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The current work investigates the protonation constants of gabapentin (GP), 2-[1-(aminomethyl)cyclohexyl]acetic acid and the stability constants for the binding of GP to Ca2+ and Mg2+ in a wide range of temperature and ionic strength conditions [283.15 ≤ T/K ≤ 318.15 and ionic strengths of NaCl(aq), 0.12 ≤ I/mol·dm−3 ≤ 4.84]. The pH-potentiometric titration method was applied for gathering experimental data and determination of solution equilibrium constants. The ΔpK method was used for the determination of calcium and magnesium stability constants due to the low values which were predicted. A Debye–Hückel type equation, Specific Ion Interaction Theory, Pitzer and van't Hoff equations were used for the modeling of ionic strength and temperature effects. Two species, ML and MHL, were found according to the best model for Ca2+ and Mg2+. Both protonation processes are exothermic based on the enthalpy values at 298.15 K and infinite dilution. A case study has been performed taking into account the speciation of GP in seawater. [ABSTRACT FROM AUTHOR]

Protonation of nicotinic acid has been investigated by means of potentiometric titrations at different temperatures 283.15 ≤ T (K) ≤ 383.15 and ionic strengths of NaCl(aq), 0.12 ≤ I (mol·kg−1) ≤ 4.84 and mixed NaCl with MgCl2 or CaCl2. Stability constants of the CaL+ and MgL+ species were obtained by means of the ΔpK method. Different models (e.g. Debye–Hückel type equation, Specific Ion Interaction Theory, Pitzer and van't Hoff) were applied to account for the ionic strength and temperature dependences, in order to obtain data in a standard state and parameters to calculate stability constants in any point of our experimental domain. Speciation studies were performed simulating the conditions of natural fluids. Literature data were investigated for comparison with the experimental results here obtained. [ABSTRACT FROM AUTHOR]

A potentiometric method has been used for the determination of the protonation constants of N-(2-hydroxyethyl)iminodiacetic acid (HEIDA or L) at various temperatures 283.15 ≤ T/K ≤ 383.15 and different ionic strengths of NaCl(aq), 0.12 ≤ I/mol·kg−1 ≤ 4.84. Ionic strength dependence parameters were calculated using a Debye-Hückel type equation, Specific Ion Interaction Theory and Pitzer equations. Protonation constants at infinite dilution calculated by the SIT model are log10TK1H=8.998±0.008 (amino group), log10TK2H=2.515±0.009 and log10TK3H=1.06±0.002 (carboxylic groups). The formation constants of HEIDA complexes with sodium, calcium and magnesium were determined. In the first case, the formation of a weak complex species, NaL, was found and the stability constant value at infinite dilution is log10KNaL = 0.78 ± 0.23. For Ca2+ and Mg2+, the CaL, CaHL, CaL2 and MgL species were found, respectively. The calculated stability constants for the calcium complexes at T = 298.15 K and I = 0.150 mol·dm−3 are: log10βCaL = 4.92 ± 0.01, log10βCaHL = 11.11 ± 0.02 and log10βCaL2 = 7.84 ± 0.03, while for the magnesium complex (at I = 0.176 mol·dm−3): log10βMgL = 2.928 ± 0.006. Protonation thermodynamic functions have also been calculated and interpreted. [ABSTRACT FROM AUTHOR]

Abstract  Protonation constants of one thiocarboxylate (thioacetate) and four sulfur-containing carboxylates (2-methylthioacetate, thiolactate, thiomalate, 3-mercaptopropionate) were determined by potentiometric measurements in a wide ionic strength range [0≤I≤5 mol⋅L−1 in NaCl and 0 ≤I≤3 mol⋅L−1 in (CH3)4NCl] at t=25 °C. For two of these ligands (2-methylthioacetate and thiolactate), the protonation enthalpies were also determined by calorimetric measurements in NaCl ionic medium [0 ≤I≤5 mol⋅L−1] at t=25 °C. Individual UV spectra of the protonated and unprotonated 3-mercaptopropionate species, together with values of the protonation constants, were obtained by spectrophotometric titrations. Results were analyzed in terms of their dependence on the ionic medium by using different thermodynamic models [Debye-Hückel type, SIT (Specific ion Interaction Theory) and Pitzer’s equations]. Differences among protonation constants obtained in different media were also interpreted in terms of weak complex formation. [ABSTRACT FROM AUTHOR]

Abstract  The formation constants of dioxouranium(VI)-2,2′-oxydiacetic acid (diglycolic acid, ODA) and 3,6,9-trioxaundecanedioic acid (diethylenetrioxydiacetic acid, TODA) complexes were determined in NaCl (0.1≤I≤1.0 mol⋅L−1) and KNO3 (I=0.1 mol⋅L−1) aqueous solutions at T=298.15 K by ISE-[H+] glass electrode potentiometry and visible spectrophotometry. Quite different speciation models were obtained for the systems investigated, namely: ML0, MLOH−, ML22−, M2L2(OH)−, and M2L2(OH)22−, for the dioxouranium(VI)–ODA system, and ML0, MLH+, and MLOH− for the dioxouranium(VI)–TODA system (M=UO22+ and L = ODA or TODA), respectively. The dependence on ionic strength of the protonation constants of ODA and TODA and of both metal-ligand complexes was investigated using the SIT (Specific Ion Interaction Theory) approach. Formation constants at infinite dilution are [for the generic equilibrium pUO22++q(L2−)+rH+ ⇌(UO22+) p (L) q H r (2p−2q+r);β pqr ]: log 10 β 110=6.146, log 10 β 11−1=0.196, log 10 β 120=8.360, log 10 β 22−1=8.966, log 10 β 22−2=3.529, for the dioxouranium(VI)–ODA system and log β 110=3.636, log 10 β 111=6.650, log 10 β 11−1=−1.242 for dioxouranium(VI)–TODA system. The influence of etheric oxygen(s) on the interaction towards the metal ion was discussed, and this effect was quantified by means of a sigmoid Boltzman type equation that allows definition of a quantitative parameter (pL 50) that expresses the sequestering capacity of ODA and TODA towards UO22+; a comparison with other dicarboxylates was made. A visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to better characterize the compounds found by pH-metric refinement. [ABSTRACT FROM AUTHOR]

Abstract  Results of an investigation on phytate interactions with zinc(II) cation in NaNO3aq at different ionic strengths (0.1≤I/mol⋅L−1≤1.0) are reported. Stability constants of various Zn i H j Phy(12−2i−j)− species were determined by potentiometry (ISE-H+ glass electrode) and the corresponding formation enthalpies by direct calorimetric titrations. Data obtained were used to provide an exhaustive speciation scheme of zinc(II) in the presence of phytate, as well as a comprehensive representation of the binding ability of phytate toward zinc(II) in different conditions. Different pL50 values [an empirical parameter already proposed, expressed as the −log 10 C Phy, where C Phy is the total phytate concentration necessary to bind 50% zinc(II)] were calculated in several conditions, and equations were formulated to model its dependence on different variables, such as ionic strength, temperature and pH. Other empirical predictive relationships are also proposed. [ABSTRACT FROM AUTHOR]

Abstract  The protonation constants for oxidized glutathione, H i−1L(4−i)−, K i H=[H i L(4−i)−]/[H i−1L(4−i)−][H] i=1,2,…,6 have been measured at 5, 25 and 45 °C as a function of the ionic strength (0.1 to 5.4 mol⋅[kg(H2O)]−1) in NaCl solutions. The effect of ionic strength on the measured protonation constants has been used to determine the thermodynamic values (K i H0) and the enthalpy (ΔH i ) for the dissociation reaction using the SIT model and Pitzer equations. The SIT (ε) and Pitzer parameters (β (0), β (1) and C) for the dissociation products (L4−, HL3−, H2L2−, H3L−, H4L, H5L, H6L2) have been determined as a function of temperature. These results can be used to examine the effect of ionic strength and temperature on glutathione in aqueous solutions with NaCl as the major component (body fluids, seawater and brines). [ABSTRACT FROM AUTHOR]

Abstract  The formation constants of dioxouranium(VI)—1,2,3,4,5,6-benzenehexacarboxy- late [mellitate(6−)] complexes were determined in NaCl aqueous solutions at 0.1 ≤ I ≤ 1.0 molL−1 and t = 25 ∘C by ISE-[H+] glass-electrode potentiometry. The speciation model obtained at each ionic strength includes the following species: ML4−, MLH3−, MLH2 2 −, MLH3 −, M2L2−, MLOH5− and ML(OH)2 6− (M = UO2 2 + and L6− = mellitate). The ionic strength dependence of the protonation constants of mellitate and of the metal-ligand complexes was investigated using the SIT (Specific Ion Interaction Theory) approach. Formation constants at infinite dilution are [for the generic equilibrium $$p{\rm UO}_{\rm 2}^{{\rm 2 + }} + q({\rm L}^{{\rm 6} - }) + r{\rm H}^{\rm + }\rightleftharpoons({\rm UO}_{\rm 2}^{{\rm 2 +}})_p({\rm L})_q {\rm H}_r^{(2p - 6q + r)};\,\beta _{pqr}$$]: log10 β 110 = 10.155, log10 β 111 = 16.084, log10 β 112 = 20.749, log10 β 113 = 24.038, log10 β 210 = 17.936, log10 β 11−1 = 2.327 and log10 β 11−2 = −6.804. Simple linear relationships between the formation constants and the stoichiometric coefficients of reactants are reported. The sequestering capacity of mellitate towards UO2 2+ was quantified using a sigmoid Boltzman-type equation. [ABSTRACT FROM AUTHOR]

Complex formation constants were determined potentiometrically (by a ISE-H+, glass electrode) in the systems, M2+ – Lz – H+ [M2+ = (C2H5)2Sn2+, Lz = malonate, glycinate and ethylenediamine] at t = 25 ∘C and 0.1 mol-L−1≰ I/ ≰ 1 mol-L−1 in NaClaq (0.1 mol-L−1 ≰ I ≰ 0.75 mol-L−1 for the ethylenediamine system). Thermodynamic values of formation constants, at infinite dilution, are [± 95% confidence interval, Tβpqr refer to the equilibrium, pM2+ + qLz + rH+ = MpLqHr(2+z+r)]: for malonate, log10Tβ110 = (5.47 ± 0.10); for glycinate, log10Tβ110 = (9.54 ± 0.08), log10Tβ111 = (12.97 ± 0.10); and for ethylenediamine, log10Tβ110 = (10.47 ± 0.10), log10Tβ120 = (16.17 ± 0.12) and log10Tβ111 = (15.46 ± 0.10). The dependence on ionic strength of the formation constants was modeled by a simple Debye–Hückel type equation and by the SIT approach. By analyzing the stability of the species in the three different systems we found a simple additivity rule that can be expressed by the relationship: log10K = 6.46 nN + 3.96 nO − 0.60 (nN2+ nO2), with a mean deviation, ɛ(log10K) = 0.15 (K = equilibrium constant for the interaction of the organometal cation with the unprotonated or protonated ligand, nN = number of amino groups and nO = number of carboxylic groups of the ligand(s) involved in the formation reaction of complex species). [ABSTRACT FROM AUTHOR]

Enthalpies of protonation of polyacrylates and polymethacrylates with different molecular weights in aqueous NaCl solutions, 0 ≤ I ≤ 2 mol-L−1 were determined by titration calorimetry at 25°C. Δ H values are dependent on both the neutralization degree, α, and the molecular weight of polyacids. TΔ S of protonation was obtained using pK values already reported and the present Δ H results. Empirical equations for the dependence on I, α, and molecular weight are reported for both Δ H and TΔ S. [ABSTRACT FROM AUTHOR]

The hydrolysis of trimethyltin(IV) has been studied by potentiometry (H+ -glass electrode) and calorimetry in various salt media (NaNO3, NaCl, KCl, Na2SO4, and NaNO3—NaCl mixtures). The effect of ionic strength on the hydrolysis constants is accounted for by a simple Debye–Hückel type equation and by Pitzer equations. The results allow us to obtain Δ H for hydrolysis and the temperature dependence of the Pitzer parameters. The resulting coefficients can be used to examine the speciation of (CH3)3Sn+ in multicomponent electrolyte solutions, such as natural waters, over a wide range of temperature and ionic strength. [ABSTRACT FROM AUTHOR]