-
[show abstract]
[hide abstract]
ABSTRACT: Quantitative parameters for the interactions between phytate (Phy) and Al(3+), Fe(3+), and Cr(3+) were determined potentiometrically in NaNO(3) aqueous solutions at I = 0.10 mol L(-1) and T = 298.15 K. Different complex species were found in a wide pH range. The various species are partially protonated, depending on the pH in which they are formed, and are indicated with the general formula MH(q)Phy (with 0 ≤ q ≤ 6). In all cases, the stability of the FeH(q)Phy species is several log K units higher than that of the analogous AlH(q)Phy and CrH(q)Phy species. For example, for the MH(2)Phy species, the stability trend is log K(2) = 15.81, 20.61, and 16.70 for Al(3+), Fe(3+), and Cr(3+), respectively. The sequestering ability of phytate toward the considered metal cations was evaluated by calculating the pL(0.5) values (i.e., the total ligand concentration necessary to bind 50% of the cation present in trace in solution) at different pH values. In general, phytate results in a quite good sequestering agent toward all three cations in the whole investigated pH range, but the order of pL(0.5) depends on it. For example, at pH 5.0 it is pL(0.5) = 5.33, 5.44, and 5.75 for Fe(3+), Cr(3+), and Al(3+), respectively (Fe(3+) < Cr(3+) < Al(3+)); at pH 7.4 it is pL(0.5) = 9.94, 9.23, and 8.71 (Al(3+) < Cr(3+) < Fe(3+)), whereas at pH 9.0 it is pL(0.5) = 10.42, 10.87, and 8.34 (Al(3+) < Fe(3+) < Cr(3+)). All of the pL(0.5) values, and therefore the sequestering ability, regularly increase with increasing pH, and the dependence of pL(0.5) on pH was modeled using some empirical equations.
Journal of Agricultural and Food Chemistry 07/2012; 60(33):8075-82. · 2.82 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The acid-base properties of γ-L-glutamyl-L-cysteinyl-glycine (glutathione, GSH) were determined by potentiometry (ISE-H(+), glass electrode) in pure NaI((aq)) and in NaCl((aq))/MgCl(2(aq)), and NaCl((aq))/CaCl(2(aq)) mixtures, at T = 298.15 K and different ionic strengths (up to I(c) ~ 5.0 mol L(-1)). In addition, the activity coefficients of glutathione were also determined by the distribution method at the same temperature in various ionic media (LiCl((aq)), NaCl((aq)), KCl((aq)), CsCl((aq)), MgCl(2(aq)), CaCl(2(aq)), NaI((aq))). The results obtained were also used to calculate the Specific ion Interaction Theory (SIT) and Pitzer coefficients for the dependence on medium and ionic strength of glutathione species, as well as the formation constants of weak Mg(j)H( i )(GSH)((i+2j-3)) and Ca(j)H(i)(GSH)((i+2j-3)) complexes. Direct calorimetric titrations were also carried out in pure NaCl((aq)) and in NaCl((aq))/CaCl(2(aq)) mixtures at different ionic strengths (0.25 ≤ I (c )/mol L(-1) ≤ 5.0) in order to determine the enthalpy changes for the protonation and complex formation equilibria in these media at T = 298.15 K. Results obtained are useful for the definition of glutathione speciation in any aqueous media containing the main cations of natural waters and biological fluids, such as Na(+), K(+), Mg(2+), and Ca(2+). Finally, this kind of systematic studies, where a series of ionic media (e.g., all alkali metal chlorides) is taken into account in the determination of various thermodynamic parameters, is useful for the definition of some trends in the thermodynamic behavior of glutathione in aqueous solution.
Amino Acids 10/2011; 43(2):629-48. · 3.25 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The formation constants of various M/Phy systems (M = Cu2+, Zn2+, Pb2+, Ni2+; Phy = Phytate) were determined in NaNO3 solutions at 0.1 ≤ I/mol·L−1 ≤ 1.0 and T = 298.15 K, by potentiometry and voltammetry. The formation constants of the Cu/Phy and Zn/Phy species, already determined, were reanalyzed together with new voltammetric and potentiometric experiments at low concentrations. A new potentiometric and voltammetric study was carried out on the Ni/Phy and Pb/Phy systems. For all of the investigated systems, the dependence on ionic strength was modeled by the Debye−Hückel and the specific interaction theory (SIT) approaches. The sequestering ability of phytate was evaluated toward the considered cations by calculating the pL50 values (i.e., the total ligand concentration necessary to bind 50 % of the cation present in trace) at different ionic strengths and pH. A complete set of “suggested” formation constants was provided. A comparison between the formation constants obtained for the Zn/Phy, Cu/Phy, and Pb/Phy systems reveals that the interactions of phytate with Zn2+, Cu2+, and Pb2+ are quite similar, while the Ni/Phy species showed a weaker complexation. For example, for the MH3Phy species, log K3 = 7.81, 7.51, 7.10, and 5.97 for Zn2+, Cu2+, Pb2+, and Ni2+, respectively. The same trend is observed concerning the pL50. Their dependence on pH and ionic strength was modeled by two empirical equations.
Journal of Chemical & Engineering Data 08/2010; 55(11):4757–4767. · 1.69 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Solubility and protonation constants of ethylenediaminetetraacetic acid (EDTA) were determined in aqueous NaCl solutions at different ionic strengths (0 ≤ I ≤ 6.0 mol·kg−1), at T = 298.15 K. Dependence on the ionic strength of protonation constants was modeled by the modified SIT approach on the molal concentration scale. From total solubility, we calculated the solubility of the neutral species S0, and from these values, as a function of NaCl concentration, the Setschenow (km) coefficient was obtained, together with the values of KS0.
01/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: Protonation constants of succinic, 1,2,3-propanetricarboxylic and 1,2,3,4-butanetetracarboxylic anions were determined in NaCl(aq)+KCl(aq) mixtures, at three ionic strengths, I=1.2, 3 and 4.5molL(-1). Experimental evidences showed that the function log K(H)=f(y) (y=[Na(+)]/([Na(+)]+[K(+)])) is not linear, indicating mixing effects on the protonation constants. The Guggenheim zeroth approximation holds that the above function can be written as:where K(Na)(H)andK(K)(H) represent protonation constants in pure salt solutions and Delta is a parameter that accounts for the mixing effect. Fitting of protonation constants to the above function gives excellent results. The Delta values can be treated in terms of mixing free energy. The behaviour of protonation constants in mixed salt solution can be interpreted by considering the formation of simple and mixed weak complexes; the protonation constants in mixed NaKCl electrolytes can be fitted to the equation: log(10)K(Na-K)(H)=log(10)K(K)(H)-log(10)(1+A(1)C(Na)+A(2)C(Na)C(K)), where A(1) is a measure of the interaction of Na(+) with the carboxylic anion and A(2) is proportional to the triple interaction Na(+)-K(+)-L(z-). Moreover, by using suitable calculation methods, it is possible to calculate the formation constants of simple and mixed ion pairs. As an example, for 1,2,3,4-butanetracarboxylic anion (L(4-)), we calculated K(Na(+)+H(i)L((4-i)-)=NaH(i)L((3-i)-))=0.67, 0.33 and 0.13; K(K(+)+NaH(i)L((3-i)-)=KNaH(i)L((2-i)-))=1.41, 1.29 and 0.9 for i=0, 1 and 2, respectively, indicating a significant tendency to form mixed alkali metal ion pairs.
Talanta 06/2007; 72(3):1059-65. · 3.79 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The solubility of two dicarboxylic acids (COOH)–(CH2)n–(COOH) (n = 4, 5; namely: adipic and pimelic acids, respectively) was determined in NaCl(aq) (0.5–4.5) mol L−1, (CH3)4NCl(aq) (0.25–3) mol L−1, (C2H5)4NI (0.25–1) mol L−1 and in pure water, at t = 25 °C. The activity and Setschenow (k) coefficients of neutral species were calculated in all ionic media from solubility data. These coefficients, in NaCl ionic medium, were also calculated by distribution (2-methyl-1-propanol/aqueous solution) measurements. The solubility trend is (C2H5)4NI > (CH3)4NCl > NaCl. To fully characterize the acid–base behaviour of the dicarboxylic acids, the self-interaction (ki) parameters from solubility and distribution data and the dimerization constant (KD) from the protonation constants in the same experimental conditions, were calculated. From these data, together with solubility ones, Pitzer and SIT parameters were calculated.
Fluid Phase Equilibria.
