The Heat Effects of Malonic Acid Dissociation

The heat effects of malonic acid dissociation at 298.15 K and several ionic strength values in the presence of NaClO₄ were determined calorimetrically. The thermodynamic characteristics of dissociation at fixed ionic strength values and at I = 0 were calculated.     

The influence of the nature of the background electrolyte on the heat effects of dissociation of succinic acid

The results of calorimetric measurements were used to determine the heat effects of dissociation of succinic acid at 298.15 K and several ionic strength values in the presence of NaNO₃. The thermodynamic characteristics of dissociation at fixed ionic strength values and I = 0 were calculated.     

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.     

The heat effects of dissociation of maleic and fumaric acids

The heat effects of dissociation of maleic and fumaric acids at 298.15 K and several ionic strength values were determined calorimetrically in the presence of NaNO₃. The thermodynamic characteristics of dissociation at fixed ionic strengths and under standard conditions were calculated.     

Enthalpies of Dissolution of Crystalline Naproxen Sodium in Water and Potassium Hydroxide Aqueous Solutions at 298 K

The enthalpies of dissolution of crystalline naproxen sodium in water and aqueous solutions of KOH at 298.15 K are measured by direct calorimetric means in a wide range of concentrations. The acid–base properties of naproxen sodium at ionic strength I ~ 0 and I = 0.1 (KNO₃) and a temperature of 298.15 K are studied by spectrophotometric means. The concentration and thermodynamic dissociation constants are determined. The standard enthalpies of the formation of naproxen sodium and the products of its dissociation in aqueous solution are calculated.     

Thermochemistry of gaseous ethylsilanes and their radical cations

The dissociation behavior of energy-selected tetraethylsilane, triethylsilane, and diethylsilane photocations is studied using the threshold photoelectron-photoion coincidence (TPEPICO) technique. In the 8–12. 5 eV photon energy range, 0 K dissociation onsets have been measured from the TPEPICO data. The dissociation channels observed include loss of ethane, hydrogen molecule, ethyl radical and hydrogen atom, depending upon the molecular ion under investigation. The thermochemistry of the molecular ions and dissociation fragments is obtained by an analysis that takes into account the kinetics and internal energy distributions of the ions. The various dissociation onsets permit the reevaluation of both neutral and ionic silane thermochemistry. We observed 298-K ethyl group values of 60±10 and 94±10 kJ mol^?1 for neutral and ionic silanes, respectively. These values are considerably smaller than the previously reported values of 86 and 130 kJ mol^?1, respectively. Finally, a Δ _f H ^° (298 K) of ?141.5 ± 21 kJ/mol for neutral diethyl silane is derived from the dissociative ionization onset of diethylsilane.     

The heat effects of dissociation of N-(carboxymethyl)aspartic acid

The heat effects of dissociation of N-(carboxymethyl)aspartic acid were determined calorimetrically at 298.15 K and various ionic strength values. The standard thermodynamic characteristics of dissociation of the complexone at fixed and zero ionic strengths were calculated.     

Protonation equilibria studies of the standard α-amino acids in NaNO3 solutions in water and in mixtures of water and dioxane

The protonation equilibria for 20 standard α-amino acids in solutions have been studied pH-potentiometrically. The dissociation constants (pK_a) of 20 amino acids and the thermodynamic functions (ΔG^∘, ΔH^∘, ΔS^∘, and δ) for the successive and overall protonation processes of amino acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol · dm⁻³ NaNO₃, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature and ionic strength influencing the protonation processes of amino acids is presented and discussed to determine the factors which control these processes.     

Dissociation constants for the zinc(II)-8-hydroxyquinoline complexes in aqueous and sodium dodecyl sulfate micellar solutions

The constants of dissociation of 8-hydroxyquinoline (pK ₁ and pK ₂) and of its complexation with zinc ions (logβ₁, logβ₂) in sodium nitrate aqueous solutions at 293 K were calculated by modeling potentiometric titration data. The values of logβ were obtained under conditions such that the complexes occurred simultaneously in the solution and at optimum values of the ionic strength and the ratio between the metal and ligand concentrations. The apparent constants of dissociation of 8-hydroxyquinoline and of its complexation with zinc ions in a 0.01 M sodium dodecyl sulfate solution were calculated. An increase in pK and logβ was explained by the stabilization of the different forms of the ligand and its complexes in the surfactant micellar phase.     

Thermodynamic characteristics of acid–base equilibria of glycyl-glycyl-glycine in water–ethanol solutions at 298 K

The enthalpies of the acid dissociation of glycyl-glycyl-glycine zwitterions and triglycinium ions are determined calorimetrically in water–ethanol solvents containing 0.0, 0.10, 0.30, and 0.50 molar fractions of ethanol at ionic strengths of 0.1 (maintained by sodium perchlorate) and Т = 298.15 K. It is found that increasing the ethanol content in the solvent enhances the endothermic effect of triglycinium ion dissociation and reduces the endothermic effect of glycyl-glycyl-glycine dissociation. The results are discussed in terms of the solvation thermodynamics.     

Dissociation of acetic acid in 50 mass % ethylene carbonate-water from 20 to 55°C

The dissociation constant of acetic acid in 50 mass % ethylene carbonate-water solvents has been determined at eight temperatures from 20 to 55°C by emf measurements of cells without liquid-junction containing hydrogen electrodes and silver-silver chloride electrodes. Acetic acid is weaker in the mixed solvent than in pure water. At 25°C, pK is 5.645 as compared with 4.756 in water, in spite of the fact that the dielectric constants of the two media do not differ materially. Standard changes in Gibbs energy, enthalpy, entropy, and heat capacity for the dissociation processes at 25°C were evaluated. The decrease in acidic strength when ethylene carbonate is added to the water medium appears to be largely the results of a destabilization of the two ionic species, especially a partial desolvation of the hydrogen ion.     

Determination of stoichiometric dissociation constants of lactic acid in aqueous salt solutions at 291.15 and at 298.15 K

Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant, K_m, of lactic acid in aqueous salt solutions at 291.15 and 298.15 K from the thermodynamic dissociation constant, K_a, of this acid and from the ionic strength, I_m, of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study, and the equations for K_m were based on the single-ion activity coefficient equations of the Hückel type. New data measured by potentiometric titrations in a glass electrode cell at 298.15 K and the literature data obtained by Larsson and Adell with quinhydrone (qh) electrode cells at 291.15 K were used in the estimation of the parameters for the Hückel equations (HE) of lactate ions in NaCl and KCl solutions. The Harned cell data measured at 298.15 K by Nims and Smith were used to obtain the activity parameters for lactate ions in dilute LiCl, BaCl₂ and SrCl₂ solutions. The conductance data measured at 298.15 K by Martin and Tartar were used to determine the thermodynamic value of the dissociation constant of lactic acid. By means of the calculation method suggested in this study, K_m can be obtained almost within experimental error at least up to I_m of about 0.5 mol kg⁻¹ for lactic acid in NaCl and KCl solutions at the two temperatures considered. The K_m values obtained by this method were compared to those obtained by the calculation methods presented recently in the literature for a general treatment of thermodynamics of weak acids in NaCl and KCl 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.     

13C NMR measurements of acid-base equilibrium constants in alkaline aqueous solutions of D-glucose anomers

The concentration dissociation constants pK _c of the α and β D-glucose anomers were measured at pH 12–14 by the ¹³C NMR method. The pK _c values for L ? H_?1L + H equilibria at 25°C and an ionic strength of 1.0 mol/l (NaCl) were pK _c ^α = 12.31 ± 0.03 and pK _c ^β = 12.03 ± 0.01. Dissociation was found to shift all the spectrum lines downfield. The most probable hydroxyl group responsible for the dissociation of β-D-glucose was determined.     

Thermochemical Study of Acid-Base Reactions of L-Valine in Salt Solutions

Heat effects of protonation and neutralization of L-valine were measured at 298.15 K, ionic strengths of the solutions of 0.25, 0.50, and 0.75, and on lithium, sodium, and potassium nitrate backgrounds. The heat effects of dissociation and neutralization of L-valine were extrapolated to zero ionic strength by a one-parameter equation to find standard enthalpies of dissociation and neutralization of L-valine in aqueous solution. Temperature-dependent and temperature-independent components of thermodynamic values were determined.     

The step dissociation constants of ethylenediamine-N,N′-diacetic-N,N′-dipropionic and ethylenediamine-N,N,N′,N′-tetrapropionic acids

The step dissociation constants of ethylenediamine-N,N′-diacetic-N,N′-dipropionic and ethylenediamine-N,N,N′,N′-tetrapropionic acids at 298.15 K and ionic strengths of 0.1, 0.5, and 1.0 (KNO₃) were determined potentiometrically. The results were extrapolated to zero ionic strength by the equation with one individual parameter. The thermodynamic constants of dissociation were calculated. The results are compared with the corresponding data on related compounds.     

Evaluation of the chloramine-T membrane electrode response in acidic solutions. The determination of the pKa of N-chloro-p-toluenesulfonamide (chloramine-T acid)

Measurements made with a combination of glass and chloramine-T ion-selective membrane electrodes in acidified chloramine-T solutions varying in ionic strength have been used to determine the dissociation constant of N-chloro-p-toluenesulfonamide (chloramine-T acid). Experimental data agree satisfactorily with reported values.     

Thermodynamics of stepwise dissociation of D,L-tryptophan in aqueous solution

The heat effects of the reaction of a solution of D,L-tryptophan with solutions of KOH and HNO₃ were measured by calorimetry at temperatures of 288.15, 298.15, and 308.15 K and ionic strength values of 0.25, 0.5, and 0.75 (KNO₃). The effect of the temperature and concentration of the background electrolyte on the heats of dissociation of amino acid was considered. The standard heat effects of ionization of D,L-tryptophan were determined by extrapolation to zero ionic strength. The standard thermodynamical characteristics of the stepwise dissociation of D,L-tryptophan were calculated.     

Effect of temperature on the thermodynamic characteristics of the protolytic equilibria of DL-α-alanyl-β-alanine in aqueous solutions in the range of 298 to 318 K

Protolytic equilibria in aqueous solutions of DL-α-alanyl-β-alanine are studied by calorimetry. Measurements are performed at temperatures of 298.15, 308.15, and 318.15 K; and at ionic strengths of 0.5, 1.0, and 1.5 (against a background of potassium nitrate). Thermodynamic characteristics (pK, ΔG, ΔH, and ΔS) of the stepwise dissociation of dipeptides both in water-salt and standard solutions are obtained for the first time.