Protolytic Equilibria Research Articles

The thermal effects of acid-base interactions in aqueous solutions of D,L-α-alanyl-glycine

The heat effects of protolytic equilibria of D,L-α-alanyl-glycine at 298.15 K and ionic strengths of 0.1, 0.3, 0.5, and 1.0 (KNO3) were determined by direct calorimetry. The standard thermodynamic characteristics of these equilibria were calculated. The results were compared with the data on related compounds.

Multiwavelength spectrophotometric determination of acidity constants of some azo dyes

A multiwavelength spectrophotometric titration method was applied to study the acidity constants of some azo dyes in water. The UV–vis absorption spectra of azo dye solutions were recorded in the course of their pH-metric titration with a standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated. The quantitative effects of the substituents on the acidity of the studied azo dyes were investigated by the linear free energy relationship (LFER) using Hammet sigma constant ( σ) and field and resonance effects of Kamlet and Taft ( f and ℜ, respectively).

Thermodynamic characteristics of protolytic equilibria of β-alanylglycine in aqueous solution

Thermodynamic characteristics of protolytic equilibria of β-alanylglycine in aqueous solution

Protolytic Equilibria in Tetrazoles

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Effect of Ethanol on the Protolytic Properties of the Vitamins B Group

A multiwavelength spectrophotometric titration method was applied to study protolytic constants of four water-soluble vitamins, folic acid(vitamin B 9 or B 0 ), thiamine(vitamin B 1 ), riboflavin(vitamin B 2 ) and pyridoxal (vitamin B 6 ) in binary ethanol-water mixtures at 25°C and an ionic strength ofO.lM NaNO 3 . The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components has been calculated from the curve fitting of the pH-absorbance data with appropriate mass balance equations by an established factor analysis model. DATAN program was used for determination of acidity constant and SPECFIT program was used for calculation of standard deviations and partial correlation coefficients. A glass electrode calibration procedure based on the four parameter equation pH=α+SpcH+JH + [H + ]+ JOH - K w /[H + ] based on the Gran's plots was used to obtain pH-readings in the concentration scale (p c H). The effect of the solvent on the protolytic constants was discussed.

Protolytic equilibria in tetrazoles

Published data on protolytic equilibria involving tetrazole hetero ring are summarized. The information is systematized according to the types of protolytic equilibria.

The thermodynamic characteristics of complex formation between vanadium(V) and malonic acid

Coordination and protolytic equilibria in the system vanadium(V)-malonic acid were studied calorimetrically at 298.15 K and I = 1.0(NaClO4). The heat effects of formation of the VO2CH2(COO) 2 − complex and step protonation of the malonate ion were determined for the first time. The data obtained were used to calculate the thermodynamic characteristics (logK, ΔG, ΔH, and ΔS) of the equilibria studied.

Protolytic properties of silica modified with maleinized linseed oil

The protolytic equilibria on the surface of silicas coated with a layer of maleinized linseed oil (MLO), a new type of carboxylic cation exchangers, were studied using potentiometric titration. The ionexchange capacity of the sorbents was determined, a parameter that first increases and then decreases with increasing concentration of MLO on the silica surface. For the dibasic carboxy groups of MLO, pKa=3.99±0.04 and 4.73±0.04, values close to pKa for succinic acid, a homogeneous analogue of the system under study.

3,3,3-Trifluoro-N′-(3-trifluoromethylphenyl)-1,2-propanediamine and its N-mono-and N,N-dicarboxyethyl derivatives: synthesis, protolytic and complexation properties

3,3,3-Trifluoro-N′-(3-trifluoromethylphenyl)-1,2-propanediamine (5) was synthesized by the reaction of 2-diazo-1,1,1-trifluoro-3-nitropropane or 3,3,3-trifluoro-1-nitropropene with 3-aminobenzotrifluoride followed by the reduction of the nitro group. The Michael 1,4-addition of diamine 5 to acrylic acid occurs only at the N(1) atom and affords N-mono-or N,N-dicarboxyethyl derivatives 6 and 7, depending on the reactant ratio. Protolytic equilibria 5–7 in aqueous solutions were studied by pH-potentiometry and UV spectroscopy. Only the aliphatic amino group can be protonated in an aqueous solution, while the aromatic amino group remains unprotonated even in 12 M HCl. The stability constants of transition metal (Cu2+, Ni2+, Zn2+) complexes with ligands 5–7 were determined by pH-potentiometric titration. The stability of the complexes and selectivity of the ligands toward Cu2+ ions increase with an increase in the number of N-carboxyethyl groups.

Thermodynamic Characteristics of Acid-Base Reactions of Aromatic Derivatives of α-Aminopropionic Acid in Aqueous Solution

The thermal effects of acid-base reactions of phenylalanine and tryptophan in aqueous solutions (ionic strength 0.5, 1.0, and 1.5; supporting electrolyte KNO3) at 298.15 K were determined calorimetrically. The thermodynamic characteristics of stepwise dissociation of the amino acids in aqueous solution were calculated. The influence of the structure of the amino acids on the thermodynamic parameters of the protolytic equilibria was examined.

Effect of the Medium on the Acid-Base Properties of α-Aminophosphoryl Compounds

Structural thermodynamic analysis of the acidity of α-aminophosphoryl compounds in water-isopropanol systems was carried out by means of evaluation of the Gibbs energies of transfer of species involved in protolytic equilibria. Protonated (deprotonated) forms of structurally diverse aminophosphoryl compounds similarly interact by the mechanism of specific and universal solvation with each of the water-isopropanol solvents (100, 50, and 25 vol % of water) in the reaction series used. For this reason, there are significant linear correlations between acidity parameters for various media, which makes possible successful predictions of the acid-base properties of structurally diverse compounds of the class studied in water-isopropanol media with various water contents.

Properties of silicas chemically modified by monodentate amines studied by conductometry

The protonation of aminoalkyl groups covalently bonded on the silica surface was studied by the conductometric titration method. Porous varieties of silica can adsorb HCl from an aqueous solution. Conductometric titration was proposed for the determination of concentrations and constants of protolytic equilibrium of grafted amino groups. During HCl chemisorption the effect of temperature on the electric conductivity of suspensions of the modified silicas was studied.

Copper(II) complexes with N-(2-carboxyethyl)anthranilic acid H2CEAnt. Synthesis and crystal structure of [Cu(CEAnt)(H2O)] ⋅ H2O

Protolytic equilibria and complexation of N-(2-carboxyethyl)anthranilic acid (H2CEAnt) with copper(II) ions in aqueous solutions were studied by UV spectroscopy and pH potentiometry. The H2CEAnt compound has no zwitterionic structure, and the protons are localized on the carboxy groups. The acid ionization constants of H3CEAnt+ (T = 25 °C, I = 0.1 M KNO3) are pK0 = 1.3±0.2 (=NH2+), pK1 = 3.88±0.02 (Alk-COOH), and pK2 = 5.28±0.02 (Ar-COOH). The model of complexation of H2CEAnt with copper(II) ions involves two deprotonated complexes [Cu(CEAnt)] and [Cu(CEAnt)2]2− (logβ = 6.31±0.04 and 8.0±0.2, respectively). The [Cu(CEAnt)(H2O)]⋅H2O complex was synthesized by the reaction of H2CEAnt with (CuOH)2CO3, and its structure was established by X-ray diffraction. The coordination polyhedron of Cu is intermediate between the tetragonal pyramid and trigonal bipyramid. The CEAnt2− ligand serves as a tetradentate chelating bridging ligand (Cu-O, 1.944(3) and 1.950(3) A; Cu-O', 2.195(4) A; Cu-N, 2.016(5) A), and the fifth position of the polyhedron is occupied by a water molecule (Cu-Ow, 1.976(4) A).

PHOTO-PK OF INDOLINE-2-THIONE

The photochemistry and photokinetics of indoline-2-thione (1) has been investigated by continuous illumination at 334 nm and flash photolysis in aqueous solutions as a function of pH. The photokinetic behaviour and product distribution were almost independent on the presence of oxygen in the solution. Whereas indole is formed as the main product in acidic solutions biindolyl was found as the main product in unbuffered solution. For the quantum yield f of photodegradation of 1 a marked increase was observed in acidic solutions. The pH dependence follows a Henderson-Hasselbalch relation with a pKa of 2.2. This photo-pK does note correspond to a pKa in the ground state. It is tentatively assigned to the protolytic equilibrium 31 + H+ ® 31H+ in the excited triplet state, evidence for which is found in the pH-dependence of transient absorption observed in laser flash photolysis experiments. Our findings support the intermediacy of a 2-mercapto-indole tautomer of 1 in the photochemical desulfurization mechanism as suggested previously by Nishio.11

Preparation and Photophysics of 2-(1-Pyrenyl)acrylic Acid and Its Methyl and 2′,2′,6′,6′-Tetramethyl-4′-Piperidyl Esters

Novel probes represented connection of pyrene as chromophore and sterically hindered amine stabilizers (HAS) in the form of esters of 2-(1-pyrenyl)acrylic acid were synthesized. HAS was in the form of parent amine (PAP) as well as stable nitroxyl radical form (PAP-NO.). Photophysics of these probes were compared with their precursor as 2-(1-pyrenyl)acrylic acid (PAA) and its methyl ester (PAM). The fluorescence spectrum of PAA strongly depends on the acidity of the solution. The spectrum in neutral methanol indicates that it originates from the anionic form -COO(-). Changes of acidity or basicity of methanol solution resulted in the changes of shape, position as well as the intensity of fluorescence band. This is due to the presence of protolytic equilibria, either in the ground state or in the singlet excited state, leading to the formation of molecular form -COOH and the cationic form -COOH(2) (+). The ester analogues did not show any changes in various pH conditions. Fluorescence of all probes depends on the polarity of solvents and the presence of oxygen. Intermolecular quenching was studied with external quenchers TEMPO and oxygen and the data were compared with the intramolecular quenching using 1'-oxo-2',2',6',6'-tetramethyl-4'-piperidinyl-2-(1-pyrenyl)acrylate (PAP-NO.).

Multiwavelength spectrophotometric determination of protolytic constants of 4-(2-pyridylazo) resorcinol (PAR) in binary DMF–water mixtures

A multiwavelength spectrophotometric titration method was applied to study the protolytic constants of 4-(2-pyridylazo) resorcinol(PAR), in binary DMF + water mixtures. UV–vis absorption spectra of PAR solution were recorded in the course of pH-metric titration of acidic solutions of PAR with standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated from the fitting of the pH-spectral titration data with appropriate mass balance equations by a home written program according to an established target factor analysis. To precise determination of number of absorptive components a recently developed statistical indicator function (IND function) was used. A glass electrode calibration procedure based on a four-parameter equation pH = α + S p c H + J H + [ H + ] + J O H − K w / [ H + ] based on the Gran's plots was used to obtain pH readings in the concentration scale (p cH). It has been observed that there is an inverse relationship between second and third protolytic constants and mole fraction of DMF. The effect of the solvent on the protolytic constants was discussed.

Protolytic equilibria in homogeneous and heterogeneous systems of ketoconazole and its direct spectrophotometric determination in tablets

The acid-base equilibria of a diprotic, slightly hydrosoluble base ketoconazole were studied in homogeneous and heterogeneous water systems. The determinations were performed at 25?C at a constant ionic strength of M(NaCl). The acidity constant Ka1 was determined by potentiometric (pKa1 3.20) and spectrophotometric (pKa1 3.26) methods. A pKa2 constant of 6.10 was obtained based on the equilibrium constants pKs0 4.84 and pKs1 -1.26, determined in a heterogeneous ketoconazole system. The obtained values of the constants served to calculate the solubility and the distribution of the equilibrium forms of ketoconazole as a function of pH. On the basis of the distribution of the equilibrium forms of ketoconazole, a spectrophotometric method for the determination of its content in commercial tablets was developed. The determinations were performed at 225 nm in 0.1 M HCl. The method is simple and rapid and enables the direct spectrophotometric determination of the content of ketoconazole without previous isolation.

Ion pair formation in copper sulfate aqueous solutions at high temperatures

Ionic association between Cu 2+(aq) and SO 4 2 - ( aq ) has been studied in the temperature range (298 to 473) K using a spectrophotometric technique. Experiments were designed to minimize the contribution of other protolytic equilibria in solution. The values of the ionic association equilibrium constant at different temperatures and pressures were fitted to an appropiate equation that allows the calculation of the thermodynamic quantities for states close to the saturation line. Using Bjerrum’s model for ionic association evidence for two ion pair populations was obtained. The process of ion pairing is discussed and a possibility to reconcile the continuum model (Bjerrum) with molecular simulation results is suggested.

Reichardt betaines as combined solvatochromic and acid-base indicators in microemulsions

Protolytic equilibria and absorption spectra of four solvatochromic Reichardt betaines different in their hydrophobicity are studied in direct microemulsions hydrocarbon-alcohol-surfactant-water (three different hydrocarbons, three alcohols, and seven surfactants including nonionic, cationic, and anionic ones were tested). Localization of even the most lipophilic betaines corresponds to the polarity index E of no less than 0.4, so that the occurrence of a purely hydrocarbon core in the microdroplets is unlikely probable. The apparent dissociation constants of the indicators are determined, and their dependence on the composition and concentration of the microemulsions is discussed.

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3Solutions

The acid-catalyzed aquation of [Cr(pic)(H2O)4]22+ and [Cr(dpic)(H2O)3]+(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO3 range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H+] have been established:kobs=a+b[H+](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and kobs=b[H+]+c[H+]2(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.