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at Different Ionic Strengths and at T = 25 °C
No full textThe formation constants of dioxouranium(VI) - 1,2,3-propanetricarboxylate [tricarballylate (3-), TCA] complexes were determined in NaCl aqueous solutions at 0 ≤ I / mol L-1 ≤ 1.0 and t = 25 °C, by potentiometry, ISE-[H+] glass electrode. The speciation model obtained at each ionic strength includes the following species: ML-, MLH0, ML24- and ML2H3- (M = UO22+ and L = TCA). The dependence on ionic strength of protonation constants of 1,2,3-propanetricarboxylate and of the metal-ligand complexes was modeled by the SIT (Specific ion Interaction Theory) approach and by the Pitzer equations. The formation constants at infinite dilution are [for the generic equilibrium p UO22+ + q (L3-) + r H+ = (UO22+)p(L)qHr(2p−3q+r); βpqr]: log β110 = 6.222±0.030, log β111 = 11.251±0.009, log β121 = 7.75±0.02, log β121 = 14.33±0.06. The sequestering ability of 1,2,3-propanetricarboxylate towards UOUO22+ was quantified by using a sigmoid Boltzman type equation
Modelling the dependence on medium, ionic strength and temperature of N-acetyl-L-cysteine acid-base properties, and its interactions with Na+, Mg2+, Ca2+ and Zn2+
No full textN-acetyl-L-cysteine (NAC) is the acetylated precursor of L-cysteine, and it is strictly related to glutathione activity. NAC is used in medicine since more than 30 years, primarily as a mucolytic, though its properties are exploited in the treatment of many diseases, such as acute poisoning by acetaminophen (restoring protective levels of glutathione), HIV, contrast-induced nephropathy, type-2 DM (diabetes mellitus), and it also looks promising in the treatment of several psychiatric disorders [1]. This resulted in a huge number of scientific contributions published during the years on NAC (searching in some scientific databases, more than 10000 papers appear).
As expected, most of them are focused on the biological and therapeutic activity of NAC, while relatively few data are reported on the solution chemistry of this molecule, despite the thorough knowledge of its acid-base and coordination behaviour are of fundamental importance for the understanding of its properties in aqueous solution, as biological fluids are. Moreover, the relatively few thermodynamic data available (necessary to assess the speciation of this ligand in the system under study) are reported in single, specific conditions, while it is well known that the most of biological fluids (and natural waters) are, from a chemico-physical perspective, multielectrolyte aqueous solutions of very variable composition, ionic strength, and temperature [2].
Therefore, in this contribution we report the results of an investigation on the modelling of the acid-base properties of NAC in different ionic media (sodium and tetramethylammonium chlorides and tetraethylammonium iodide) at different ionic strengths (0 < I / mol dm-3 ≤ 3.0) and temperatures (283.15 ≤ T / K ≤ 318.15). Due to their importance from a biological perspective, we also report the results on the binding ability of NAC towards Na+, Mg2+, Ca2+ and Zn2+ in NaClaq at T = 298.15 K and different ionic strengths (0 < I / mol dm-3 ≤ 2.0). Surprisingly, despite the importance of these cations and their involvement in many biological mechanisms strictly connected with NAC activity, to our knowledge almost no data (only some for Zn2+) are present in literature on these interactions. The dependence on medium, ionic strength and temperature of the protonation and complex formation constants obtained has been modelled by classical approaches, such as the Extended Debye-Hückel (EDH), Specific ion Interaction Theory (SIT), Pitzer, and by the van’t Hoff equation
Sequestering ability of landfill leachate towards toxic metal ions
No full textThe great part of municipal solid wastes is worldwide stored in sanitary landfills. The interaction of organic and inorganic wastes with rainwater produces in the landfill a leachate of extremely variable composition. It depends on several variables such as the type of wastes, the age of landfill, the pH, the redox potential, etc [1-2]. Four are the recognized categories of pollutants in landfill leachate: inorganic macrocomponents, dissolved organic matter (DOM), heavy metals and xenobiotic organic compounds [3]. In particular, heavy metals (arsenic, cadmium, copper, lead, mercury, etc.) are usually present at concentration of few ppb and up to some ppm.
A variable but consistent fraction of DOM is constituted by humic and fulvic-like compounds, formed through a series of biological and chemical processes that involve DOM, especially during the methanogenic phase of wastes decomposition [3]. The great number of binding sites in humic like substances, mainly carboxylic and phenolic groups, are responsible of the high sequestering ability of leachate towards metal ions. As consequence, the speciation scheme of all the toxic metal ions contained in the landfill is strictly influenced by the amount and composition of DOM fraction in leachate.
Due to the imperfections or the possible damages of landfill liners, the leachate can contaminate superficial and ground waters in proximity of the landfill causing serious environmental pollution and human health risks. In a previous study the presence and the amount of landfill leachate, also in trace, on some groundwater samples collected in proximity of Bellolampo landfill was evaluated [4]. The obtained results showed a leachate concentration of ~ 130 μg L-1. Landfill of Bellolampo is a solid waste sanitary landfill near Palermo (Sicily). The landfill has been created in 1960s and become a controlled dump site in 1980s. Previous ICP-OES measurements on Bellolampo leachate have been shown a Pb, Cd and Cu concentrations in the ranges 0.05 - 2.3, 0.02 - 0.12 and 0.02 - 0.6 mg L-1 respectively.
The environmental and health risks related to the presence of toxic heavy metal ions in leachate depends on the species that metals form with the other components and at the conditions of the contaminated groundwater, with particular attention to the amount of free metal ions, the most dangerous among their possible species in aqueous solution.
For this reason, the study of the sequestering ability of landfill leachate towards toxic metal ions is of fundamental importance to establish the environmental impact of leachate contamination. Here we present a potentiometric and voltammetric (Differential Pulse Anodic Stripping Voltammetry, DP-ASV) study on the acid-base properties and on the sequestering ability of leachate samples collected in Bellolampo landfill towards Cd(II), Pb(II) and Cu(II) ions. The voltammetric and potentiometric titrations were carried out in NaCl aqueous solution, at I = 0.1 mol L-1 in order to simulate the mean experimental conditions typical of groundwaters. The acid – base properties of the filtered leachate were compared to those of the soluble fraction of humic substances, with carboxylic and phenolic groups as main binding sites of the macromolecules. Two units namely leach1 and leach2 containing COOH and phenol OH groups, respectively were considered and the Diprotic Like model has been used to process the ISE-H+ potentiometric data [5].
The voltammetric titrations of toxic metal ions solutions with leachate (diluted or as it is) were carried out adjusting the pH of titrand solutions at 5. The results shown an appreciable sequestering ability of leachate towards the heavy metal ions considered and an estimation of the metal complexes stability has been done by calculating rough metal – leachate formation constants
Prediction of water solubility and Setschenow coefficients by tree-based regression strategies
No full textThe experimental determination of water solubility (log S 0 ) and Setschenow coefficient (k m ) of a compound is a time-consuming activity, which often needs large amounts of expensive substances. This work aims at establishing two “open-source” chemometric models based on a regression tree that is able to predict the two abovementioned quantities. The dataset used is the largest to appear up to now for the collection of k m values, containing information on 295 molecules and it is relevant also for the collection of logS 0 values (321 molecules); for each of them 32 descriptors were taken from freely available databases. Information about water solubility and Setschenow coefficients, necessary to train the models, were taken from available literature. Validation was performed on a separate test set of molecules. The precision reached in the prediction is fully satisfying, being RMSEP = 0.6086 and 0.0441 for logS 0 and k m , respectively
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Thermodynamic study on 8-hydroxyquinoline-2-carboxylic acid as a chelating agent for iron found in the gut of Noctuid larvae
No full textThe recognition of quinolinic carboxylic acids as natural chelants and the recent observation of a high production of 8-hydroxyquinoline-2-carboxylic acid (8-HQA) in the gut of several Noctuid larvae (e.g. Spodoptera littoralis) has inspired the study of the chelation properties of 8-HQA towards Fe2+ and Fe3+. Here, we report a detailed characterization of the thermodynamic solution behaviour of Fe2+/8-HQA and Fe3+/8-HQA systems as a function of the pH value. The acid-base properties of 8-HQA and its binding ability towards Fe2+ and Fe3+ have been investigated over a wide range of pH values (2.0 ≤ pH ≤ 11.0) by ISE-H+ (glass electrode) potentiometric titrations in KCl(aq) at I = 0.2 mol dm-3 and at T = 298.15 K. For both oxidation states, various FeLqHr species are formed, with q = 1, 2 (and 3), and -2 ≤ r ≤ 1. The presence of the main FeLqHr species was confirmed by HESI-HRMS. ESR measurements have also been performed to get some extra information on the Fe3+ coordination, indicating a distorted octahedral symmetry around the metal center. Quantum mechanical calculations have been carried out in order to characterize the structural features of selected metal complexes. The complexing ability of 8-HQA is generally much higher for Fe3+ than Fe2+. Nevertheless, the sequestering ability of 8-HQA towards these two oxidation states of this metal ion, obtained by the calculation of several pL0.5 values, resulted in it being highly dependent on the pH value: (i) at relatively low pH values, it is higher for Fe3+ (pL0.5 = 6.3 at pH = 3.0) than for Fe2+ (pL0.5 = 3.1 at pH = 3.0); (ii) it is almost the same at pH = 8.1 (Fe3+: pL0.5 = 8.3; Fe2+: pL0.5 = 8.1); (iii) it is higher for Fe2+ at high pH values (pL0.5 = 8.9 for Fe2+ and pL0.5 = 6.2 for Fe3+ at pH = 10.0). The determination of the stability constants of the Fe2+/8-HQA and Fe3+/8-HQA complexes was also complemented by data obtained by the ligand-competition approach, using EDTA as a competing ligand over a wide range of cation and ligand concentrations and ratios. This also allowed a more thorough investigation of both the Fe2+/EDTA and Fe3+/EDTA systems, providing an accurate stability constant dataset for the Fep(EDTA)qHr species under the above-mentioned experimental conditions, which are commonly used in biological studies
A novel thermodynamic approach for the complexation study of toxic metal cations by a landfill leachate
Full text linkLandfill leachates can contaminate nearby aquifers. The hazards deriving from this contamination also depend on the chemical speciation of various contaminants. A novel approach is proposed here to face this problem from a chemical thermodynamics point of view. The complexing ability of the soluble fraction of a landfill leachate (collected from Bellolampo, Palermo, Italy) towards Pb2+, Cd2+and Cu2+has been investigated at T = 298.15 K in NaClaqat I = 0.1 mol dm-3. The soluble fraction of the landfill leachate was first characterized by different analytical techniques. Then, its acid-base properties were studied by ISE-H+potentiometric titrations and modelled by the so-called diprotic-like model. Differential Pulse Anodic Stripping Voltammetry (DP-ASV) titrations of metal ion aqueous solutions with a diluted landfill leachate were carried out, successively, in order to determine the stability constants of the leachate-metal complexes. The stability of the Pb2+/OH-and Pb2+/Cl-complexes was also studied by the same technique. Finally, the sequestering ability of the leachate towards the investigated metal cations has been quantified by the calculations of various pL0.5values under different pH conditions. The results proved that the landfill leachate is a good sequestering agent toward those cations, reducing the fraction of the free cations in solution, and that it can be a selective sequestrant at different pH
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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