be expected that human intake of REEs increases by the huge increase of REEs applications during last years. Very little is known of possible injurious effects exerted by REEs on human health. When considering the load of the environment by REEs, special attention should be paid to the fertilizer industry. In this industry millions of tons of natural phosphates are annually processed. Both magmatic and deposit calcium phosphate are used for the production of phosphate fertilizers, which means that the REE contents of the row material usually varies between 1% and 0.1%. After the recovery procedures of REE, the resulting phosphoric acid solution, which is used for the production of the fertilizer, contains the remaining 40 to 20% of the REEs. Even if only a minor part of the REEs present in the raw material reaches the fertilizer, a continuously increasing load of the arable land by REEs results, which will inevitably lead to an increase of human intake. Recently agricultural studies indicate effects of REE concentrations on growth and nitrogen metabolism of plants. The complexing interactions between the cations and ligands in the soil are important in the respect of agricultural studies because these interactions affects the metabolism of plants. Several groups of compounds which act as pesticides are potential ligands for the cations present in the soil. The use of pesticides (insecticides, fungicides, herbicides etc.) in agriculture is rapidly increasing, whereas the quantitative knowledge of the interactions between these compounds and cations present in the soil unsatisfactory. It is therefore found interesting to investigate the complexing ability of some pesticides with respect to REEs accumulating to the soil by fertilizers. By this purpose, phenoxyacetic acid which is the most common used herbicide was chosen as the model ligand. It can be suggested that this study will be of some interest in the chemical modeling of natural systems in which such types of compounds are involved. Complex formation leads to changes in numerous physical chemical properties of solutions. In principle, the measurement of any parameter which varies in response to complex formation provides a possibility for calculation of the compositions and formation constants of the complexes. In practice, however, the following conditions must also be satisfied if quantitative conclusions are to be drawn from the changes. (a) The correlation between the extent of complex formation and the measured parameter must be known exactly. (b) Complex formation must cause the parameter in question to change to an extent much higher than the experimental error. (c) Measurements must be made in the range of total concentrations of the components where complex formation is well measurable, but not complete. A variety of experimental methods have been developed for the determination of the formulas of the complexes and of the stability constants. The methods available for determining stability constants are mainly based on the preparation of series of solutions containing known amounts of the complex- forming components, in which the concentration of one component is gradually varied and the concentration of one of the reactants or products is followed directly or indirectly by a suitable analytical method. Usually the concentration of the metal ion xu is kept constant and that of ligand varies within wide limits. In order to make the stability constants calculated from analytical concentrations, the ionic strength should be the same in each solution. Instrumental method of analysis are mostly based on the measurement of an intensive physico-chemical property which is proportional to the concentration of substance. If suitable electrodes are available, e.m.f.s methods are the most important methods for the study of complex equilibria. In general, a high accuracy can be obtained and the experimental technique is very simple especially if the measurement are performed as e.m.f titrations. A fairly large number of methods for calculating constants from experimental data have been reported in the literature. In order to evaluate these stability constants, it is necessary to find a relationship between them and the experimentally determined variables. This relationship is often established via the basic functions w (the complex formation function), a(the degree of formation), (j)(Leden fonksiyonu). For the present study the potentiometric method was found more suitable and measurements have been carried out by means of a glass electrode. Thus, the free ligand concentration has been determined via pH measurement in metal ion solutions, containing phenoxyacetate-phenoxyacetic acid buffers. To do this, acidity constant of phenoxyacetic acid has been determined under the same conditions. All measurement have been performed at 25°C and in an aqueous medium of unit ionic strength with sodium perchlorate as inert salt. The calculations are based on the e.m.f. of the following cell. ( - ) RE// CM M Me(C104)3, CH M HC104, CL M L(phenoxyacetate), NaCKX, to I = 1.00 /GE ( + ) where RE // Ag, AgCl / 0.025 M NaCl, 0.075 M NaC104 / 1.00 M NaC104 /. Here, Me denotes metal ion. The e.m.f. was measured by a Metrohm 654 digital pH meter. The glass electrode, a Metrohm electrode, had the theoretical slope in the pH region in question. The Ag, AgCl reference electrode was prepared according to Brown. A magnetic stirrer was used during the titrations. Within a few minutes after addition of titrant, the e.m.f.s reached a stable value. All titrations were carried out at least four times. The reproducibility of the e.m.f. readings was ± 0.2 mV. The cell was kept at (25.0 ± 0.1) °C by means of a water thermostat. The measurements were carried out by potentiometric titrations, the metal ion and acid concentrations, Cm and Ch, respectively, being kept constant and the phenoxyacetate concentration being varied during titration. Equal volumes of solutions Ti and T2 were added from a Metrohm 654 Multi Dosimat to Vo ml of solutions S. These solutions had the following compositions. S : CM M Me(C104)3, CH M HC104, (1-CH -6CM) M NaC104 Tj : CL M CgHjANa, (1-CL) M NaC104 T2 : 2CM M Me(C104)3, 2CH M HC104, (1-2CH -12CM) M NaC104 The ionic strength of the resulting solution will then be 1.00 M. The e.m.f. of the above cell is given in mV, at 25°C by X1U RT EH = E°H +Ej +2.303- logA 0) where E°H is the cell constant, Ej is the liquid junction potential, EH is the elektromotor force of the cell, h the free hydrogen ion concentration. The measured e.m.f. values were converted to hydrogen ion concentrations from a calibration curve of the glass electrode which was obtained using solutions of known hydrogen ion concentrations. By determining the values of h, the free ligand concentration [L] and the average ligand number n can be calculated from C"-h (2) _ CL-CH+h-[L] n- - C (3) M where Cu is the total acid concentration and Ci is the total phenoxyacetate concentration after the addition of ligand buffer to the titration vessel. The concentration used did not result in formation of ML-2 for rare earth cation expect four metal ions. A linear least squares analysis of the equation n (1-S) fiM (4) gave the values of fi\. For the calculation of two stepwise stability constant of the systems for which two complexes formed the below equation was used. (l-g) (2-n)[L] n (2-n)[L}\ K,~K2 (5) If the left-hand side of equation is plotted against n /(2-« )[L]2 a straight line results. The y-intercept gives -K2 and the slope l/Ki, the x-intercept gives K1K2. Stability constant of rare earth metal ions were also calculated using Froneaus and Leden methods. From graphical integration of w/[L] vs. [L] plot, Fronaeus integral X can be evaluated. XIV (6) The overall constants are then found by plotting the functions Xj = (XiA - fiA )/[L] vs. [L], where 1 Mg2+ > Li+ > Na+ > K+. In this order, the highest stability constant value is for Ca2+ as 6.45 and the lowest is for Na+ and K+ as 1.12. Since the interaction between hard acceptors and donors are XV mainly electrostatic, as expected 3+ charged rare earths more stable complexes with phenoxyacetic acid than alkali and alkali earth cations. By comparing the present results and the literature data for benzoic acid, it can be suggested that the oxygen donor in phenoxyacetic acid is not involved in coordination and this ligand is acting as the monodendate. Even if REEs show similar features, as can be seen in Table 1, the differences between stability constant values are sufficient to permit a correct speciation of a fluid containing phenoxyacetate and REE cations.