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A Potentiometric investigation on the behaviour of taurine as a ligand
No full textThe ligand capability of taurine was studied at 25 degrees C towards cadmium(II), cobalt(Il) and nickel(II) in 1.00 mol/L NaCl as constant ionic medium and towards cobalt(II), nickel(II), calcium(II) and magnesium(II) in 1.00 mol/L NaClO4 as constant ionic medium. In the former ionic medium, potentiometric measurements were carried out by means of galvanic cells with a glass electrode and, for cadmium(ll), with an amalgam electrode, as well. In the latter ionic medium, free hydrogen ion concentration and free taurine concentration were obtained from electromotive force measurements of cells containing a glass and a silver electrode, respectively. The experimental data were explained by assuming the formation of complexes of the type ML and ML2, where M is the cation and L the taurine in basic form. The inspection of the values obtained for the stability constants seems to exclude the formation of chelates. A comparison among the values of the constants shows differences between the two ionic media and can be deduced a stability order
Formazione di complessi fra cistina e ioni sodio.
No full textLa solubilità della cistina (H2L = -O2CNH3+CHCH2SSCH2CHNH3+CO2-) è stata determinata a 25°C ed in mezzo ionico costante costituito da NaCl a 5 differenti concentrazioni ( 3,00; 2,00; 1,00; 0,500 e 0,150 mol dm-3).
Negli stessi mezzi ionici sono state determinate le quattro costanti di protonazione della specie L=.
E’ stato osservato che la solubilità di H2L aumenta con l’aumentare della concentrazione di NaCl usato come mezzo ionico. Anche le costanti di protonazione di L= sono differenti alle diverse concentrazioni di NaCl. I loro valori presentano un minimo quando la concentrazione di NaCl è 0,500 mol dm-3.
E’ stata formulata l’ipotesi che le variazioni riscontrate potevano essere spiegate sia mediante la variazione dei coefficienti di attività dei reagenti, sia mediante interazioni fra lo ione sodio e la cistina.
A questo scopo è stato scelto un mezzo ionico differente, questa volta costituito da N(CH3)4Cl 3,00 mol dm-3 che consentiva di variare ampiamente la concentrazione dello ione sodio.
Sono state determinate le costanti di protonazione della cistina (L=) in tale mezzo ionico, mediante misure di f.e.m. di una cella galvanica contenente un elettrodo di vetro sensibile allo ione H+.
Successivamente è stata studiata la competizione fra ione sodio e protoni in soluzioni contenenti cistina a diverse concentrazioni e con un elevato rapporto Na+/Cistina.
I risultati di misure di f.e.m. di una cella galvanica contenente un elettrodo di vetro sensibile allo ione idrogeno sono stati spiegati assumendo la formazione di specie complesse fra cistina e ione sodio
Protonation equilibria and solubility of L-cystine.
No full textL-Cystine is the natural least soluble of the naturally occurring amino acids. Its solubility is a function of the hydrogen ion concentration depending on its protonation. This paper presents an investigation on properties of L-Cystine in a constant ionic medium of NaCl at different concentrations (3.00. 2.00, 1.00. 0.500, 0.150 mol dm(-3)) and at 25 degreesC. In such experimental conditions, solubility and its changes as a function of hydrogen ion concentration were determined. Protonation of cystine was studied in acid and basic solution, by measuring the electromotive force of galvanic cell involving a glass electrode. Four protonation constants for L-cystine were determined in all the selected concentrations of ionic medium. The different values obtained for the constants in ionic medium of different concentration could be explained both with the variation of activity coefficients or with the complex formation between L-Cystine and sodium ions. Variations of solubility for different concentration of NaCl were observed as well. (C) 2002 Elsevier Science B.V. All rights reserved
Protonazione della cistina e sua solubilità.
No full textA causa della sua scarsa solubilità, non vi sono molti dati di letteratura sulle costanti di protonazione della cistina.
Tenendo però presente che la solubilità della cistina è funzione della concentrazione idrogenionica, è stato eseguito questo studio, in cui viene determinata la solubilità della cistina a diversi valori di concentrazione idrogenionica a 25°C ed in NaCl 3,00 M.
Cistina solida è stata messa a dibattere con soluzioni a differente concentrazione idrogenionica e, ad equilibrio raggiunto, è stata misurata la concentrazione libera dello ione idrogeno usando l’elettrodo di vetro e la concentrazione della cistina mediante misure polarografiche.
La protonazione della cistina veniva studiata anche mediante misure di f.e.m. condotte con l’uso di un elettrodo di vetro in soluzioni acide, -log cH 3, o in soluzioni alcaline –log cH 8, in cui la cistina era solubile a concentrazioni apprezzabili (-log Ccistina ~3).
I due procedimenti fornivano sia la solubilità che i valori delle costanti di protonazione che erano in buon accordo tra loro
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
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
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