The dissociation constant values of l-histidine, proline and tryptophane were determined at 25 ± 0.1°C by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, acetone and dioxane. Initial estimates of the dissociation constant values of the different amino acids studied have been refined with ESAP2M computer program. It was observed that changing the medium permittivity as the solvent is enriched in methanol or ethanol has little influence on the pK*a values of the amino acids studied. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents and the possible variation in microheterogeneity of the salvation shells around the solute. © 1994.

The second stage dissociation constants of N-[Tris-(hydroxymethyl)methyl]-2-aminoethane sulphonic acid (TES) were determined at 25±0.1 °C by potentiometric pH-titration in different mixed solvent mixtures. The organic solvents used were methanol, ethanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetone, and dioxane. The ionization of TES depends on both the proportion and the nature of the organic co-solvent used. It was observed that the pKa2 value is slightly influenced as the solvent is enriched in methanol, ethanol and remains practically constant in presence of different amounts of DMF and DMSO. A pronounced change in the pKa2 is observed as the solvent is enriched in acetone or dioxane. These results are discussed in terms of various solvent characteristics. It is concluded that the electrostatic effect has only a relatively small influence on the dissociation equilibrium of the cationic group - {Mathematical expression}H2. Other solvent effects such as solvent basicity, different stabilization of the conjugate acid free base by hydrogen bonding interactions in aquo-organic solvents media relative to pure aqueous media as well as proton-solvent interaction play an important role in the acid dissociation equilibrium. © 1994 Springer-Verlag.

The second-stage dissociation constants of N,N-bis(2-hydroxyethyl)glycine (bicine) were determined at 25.00 ± 0.02 °C by potentiometric pH titration in pure water and different mixed solvent mixtures (methanol, ethanol, -dimethylformamide, dimethyl sulfoxide, acetone, and dioxane). It is observed that the pKa2 value is slightly influenced as the solvent is enriched in methanol or ethanol and remains practically constant in the presence of different amounts of DMF and DMSO. A pronounced change in the pKa2 value is observed as the solvent is enriched in acetone or dioxane. These results are discussed in terms of various solvent characteristics. It is concluded that the electrostatic effect has only a relatively small influence on the dissociation equilibrium of the imino group (-NH+). Other solvent effects such as solvent basicity and different stabilization of the conjugate acid free base by hydrogen-bonding interactions in aquo-organic solvent media relative to pure aqueous media as well as proton-solvent interaction play an important role in the acid dissociation equilibrium. © 1994, American Chemical Society. All rights reserved.

Potentiometric equilibrium measurements have been made at 25.0±0.1 °C and ionic strength I=0.1 mol dm-3 KNO3 for the interaction of adenosine 5′-mono-, 5′-di-, and 5′-triphosphate (AMP, ADP and ATP) and Cu(II) with biologically important secondary ligand zwitterionic buffers (N,N-bis-(2-hydroxyethyl)-2-aminoethanesulphonic acid (BES), N-tris-(hydroxymethyl)-methyl-2-aminoethanesulphonic acid (TES), N,N-bis-(2-hydroxyethyl)-glycine (Bicine) and tris-(hydroxymethyl)-methylaminopropane sulphonic acid (TAPS)) in a 1:1:1 ratio and the formation of various 1:1:1 mixed ligand complex species inferred from the potentiometric pH titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of AMP, ADP, ATP, and secondary ligands have been refined with the SUPERQUAD computer program. Negative and positive Δ log K values were obtained for the ternary systems studied. In some Cu(II) ternary systems studied the interligand interactions or some cooperativity between the coordinate ligands, possibly H bond formation, has been found to be most effective in deciding the stability of the ternary complexes formed in solution. Stabilities of mixed ligand complexes increase in the order AMP<ADP<ATP. The trend in stability constants of the mixed-ligand complexes of the title zwitterionic buffer ligands is found to be TAPS>Bicine>TES>BES. © 1994 Springer-Verlag.

Potentiometric equilibrium measurements have been made for the interaction of adenosine-5′-mono-,-di-, and -triphosphate, and Co(II) with biologically important secondary ligands (malic, maleic, succinic, tartaric, citric, and oxalic acid). The formation of various 1:1:1 mixed ligand complexes inferred from the potentiometric titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of AMP, ADP, ATP and the secondary ligand acids have been refined with a computer program. In some systems, the ternary complexes are found to be more stable than the corresponding binary ones. In some ternary systems studied, interligand interactions or some cooperativity between the coordinate ligands, possibly H bond formation, have been found to be most effective in deciding the stability of the complexes formed in solution. Stabilities of mixed ligand complexes increase in the order AMP <ADP <ATP. With respect to the secondary ligands, the formation constants of the mixed ligand complexes decrease in the following order: succinic > maleic > tartaric > malic > citric > oxalic acid. © 1994 Springer-Verlag.

The acidity constants of adenosine-5′-mono- and diphosphate (AMP and ADP) were determined at 25.00±0.1°C by potentiometric titration in pure water and different solvent mixtures (methanol, ethanol, N,N-dimethylformamide, dimethylsulfoxide, acetone, and dioxane). The ionization of AMP and ADP depends on both the proportion and the nature of the organic solvent used. The pKa1 values for both AMP and ADP are slightly influenced as the solvent is enriched in ethanol and methanol and remains practically constant in presence of different amounts of DMF and DMSO. A pronounced change in the pKa1 values is observed as the solvent is enriched in acetone or dioxane. It is concluded that the electrostatic effect has only a relatively small influence on the dissociation equilibrium, whereas other solvent effects such as solvent basicity, hydrogen bonding and protonsolvent interactions play an important role. © 1994 Springer-Verlag.

The dissociation constant values of glycylglycine were determined at 25 ± 0.1 °C (I = 0.1 mol L−1 KNO3) with Potentiometric pH titration in pure water and various mixture of water and organic solvents. The organic solvents used were methanol, ethanol, N, N‐dimethylformamide, dimethylsulfoxide, acetonitrile and dioxane. Initial estimates of the dissociation constants of glycylglycine were refined with ESAP2M computer program. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents and the possible variation in microheterogeneity of the solvation shells around the solute. Copyright © 1995 The Chemical Society Located in Taipei & Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim, Germany

Potentiometrie measurements have been made at 25.0 ± 0.1 °C and ionic strength I = 0.1 mol dm‒3 KNO3 for the interaction of adenosine 5’-mono-, 5’-di-, and 5’-triphosphates (AMP, ADP, and ATP) and Co(II) with biologically important secondary ligand zwitterionic buffers N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), N-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid (TES), N,N-bis(2-hydroxyethyl)glycine (Bicine), and N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS) in a 1:1:1: ratio and the formation of various 1:1:1 mixed ligand complex species inferred from the Potentiometrie pH titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of the resulting species and the acid dissociation constants of AMP, ADP, ATP, and secondary ligands have been refined with the SUPERQUAD computer program. In some Co(II) ternary systems the interligand interactions have been found to be most effective in deciding the stability of the ternary complexes formed in solution. Stabilities of mixed ligand complexes increase in the order AMP < ADP < ATP. The trend in stability constants of the mixed-ligand complexes of the zwitterionic buffer ligands is in the order TAPS > Bicine > TES > BES. © 1995, American Chemical Society. All rights reserved.

The solvation number and hydration constant of Th4+ in ethanol-water medium were determined at 25°C using UV-spectral and electrochemical measurements. A solvate formation equilibrium is demonstrated and characterized. Three molecules of ethanol (S) can bond to the metal cation with strengths comparable to that for H2O to form ThS3(H2O)34+ Formation of thorium monochelate with lawsone (2-hydroxy-1,4-naphthoquinone) eliminates bonding with alcohol molecules. The dissociation constant of the chelating agent 5Ka and the formation contant of the monochelated metal ion ,K1* that are essentially independent of the solution composition are evaluated. Hydration titrations involving thoriumlawsone monochelate are performed and the data obtained from the changes of pH with solvent composition are analyzed. The solution independent constant, sKf* for thorium-lawsone complex formation in mixed aqueous ethanol is given by log sKf* = vpKa + logsKh - log [LH] - vpH + 3 log v where vpKa is the dissociation constant of the chelating agent LH in the solvent system of v volume fraction of water and sKh is the solution-independent hydration constant of thorium (IV) in the solvent system. Log-values for the constants sKh, sKf* and sKa* are found to be 7.8±0.02, 11.38+0.04 and -0.753, respectively.

The kinetics of the hydrolysis of hydrazpne derived from nitropyridine 2-carboxaldehyde and 2-hydrazino-4-substituted thiazoles la-e in 10% (v/v) DMF-buffer mixture has been investigated by applying two independent and different techniques viz UV-spectrophotometry and differential pulse, polarography (DPP). The hydrolysis has been found to follow general acid-base catalysis. Mechanism for the hydrolysis in acidic medium has been postulated. The observed rate constants, activation and thermodynamic parameters for the hydrolysis have been calculated. The effects of pH, molecular structure, temperature and change in percentage of water in the DMF-buffer mixture on the rate of hydrolysis have been discussed.