Electrochemical and spectroscopic studies of the complexed species of models of nitrohumic acids derived from phthalic acid

Journal of the Brazilian Chemical Society

Lombardi, Simone Cristina¹  Mangrich, Antonio Sálvio¹  Szpoganicz, Bruno¹  Universidade Federal de Santa Catarina, Brazil¹  UFPR, Paraná, Brazil¹  Mercê, Ana Lucia Ramalho¹  Universidade Federal do Paraná, Curitiba, Brazil¹  Levy, Noel Massinhan¹ 

关键词: metal complex;    humic substances;    potentiometric titration;    cyclic voltammogram;    speciation;    organic fertilizer;   

DOI  :  10.1590/S0103-50531998000300006

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Humic substances (HS), organic material existing in soil and waters arising from the decay of plants and animals play many roles in soils. Among them, they have long been considered as a natural source of C, N, P, and S and also, due to their ability to form stable complexes with many metal ions, can be considered as a natural fertilizer1-2. Many workers have made contributions concerning the determination of dissociation constants of the acidic functional groups of HS in order to obtain a better understanding of metal complexation by these organic macromolecules since the late 70Â’s. The determination involved the use of thermochemistry2, Fourier Transform Infrared Spectrometry3, and Potentiometric studies4. Since the contribution of Schulten and Schnitzer5 for the unraveling of the structure of HS, there have been few important contributions to the HS literature. The structure of these geopolymers is complex. However, it is known for certain that aromatic structures are present as part of the structure of HS and NHS. These macromolecules contain within their structure organic functional groups such as catechols, salicylates and phthalates, capable of complexing metal ions6-10. Figure 1, showing the Eqs. 1 to 3, represents some of the structures capable of complexing the metal ions.   Many studies have been published11-17 of the complexing ability of HS towards metal ions like copper, nickel and aluminum as well as the pH dependence of these systems. Model compounds are used to overcome the difficulties of studying HS due to their physical complexity. No matter the size of the molecules chosen, they allow for a better understanding of the complexation behaviour of HS.  Another advantage in using model compounds for the study of the behaviour of HS towards metal ion complexation lies in the fact that it is very difficult to make interpretations of the acid-base properties of HS, due to the perturbations encountered in acid-base potentiometric titrations. Various models have been proposed in the literature to take into account the mixture of chemically non identical acid sites. One of them has sought to fit experimental data empirically and is called the Discrete Ligand Approach18. The empirical model presumes that the observed potentiometric properties are a consequence of the different acid strenghts of a limited number of functional sites that constitute the humic acid (HA) molecule . The present work is based on this model. The nitrohumic acids (NHA), a laboratory artifact of HA obtained by the action of nitric acid upon HA, have the advantage of a higher percentage of N than in HA.9 The complexing ability of a model compound for the phthalic derived sites of HA is compared with the models chosen for the nitrophthalic derived sites of NHA using two important plant micronutrients, one a hard (Fe(III)) and the other a softer Lewis acid (Zn(II)). The models, phthalic acid (APA), 3-nitrophthalic acid (3-NPA) and 4-nitrophthalic acid (4-NPA), are represented in Fig. 2.   Althought the models are not complex molecules, the interest for studying them, apart from what has been said above, is that there is a need to complement the data in the literature, due to the lack of certain stability constant data. Also, this study is important for evaluating the interactions of HS and NHS acid sites in the presence of metal ions, in order to understand how during their slow mineralization in soil, not only how the greater amount of N of NHS compared to HS, is released to plants but also how all the metal ions bound to these acidic sites, will be made available. A comparative study is extended by collecting data from model compounds of HA and NHA, salicylic and catecholic derivatives, previously reported in the literature19-24. The study of metal ions and HA complexes has led to few conclusive results, as there are many drawbacks to attributing the binding constants found with a specific site or reactive site that is actually linked to the metal ion. In order to gather some previously reported data concerning nitrosalicylic acids19-20 and nitrocatechols22-23, a trial titration was made in a mixture of NHA models and Cu(II). Cyclic voltammetry (CV) can give information concerning the stability of electrogenerated products in a redox system tested over a range of p[H] values. This technique is also important in helping to identify those species involved (aqua ions, complexed species, etc)25. Ultraviolet-visible spectroscopy (UV-Vis) shows a relation between the differences in the wavelength values of the spectra obtained in solutions of differing p[H], the differences corresponding to the different species proposed in the equilibria26.   Experimental Chemical reagents All chemicals used were of analytical-reagent grade and were used as received. All solutions were made with bi-distilled, deionized and CO2 - free water. All nitrosalicylic and salicylic acid solutions were described elsewhere19. The nitrophthalic (Sigma - USA) and phthalic acid (Reagen - Brazil) solutions were made in 5% v/v ethanol(Merck)/water. Metal solutions were made from their nitrate salts (Carlo Erba - Brazil) and their concentrations were determined by literature procedures27. The Fe(III) 0.01mol L-1 solution was made in 0.03 mol L-1 HCl (Merck) and the H+ ion content was determined by GranÂ’s Plot.28 The aqueous KOH (Merck - Brazil) 0.1 mol L-1, carbonate-free solution was standardized against potassium hydrogen phthalate (Carlo Erba - Spain). KNO3 (Baker & Adamson - USA) was the supporting electrolyte. Potentiometric measurements All potentiometric titrations were carried out under an inert atmosphere of water-KOH saturated nitrogen (White-Martins - Brazil) in a water-jacketed vessel maintained at 30.0 ± 0.1 °C and 0.100 mol L-1 ionic strength (KNO3). Four experiments were performed for each ligand studied. The first with the ligand alone to determine its protonation constants. The others, with the ligand in the presence of the metal ion in the proportions ligand to metal, 1:1, 2:1 and 3:1. The pKw was determined to be 13.63. A 20 mL Metrohm manual piston microburet tip was used to deliver the titrant - CO2 - free KOH standard, and the p[H]28 values were directly measured with a Micronal (SP - Brazil) model B-375 pH meter, fitted with an Analyser (SP - Brazil) blue-glass and a saturated calomel reference electrode, calibrated with a standard strong acid (HCl) to read -log[H+] directly, accuracy, 0.001 p[H] units. The data obtained in the titrations were treated in a PC computer equipped with a Fortran program Best and the results displayed as a curve of species distribution with the aid of the program SPE28. Two special potentiometric titrations were carried out in a solution obtained by mixing model compounds of NHA in the absence and in the presence of Cu2+. A quantity of 0.0717 millimole of 3-NPA and 4-NPA, 0.0540 millimole of 3-nitrosalicylic acid (3-NSA), 5-nitrosalicylic acid (5-NSA) and 3,5-dinitrosalicylic acid (3,5-DNSA), and 0.0344 millimole of 4-nitrocatechol (4-NC) were titrated against KOH. The second titration involved the same model compounds, in the above quantities, but with the inclusion of 0.0786 millimole of Cu2+. UV-visible measurements The aliquots analyzed by UV-VIS spectroscopy were taken at specific p[H] values from a second potentiometric titration, specifically done for this purpose. The UV-VIS spectra were taken with a Hewlett - Packard model 8450A (USA) - Diode array spectrophotometer, from 260 to 600 nm, using aliquots of a solution of ratio ligand to metal 3:1 of 3-NPA (7.5 x 10-4 mol L-1) and Fe+3 (2.5 x 10-4 mol L-1), ionic strength 0.100 M (KNO3), using 1.000 cm quartz cells, at a controlled room temperature of 25.0 °C, and air in the reference beam. The p[H] values of the experimental solutions were adjusted by adding small volumes of 0.1 mol L-1 KOH with a 20 mL Metrohm piston microburet attached to the vessel. Cyclic voltammetric studies The electrochemical cell employed supported a 15 mL total volume of solution. It was maintained at 0.100 M (KNO3 - Merck - Brazil) ionic strength, sufficiently great to minimize solution resistance to charge flow through the cell, and to minimize migration as a means of mass transport to the electrode. The working electrode used was a glassy carbon of 2 mm diameter. The reference electrode was Ag/AgCl, and the auxiliary electrode was a platinum wire. The system, totally deoxygenated by a stream of pure nitrogen (White-Martins - Brazil), was connected to a cyclic voltammograph of Bioanalytical System Incorporation, model BAS-27 (USA), and the results were recorded in a BAS X-Y recorder. The potential values reported in this work were increased by 0.204 V, and referred to normal hydrogen electrode (NHE)29. The temperature was 25 °C, room-controlled. The final concentration in the cell for both the ligand (4-NPA) and the metal ion, Fe+3, was 10-4 mol L-1 where the ligand to metal ratio was 1:1; 0.5 x 10-4 mol L-1 in the 2:1 solution, and 0.33 x 10-4 mol L-1 in the 3:1 solution. The cyclic voltammograms were obtained as a function of each p[H] value measured, using a glass electrode, Ag/AgCl (Analyser - SP - Brazil), 5 mm diameter, placed into the electrochemical cell and a Corning pHmeter (UK), accuracy, 0.01 p[H] units. A Gilmont (USA) microburet delivered the required amount of KOH (0.1 mol L-1) to reach to the desired p[H] values. The experimental conditions of the cyclic voltammograph were the following: a) range of swept potential was from 1.0 V to -0.20 V; b) scan rate = 10 mV/s; and c) initial potential applied = +1.0 V.   Results and Discussion The calculation of the equilibrium constants employed the microcomputer program BEST28. The results were also displayed in the form of species distribution diagrams with the aid of the program SPE28, with the metal concentration set at 100%. The BEST program was designed to solve for the set of equilibrium constants corresponding to the model selected and also makes it possible to explore all aspects and variations of the model. The model used in this work involved the choice of the chemical species shown to be present from potentiometry, UV-Vis spectrophotometry and cyclic voltammetry data. Each species concentration consists of a product of the overall stability constant (bn) and individual component concentration raised to the power of its stoichiometric coefficient. The calculation of b values continues until no further minimization of the standard deviation (s.d.), in p[H]28 units, is obtained. The overall stability constants are defined by Eq. 4:All mathematical aspects of the programs employed have been reported elsewhere28,30, and the desired ionic strength was set in all experiments by following the literature31. The equilibrium constants for the hydrolysis species of the metal ions employed in the calculations of the overall formation constants (b), were taken from the literature32. The following hydrolysis species were considered in the calculations: Fe(OH)2+, Fe(OH)2+, Fe(OH)3, Fe(OH)4-, Fe2(OH)24+, Fe3(OH)45+; Zn(OH)+, Zn(OH)2, Zn(OH)3-, Zn(OH)42- and CuOH+, Cu(OH)2, Cu(OH)3-, Cu(OH)42-, Cu2(OH)2. Figure 1 shows a potentiometric p[H] profile of 3-NPA alone and in the presence of Fe+3 in the proportions of ligand to metal: 1:1, 2:1 and 3:1. Precipitation of hydrolysis products above p[H] = 4.5 prevented further measurements of the metal systems. The protonation constants of the ligands 3-NPA and 4-NPA were determined under the same experimental conditions as for the complex formation constants in this work, 30.0 ± 0.1 °C and ionic strength 0.100 M (KNO3) in order to use them for further calculations. These constants are presented in Table 1 as well as the results reported in the literature19-20,22-23 and also the values for the ligands derived from catechol and salicylic acid.   The values in Table 1 show that the most acidic compounds are the nitro- derivatives of phthalic acid, and the least acidic, catechol, as expected. Many different methods of calculation and experimental approaches for determining apparent stability constants of Cu(II) and HA have been reported in the literature. One of them33 gives values for the binding constants of these kinds of complexes using a continuous distribution model based on the Scatchard plot, alternatively to the Bjerrum potentiometric titration method. The former method provided an average overall stability constant of 8.0, at 25 °C, m = 0.005 mol L-1, concentration of HA = 0.25 mg mL-1. The latter method employed gave the average values for the same kind of stability constant under the same conditions as 7.6. The main important feature was that Cu(II) reacts with more than one binding site , as the authors say, due to evidence for formation of CuL+ and CuL2, where L is the reactive site of the macromolecule studied. The Bjerrum approach is suitable for this study but it does not account for the heterogeneous nature of HA. The Scatchard Plot gives rise to individual K values that are considered to reflect variations in binding energies without any regard to the nature in which Cu(II) is bound. Another reported work34 dealt with the study of the formation constant for HA and Cu(II) by cross-polarization/magic angle spinning 13C-NMR. The results of the average conditional constant was 11.6. All spectra contained one major peak in the carboxyl carbon region associated with the carboxylic acid carbon. There was a sharp difference among the HA employed although the values found for the binding constants were close. In the trial titration that was carried out in this work using a mixture of nitrocompounds, models for nitrohumic substances, three protonation constants were observed due to the respective acidic groups. These groups were named g1, g2, and g3 and their average values are recorded in Table 1. The g1 acidic group was assigned to the nitrophthalic acids22, g2, to the nitrosalicylic acids19-20, and g3, to nitrocatechol22. Table 2 presents the average formation constants calculated when the mixture of the nitro- models was titrated in the presence of a metal ion. Cu(II) was chosen because there is comprehensive data available in the literature. The data in the literature were required for the attempted assignment of value found in this titration with a proper basic site.   As can be seen from Table 1, the value for the formation constants for CuL, L being the reactive site of the model, of around 3.6 was assigned to two ortho carboxyl groups in an aromatic ring, having a nitro substituent in the 3- and 4-C position (nitrophthalic acids); the value around 9.8 was assigned to one carboxyl and one hydroxyl group in ortho positions in an aromatic ring (nitrosalicylic acids); the value near 11.5 was assigned to two ortho phenolic hydroxyl groups (nitrocatechol). The basic group assigned to the nitrophthalic acids has exhibited a greater basicity towards Cu2+, as it has a larger formation constant when present in the mixture of the model compounds, than when alone with the Cu2+. The same was true of the nitrosalicylic basic sites. The basic group due to nitrocatecholate behaves the same, no matter only with the metal ion or in the mixture of the model compounds employed. Looking at these results one can infer that there are some interactions among the basic sites of the nitro-models employed when they are mixed in solution. This feature can be explained by the fact that when in a mixture , the less basic ligands will change the way they interact with metals by the presence of other more basic sites in their vicinities. This maybe the

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