Protolytic Equilibria Research Articles

Protolytic and coordination equilibria in aqueous solutions of nickel(II) complexonates

Protolytic and coordination equilibria in ternary systems containing a nickel(II) salt, ethylenediaminetetraacetic acid, and monoamine complexones, such as iminodiacetic, hydroxyethyliminodiacetic, and nitrilotriacetic acids, were studied by absorption spectroscopy against the background of NaClO4 at I = 0.1 and T = (20 ± 2)°C. Experimental data were processed by mathematical models that were able of evaluating the possibility of existence of a wide spectrum of species in solution and to select the species sufficient for the experimental data to be reproduced from them. The compositions of polynuclear heteroligand complexonates were determined, the pH regions of their existence were established, and the equilibrium constants of reactions and the stability constants of the complexes were calculated.

Thermodynamic characteristics of the acid-base equilibria of taurine in aqueous solutions, according to calorimetry data

Enthalpies of the neutralization and protonation of taurine (HL) are measured by direct calorimetry at 298.15 K and ionic strengths of 0.3, 0.5, and 1.0 (KNO3). The standard thermodynamic characteristics of HL protolytic equilibria are calculated.

Enthalpies and constants of dissociation for D,L-Alanyl-D,L-Serine at 298 K

Protolytic equilibria in aqueous solutions of D,L-alanyl-D,L-serine are studied by means of potentiometry and calorimetry. The dissociation constants and thermal effects of this reaction of the dipeptide are determined at 298.15 K and ionic strengths of 0.1, 0.3, 0.5, and 1.0 (KNO3). The standard thermodynamic characteristics (pK°, ΔrG°, ΔrH°, and ΔrS°) of the studied equilibria are calculated. The final results are compared with the corresponding data on the related compounds.

Limit of concentration constant of protolytic equilibrium

Limit of concentration constant of protolytic equilibrium

Вплив замісників та характеру середовища на аналітичні властивості асоціатів ксантенових барвників з полігексаметиленгуанідиній хлоридом

The interaction of 5 anionic xanthene dyes – 9-R-2,3,7-trihydroxy-6-fluorones (ТОF) with cationic polyelectrolyte (PE) – polyhexamethyleneguanidine chloride (PHMG) has been studied by the optical spectroscopy in a concentration range where PHMG has a polyelectrolyte effect. Hammet σ-constants were calculated. The correlations between structure of ТОF and shift of protolytic equilibrium in aqueous-polyelectrolyte solutions were established. Influence of рН, concentration of the ethanol, components ratio in the system PHMG-TOF on the character of the interaction of cationic PE with ТОF and value of the analytical signal were studied based on absorption spectra. The results were compared to those obtained in water solutions.

Thermodynamic characteristics of the protolytic equilibria of tetramethylenediamine-N,N,N′,N′-tetraacetic acid

The stepwise dissociation constants of tetramethylenediamine-N,N,N′,N′-tetraacetic acid (H4L) are determined by means of potentiometry at 298.15 K and ionic strength values of 0.1, 0.5, and 1.0 (KNO3). The heat effects of the dissociation of the betaine groups of the complexone are measured by direct calorimetry. The standard thermodynamic characteristics of the protolytic equilibria of H4L are calculated via combined use of the results from thermochemical and potentiometric studies performed under identical experimental conditions. Our results are compared with the corresponding data on relative compounds.

Thermodynamic characteristics of protolytic equilibria of L-valyl-L-valine in aqueous solution

Thermodynamic characteristics of protolytic equilibria of L-valyl-L-valine in aqueous solution

Influence of temperature on the heats of acid-base reactions in L-glutamine aqueous solution

Protolytic equilibria in L-glutamine aqueous solutions were studied calorimetrically. The measurements were carried out at 298.15, 308.15, and 318.15 K and ionic strengths of 0.5, 1.0, and 1.5 (potassium nitrate). The thermodynamic parameters (pK, ΔG, ΔH, ΔS) of stepwise dissociation of L-glutamine both in water-salt solutions and in the standard solution were obtained.

Protolytic equilibria and density of solutions of weak electrolytes

Protolytic equilibria and density of solutions of weak electrolytes

Acid-base properties of α-aminomethylphosphine oxides

Ionization constants of a series of aminomethylphosphine oxides and respective amino precursors, as well as their isostructural amino phosphonates in water-2-propanol or water medium were determined. The main regularities of the effect of the phosphoryl group on the protolytic equilibria involving mono- and bisphosphorylated amines and diamines were found. The environment effect on the ionization constants of hydrophilic aminophosphoryl compounds and amines was estimated quantitatively. The relationship between the dissociation constants of aminophosphoryl compounds and related constants of amino precursors in the framework of the principle of linearity of free energies was demonstrated.

Correlation of the Extraction Constant Values of Zn2+ and Cd2+ from Phosphoric Acid Media by Bis(2,4,4-trimethylpentyl)dithiophosphinic Acid Dissolved in Toluene

Values of the extraction constants of Zn2+ and Cd2+ from aqueous phosphoric acid solutions (0.36 to 7.31 mol⋅L−1) by Cyanex 301 in toluene, involving formation of the complexes ZnR2 and CdR2 with R being bis(2,4,4-trimethylpentyl)dithiophosphinate, have been correlated at T=298 K as a function of the ionic strength. For this purpose the activity coefficients of all of the aqueous species have been calculated taking into account both the protolytic equilibria of concentrated phosphoric acid and complexation reactions between the cations and the phosphoric acid species. Good correlations have been obtained for the extraction constant values with the ionic strength, provided the release of water molecules during the extraction processes is considered. Finally, extraction constant values are reported at infinite dilution.

The Mechanism and Kinetics of the Oxidative Polymerisation of N-Ethylaniline

In the present work an examination is made of the kinetics of the oxidative polymerisation of N-ethylaniline in an aqueous hydrochloric acid solution, with ammonium peroxydisulphate as the oxidant. The key role of protolytic equilibria in this process is shown, a kinetic model is proposed, and its parameters are determined.

Thermochemical characteristics of nicotinamide protolytic equilibria in water-dimethylsulfoxide mixtures

The heat effects of nicotinamide protonation in water-dimethylsulfoxide (DMSO) solutions over the concentration range 0–0.75 DMSO mole fractions were determined calorimetrically at 25.00 ± 0.01°C and ionic strength 0.25 (NaClO4). Changes in the enthalpy of protonation as the content of DMSO increased were found to be described by an S-shaped curve. This curve shape was caused by the dynamics of reagent solvation contributions as the concentration of DMSO grew with the predominance of the nicotinamide solvation contribution.

Protolytic equilibria of picric acid in a mixture of protophobic and protophilic polar solvents non-donors of H-bonds

A new differentiating medium for the study of prototropic equilibria is proposed: acetonitrile with a small addition of DMSO. From the dependence of the picric acid pKa on the content of DMSO in this medium the optimum composition of the mixture was found ensuring the stability of the pKa values with minimal addition of DSMO: 4 wt %. It was experimentally confirmed that the addition of water in an amount up to 0.1 wt % has virtually no effect on the measured pKa value in a mixture with the optimum composition.

Thermodynamic characteristics of protolytic equilibria of L-serine in aqueous solutions

The heat effects of the reaction of aqueous solution of L-serine with aqueous solutions of HNO3 and KOH were determined by calorimetry at temperatures of 288.15, 298.15, and 308.15 K, and ionic strength values of 0.2, 0.5, and 1.0 (background electrolyte, KNO3). Standard thermodynamic characteristics (Δr H o, Δr G o, Δr S o, ΔC o ) of the acid-base reactions in aqueous solutions of L-serine were calculated. The effect of the concentration of background electrolyte and temperature on the heats of dissociation of amino acid was considered. The combustion energy of L-serine by bomb calorimetry in the medium of oxygen was determined. The standard combustion and formation enthalpies of crystalline L-serine were calculated. The heats of dissolution of crystalline L-serine in water and solutions of potassium hydroxide at 298.15 K were measured by direct calorimetry. The standard enthalpies of formation of L-serine and products of its dissociation in aqueous solution were calculated.

Thermodynamic characteristics of protolytic equilibria of hexamethylenediamine-N,N,N′,N′-tetraacetic acid

The heats of dissociation of betaine groups of hexamethylenediamine-N,N,N′,N′-tetraacetic acid (H4L) at 298.15 K and the ionic strengths of 0.1, 0.5, and 1.0 (KNO3) were determined by direct calorimetry. The standard thermodynamic characteristics of the ptotolytic equilidria of H4L were calculated using the results from thermochemical and potentiometric measurements made under identical test conditions.

Supramolecular assembly of hoechst-33258 with cucurbit[7]uril macrocycle

Molecular assemblies of potential guest molecules through non-covalent host-guest interactions have found immense use in many applied areas. In this study supramolecular interaction of a biologically important dye Hoechst-33258 (H33258) has been investigated in aqueous solutions at different pHs, in the presence of a macrocyclic host, namely, cucurbit[7]uril (CB7). The pH dependent emission behaviour of H33258 is inherently connected with its protolytic equilibria which allow the dye in different geometrical conformations. This pH dependent structural orientation is greatly affected by the complexation with CB7. The significant structural changes in the monocationic H33258 brought out by CB7 at pH 7 have been documented in the fluorescence emission and lifetime data, which are comparatively less affected in case of the dicationic form, which is prominent in dye solutions at pH 4.5. The strong ion-dipole interactions provided by the carbonyl portals of the CB7 host adequately stabilize the CB7-H33258 complex, both in 1:1 and 2:1 stoichiometries at both the pH conditions. The Job's plot method, fluorescence anisotropy, NMR measurements and geometry optimization calculations confirm the stoichiometric arrangement and are found to be tunable with the addition of metal ions. The non-covalently stabilized assembly brings out large enhancement in the fluorescence emission due to the unique structural orientation attained by H33258, which reduces the non-radiative relaxation pathways. Comparison of the spectral data of the dye at different pH conditions in the absence and presence of CB7 proposes a large upward pK(a) shift due to CB7 encapsulation, thus providing a handy tool to modulate the photophysical characteristics of the guest molecules.

Insight into the degradation of a manganese(III)-citrate complex in aqueous solutions

AbstractKinetics of the electron transfer process between citrates and manganese(III) ions has been studied in acidic aqueous solutions. Acidification of the reaction mixture increased the reaction rate. The reaction is dependent on pH because there are two main protolytic forms of the Mn(III)-citrate complex in the studied pH range (4.5–6.5). Reduction potentials of Mn(III)/Mn(II) system in acidic and basic solutions as well as protolytic equilibria play a crucial role in understanding the pH profile of the studied system. The rate constants for Mn(III)citH and Mn(III)citH2+ species degradation processes are presented (citH3− and citH22− are trivalent and divalent anions of citric acid, citH4, respectively). Protolytic constant (expressed as pK′a) for Mn(III)citH protonation is estimated and discussed.

ChemInform Abstract: pH-Induced Effects on the Photophysics of Dipyridyl Ketones

The photophysics of dipyridyl ketones (DPKs) has been investigated in aqueous solutions as a function of pH using pulsed and steady-state emission spectroscopy and nanosecond laser flash photolysis. All six DPK isomers were found to emit phosphorescence in aqueous frozen solutions at 77 K, but only three of them exhibited phosphorescence emissions and showed triplet transient absorption at room temperature. From both phosphorescence and triplet–triplet absorption titrations, they revealed proton-transfer reactions in the excited state occurring at different pH/H0 values than the ground-state acid–base equilibrations. Comparing results from different techniques gave evidence that the carbonyl oxygen is involved in the triplet state protonation (pK*≈ 3.5–6) for the isomers where it conjugates with the ring nitrogen atom, while proton addition prevalently occurs on the nitrogen for the unconjugated meta–meta isomer. A second protolytic equilibrium (pK*≈–2 to –1.6) implies formation of nitrogen diprotonated ions for all the isomers. The presence of two positive charges in the molecules prevents further protonation at the carbonyl group in the ground states as well as in the excited states.

The influence of β-cyclodextrin on acid–base and tautomeric equilibrium of fluorescein dyes in aqueous solution

The protolytic equilibrium of fluorescein in aqueous solutions was studied in the presence of cycloheptaamylose (β-cyclodextrin, or β-CD). The constants of stepwise ionization of the dye ( H 3 R + ⇆ H 2 R ⇆ HR - ⇆ R 2 - ), K a0, K a1, and K a2 were determined using vis-spectroscopy at ionic strength 0.05 M (NaCl + buffer) and 25 °C. In the presence of 0.0086 M β-CD, the indices of ionization constants are as follows: p K a0 = 1.21 ± 0.12, p K a1 = 5.08 ± 0.03, p K a2 = 6.35 ± 0.02. The changes in these p K as, as compared with the values determined without cyclodextrin, are unequal. Namely, the p K a0 value decreases by 1.0, while the p K a1 value increases by 0.7. Thus, the introduction of β-CD allows to govern the ratios K a0/ K a1 and K a1/ K a2, which are equal to, respectively, 141 and 151 in water, and 7.4 × 10 3 and 18.6 with cyclodextrin added. Rationalization of the observed phenomenon is possible taking into account the detailed scheme of protolytic equilibrium. Conclusions concerning tautomerism of dye molecules were deduced from absorption spectra; the fractions of tautomers, tautomerization constants, and microscopic ionization constants were evaluated. These data allow concluding that the main reason for the aforementioned p K a alterations is the binding of H 2R by the cyclodextrin cavity accompanied by turning these neutral species into the colorless lactone. The host–guest interaction of neutral species of fluorescein isothiocyanate, 2,7-dichlorofluorescein, and 3′,4′,5′,6′-tetrachlorofluorescein also results in the cyclodextrin-assisted shift of tautomeric equilibrium. Such nature of interactions is proved by the addition of competing agents, camphor-4-carboxylic acid and sodium n-nonylsulfonate, which results in the removing of neutral dye species from the cycloheptaamylose cavity.