Speciation of binary complexes of Ca(II), Mg(II) and Zn(II) with L-3,4-dihydroxyphenylalanine (dopa) in (0 -60% vyv) dioxane (DOX) -water mixtures have been studied pH-metrically at a temperature of 303 K and at an ionic strength of 0.16 mol L À 1 . The models for binary species contain MLH, ML 2 H 2 , ML 2 H and ML 2 for Ca(II) and Mg(II) and MLH, ML, ML 2 H 2 , ML 2 H and ML 2 for Zn(II). The trend in the variation of stability constants with the mole fraction of dioxane was explained on the basis of electrostatic and nonelectrostatic forces. Distribution of the species with pH at different compositions of DOX -water media was also presented.
Protonation equilibria of L-Dopa and 1,10 phenonthroline have been studied in varying concentrations (0-60% v/v) of propylene glycol-water mixtures maintaining an ionic strength of 0.16 mol dm -3 at 303 K using pH metric method. The protonation constants have been calculated with the computer program MINIQUAD75 and the best fit models are arrived at based on statistical grounds employing crystallographic R factor, χ 2 , skewness and kurtosis. Dopa has three dissociable protons and one amino group which associate with proton. It exists as LH 4 + at low pH and gets deprotonated with the formation of LH 3 , LH -2and LH 2-successively with increase in pH. Phen forms LH 2 2+ at low pH and gets deprotonated with the formation of LH + and L with increase in pH. Secondary formation functions confirm the existence of 3 and 2 protonation equilibria for dopa and phen, respectively. The linear increase of log values of protonation constants of Dopa with decreasing dielectric constant of PG-water mixtures indicates the dominance of electrostatic forces in the protonation-deprotonation equilibria. Phen exhibits non-linear trend indicating the dominance of non-electrostatic forces.
Formation of metal ligand complex species of 5-sulphosalicylic acid with Ca
II
, Mg
II
and Zn
II
metal ions have been studied potentiometrically in various composition (0.0, 0.5, 1.0. 1.5, 2.0, and 2.5% w/v) of cetyltrimethylammonium bromide (CTAB). The study has been carried out at 303
K
temperature and maintaining 0.16 mol dm
−3
ionic strength using NaCl. The dynamic species belongs to 5-sulphosalicylic acid are LH
2
-
and LH
2-
. The best fit of complex speciation has been preferred on basis of statistical parameters like skewness,
χ
2
, Kurtosis and crystallographic
R
-factor. ML
2
H, ML
2
H
2
and ML
3
H
3
type complex were formed by the complexation of 5-sulphosalicylic acid with the metal ions. The change in stability of complex species with composition of the surfactant has been illustrated on the basis electrostatic grounds. Distribution of chemical species with respect to pH and various compositions medium and probable complex equilibria are also studied.
To gain more information about the effect of surfactant on salicylic acid derivatives, the stoichiometric protonation constants of 5-Sulphosalicylic acid and 5-Hydroxysalicylic acid in 0.0%-2.5% (w/v) cetyltrimethylammonium bromide (CTAB) -water mixtures were determined at an ionic strength of 0.16 mol dm -3 and at 303 K. A potentiometric method was used and the calculation of constants was carried out using the computer program MINIQUAD75. These protonation constants values have been found to shift in micellar media as compared to those in pure water. The differences in the values have been attributed to the solvent properties of the interfacial and bulk phases involving contribution from the micellar surface potential in the case of charged micelles. The trend of log values of step-wise protonation constants with mole fraction of the medium have been explained based on specific solute-solvent interactions. In this study Distributions of species, percentage of species composition, protonation equilibria and effect of influential parameters on the protonation were also discussed.
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