Ionization of aqueous dimethylbenzoic acids: Conductance and thermodynamics

Precise conductance measurements are reported for four xylic acids, 2,3-2,5-2,6 and 3,5-dimethylbenzoics. Limiting molar conductances Λ_O and pK _a obtained from a Fuoss type analysis, are reported for each of these acids at five degree intervals covering the range from 0° to 100°C. The Λ_O values for each acid are described by a polynomial in the Celsius temperature. The pK _a were smoothed as a function of the Kelvin temperature T with an equation of the form: $$pK_{\text{a}} = A{\text{ }} + {\text{ }}B/T{\text{ }} + {\text{ }}C{\text{ }}logT{\text{ }} + {\text{ }}DT$$ where the term linear in T was required only for the 2,3-acid. Standard enthalpy, entropy, and heat capacity changes were calculated by suitable differentiation of this equation. Walden products were calculated for the four anions at each of the temperatures and are compared with earlier data for the toluate and the benzoate ions. Those acids with ortho groups undergo a large decrease in enthalpy on ionization and are substantially more acidic than benzoic acid. These effects are especially large for 2,6-dimethylbenzoic acid. A methyl group in the meta position lowers acidity slightly both in m-toluic and in 2,5-dimethylbenzoic acid. However, 2,3-dimethylbenzoic acid is more acidic than o-toluic.     

Thermodynamics of the ionization of four difluorobenzoic and pentafluorobenzoic acids in water

Precision conductance measurements as a function of concentration and temperature between 0 and 100°C have been made for 2,3-, 2,5-, 3,4-, and 3,5-difluorobenzoic acids and pentafluorobenzoic acid in water. Standard state changes in enthalpy, entropy, and heat capacity as a function of temperature are reported for the ionization of these acids. Comparisons are made with other fluorobenzoic acids that have been reported earlier and with the corresponding methylbenzoic acids. The differences between the ionization of a fluorobenzoic acid and its counterpart methylbenzoic acid is almost entirely a difference in the entropies of ionization with only a small difference in the enthalpies of ionization. Each fluorine substituted in the meta or para position increases the entropy of ionization while substitution in the ortho position decreases the entropy. This is in contrast to the effect of methyl substitution where substitution in any position decreases the entropy of ionization. Walden products for the various anions follow the pattern found for the methylbenzoate anions with a sharp increase from 0 to about 30°C and with ortho substituted anions less mobile than those without ortho substitutents.     

Dimerization of some substituted benzoic acids in aqueous solution from conductance measurements

Precision molar conductances of benzoic, o-toluic, 2,6-dimethylbenzoic, 2,3,6-trimethylbenzoic, and, o-fluorobenzoic acids have been determined in aqueous solution as a function of temperature and of concentration up to near saturation (<0.035 M). At the higher concentrations molar conductances are found to be less than anticipated for the simple dissociation of a 1-1 electrolyte. Although the deviations are only 1% or less they have been interpreted to show that these acids are dimerized in solution. The interpretation includes an assumption that the dimer ionizes to produce a triple ion. Increasing numbers of methyl groups lead to increasing dimerization. For those acids with two ortho groups the dimerization increases with increasing temperature while the other three show decreasing dimerization with increasing temperature. Temperature functions have been determined for the dimerization constants and from these functions standard changes in enthalpy, entropy, and heat capacity have been determined. Comparisons are made with dimerization studies in non-aqueous solvents. From these as well as the behavior of benzene in water it is concluded that a major factor driving the dimerization is hydrophobic interaction. To provide a limiting conductance of the triple ion needed in the dimerization calculations a conductance study was also made for o-Phenylbenzoic acid on the assumption that its anion provides an approximate model of the triple ion.     

Ionization of aqueous toluic acids: Conductance and thermodynamics

Conductivities of aqueous solutions ofortho-, meta-, andpara-toluic acids have been measured for the concentration range 0.1–2 millimolar and at 5° intervals from 5 to 100°C. At each temperature pK _a(m) andA ₀ have been calculated using the paired ion model recently described by Fuoss. Thermodynamic parameters have been calculated for the ionization of each acid, and Walden products for the anions. Results are discussed in terms of contributions to acidity by enthalpy and entropy changes as well as by hydration of the various solute species.     

Thermodynamics of dissolving and solvation processes for benzoic acid and the toluic acids in aqueous solution

Experimental data are presented for the solubility in water of benzoic and toluic acids from 5° to 65°C. From the solubility the molality of the monomeric form of the acid is calculated using literature data for both ionization and dimerization of the acid. These data for the monomer combined with data from the literature for vaporization of the solid and ionization in both the gas phase and the aqueous phase yield entropy and enthalpy changes for the solvation of molecular and anionic forms of the acid. A similar procedure is also applied to literature data for the solubility of benzene in water. It is shown that the hydration entropies of the monomeric forms are a linear function of their partial molar volumes. It is concluded that hydration of the undissociated o-toluic acid may be crucial to the increased acidity of that acid compared to benzoic acid.     

Complexation of UVI with 1-hydroxyethane-1,1-diphosphonic acid in acidic to basic solutions

Complexation of UVI with 1-hydroxyethane-1,1-diphosphonic acid (HEDPA) in acidic to basic solutions has been studied with multiple techniques. A number of 1:1 (UO2H3L), 1:2 (UO2HjL2 where j = +4, +3, +2, +1, 0, and -1), and 2:2 [(UO2)2HjL2 where j = +1, 0, and -1] complexes form, but the 1:2 complexes are the major species in a wide pH range. Thermodynamic parameters (formation constants and enthalpy and entropy of complexation) were determined by potentiometry and calorimetry. Data indicate that the complexation of UVI with HEDPA is exothermic, favored by the enthalpy of complexation. This is in contrast to the complexation of UVI with dicarboxylic acids in which the enthalpy term usually is unfavorable. Results from electrospray ionization mass spectrometry and 31P NMR have confirmed the presence of 1:1, 1:2, and 2:2 UVIHEDPA complexes.     

Thermodynamics of ionization of 2,4-dinitrophenol in water-dimethylsulfoxide solvents

The ionization constants of 2,4-dinitrophenol were measured in water-dimethylsulphoxide solvent mixtures at five temperatures ranging from 20 to 40°C. The enthalpy and entropy contributions to the ionization process are discussed. The results indicate that in water-rich solvent mixtures the ionization process is controlled by the entropy factor while in the dimethylsulphoxide-rich solvent mixtures it is controlled by the enthalpy factor.     

Thermodynamical characteristics of acid-base equilibria in glycyl-glycyl-glycine aqueous solutions at 298 K

The constants and enthalpies of acid dissociation of glycyl-glycyl-glycine peptide in aqueous solutions at 298.15 K and ionic strength values of 0.1, 0.5, and 1.0 M containing NaCl as a background electrolyte, were determined by potentiometry and calorimetry. The standard values of pK ₁^° and pK ₂^° constants, the change of Gibbs energy, enthalpy, and entropy for the dissociation processes were calculated. It was found that the acidic properties of the carboxylic group in aqueous solution subside upon the transition from α-amino acids to peptides due to the lower entropy effect of dissociation. It was concluded that the increase of dissociation constant of a protonated peptide amino group upon an increase in the length of its molecule is determined by entropy factor associated with the attenuation of amino group solvation.     

Enthalpy and entropy changes associated with mixed ligand chelates of Cu(II) and Ni(II) with 4-methoxy picolinic acid N-oxide and some amino acids in aqueous medium

The stability constants of mixed complexes of Cu(II) and Ni(II) with 4-methoxy picolinic acid N-oxide, and glycine, α-alanine, proline and hydroxy-proline have been determined at various temperatures by the potentiometric method in 0·1 M ionic strength. The formation constants of the mixed complexes have been evaluated and are in good agreement with statistically expected values. The enthalpy and entropy values have been calculated from 1∶1∶1 stability constants temperature coefficient data. From the enthalpy values of the mixed complexes it may be concluded that the bond strengths are not equal to the average of the bond strengths inMA ₂ andMB ₂ type parent complexes. The entropy values have been found to be favourable for ternary complex formation.     

Study of the Enzymatic Activity of Arthrobacter ureafaciens Neuraminidase by Isothermal Titration Calorimetry

Isothermal titration calorimetry was used to determine the enzymatic activity and thermodynamic activation parameters of Arthrobacter ureafaciens sialidase with the sialyl substrates α‐2,3‐, α‐2,6‐sialyllactoses and α‐2,8‐sialic acid dimer. By monitoring the heat released during hydrolysis, the Michaelis constant (K_m), catalytic rate constant (k_cat), activation energy, activation Gibbs energy, enthalpy, and entropy for different monovalent sialyl conjugates were calculated and found to be consistent with those derived by chromatographic or colorimetric assays. The observed decreases in the activation energy and transition entropy of sialyllactoses were larger than the Michaelis activation parameters of lactose‐free di‐sialic acid because of the specific enzyme activity of A. ureafaciens sialidase.     

Calculation of the transport properties of aqueous species at pressures to 5 KB and temperatures to 1000°C

The limiting equivalent conductances at temperatures from 0° to 1000°C and pressures from 1 to 5000 bars of a large number of aqueous ions have been calculated from limiting equivalent conductances of electrolytes reported in the literature. The limiting equivalent conductances of individual ions typically increase by a factor of about 15 with increasing temperatures from 0° to 1000°C and decrease about 30 percent with increasing pressure from 1 to 5 kb. The equivalent conductance of H₂O approximated by the sum of the limiting equivalent conductances of H⁺ and OH^– is essentially independent of pressure, but increases from about 350 to a maximum of approximately 1800 S-cm²-equiv^–1 in response to an increase in temperature from 0° to 500°C at 1kb. Stokes' law radii and Walden products generated from the computed limiting equivalent conductances of ions exhibit changes over the temperature and pressure range of interest by as much as 100 percent for all of the ions except H⁺ and OH^–, which vary by an order of magnitude. Apparent solvation numbers calculated as a function of pressure and temperature from the Stokes' law radii using the volume and dielectric constant of H₂O and Born coefficients of the individual ions approach infinity at the critical point of H₂O. Residual friction coefficients as a general rule approach zero as temperatures increases to 1000°C. The excess limiting equivalent conductances of the hydrogen and hydroxyl ions computed from the differences between the limiting equivalent conductances of HCl and KCl, and NaOH and NaCl, respectively, increases with increasing pressure, and maximize at 250°C.     

Preparation of 5-substituted benzylbarburituric acids and investigation of the effect of the benzyl and substituted benzyl groups on the acidity of barbituric acid

The acidity of 5-benzylbarbituric acid and a series of 5-substituted benzylbarbituric acids has been determined in 50% ethanol/water and they were found to be more acidic than barbituric acid. The pK_as of these derivatives obey Hammett's equation indicating that their acidity is affected by substituents in the same manner as the benzoic acid ionization constants. A synthesis of these acids is described.     

枯草杆菌α-淀粉酶在一些色谱体系中的热力学和超热力学研究

通过测定不同温度范围的热力学平衡常数、焓变、熵变、自由能变和补偿温度,研究了枯草杆菌α-淀粉酶在几种色谱介质上的热力学和超热力学。结果表明,在RP-C18反相介质、Zn2+螯合的Sepharose fast-flow亲和介质和WCX-1阳离子交换介质上,当温度分别在13-30和30-50℃范围时,它们的lnKSL分别随绝对温度的倒数线性变化;而在PEG-400和修饰的PEG-400疏水色谱介质上,当温度分别在13-40和13-30℃范围时,它们的lnKSL分别随绝对温度的倒数线性减小,但当温度分别高于40℃和30℃时,它们则随绝对温度的倒数剧烈减小。通过研究不同温度范围的焓变、熵变、自由能变和α-淀粉酶构象变化之间的关系,发现在RP-C18反相和Zn2+螯合的Sepharose fast-flow亲和介质上在30- 50 ℃温度范围内,在WCX-1阳离子交换介质上在13-30 ℃温度范围内,α-淀粉酶的吸附过程由焓变和熵变共同所支配,而在Zn2+螯合的Sepharose fast-flow亲和介质上在13- 30 ℃温度范围内,在WCX-1阳离子交换介质上在30-50 ℃温度范围和在PEG-400 和修饰的PEG-400疏水色谱介质上在13-65 ℃温度范围时,α-淀粉酶的吸附过程仅仅由熵变所控制。最后,通过α-淀粉酶在这些色谱体系中的补偿温度进一步发现,它们的焓变仅仅只能通过它们构象变化所引起的熵变所补偿。     

A study of metal complexes of a naturally occurring macrocyclic ionophore-monensin

The acid-base and complexing properties of a naturally occurring antibiotic ionophore-monensin (MonH)-were studied in anhydrous methanol solutions primarily by potentiometric measurements. The pK _a of the acid was found to be 10.30±0.05 at 25°C. Complexes of silver, thallium, and alkali ions with the undissociated ligand were also studied, and their stability constants were determined. The acid dissociation constant of MonH as well as the stability constants of Mon^– Na⁺ and MonH·NaClO₄ complexes were determined in the 5–45°C temperature range, and the enthalpy and entropy of the acidity and stability constants were determined from van't Hoff's plots. The formation of MonH and Mon^–Na⁺ species are both enthalpy and entropy stabilized, but the formation of the MonH·NaClO₄ complex is enthalpy stabilized but entropy destabilized.     

Conductance study of the thermodynamics of some transition and heavy metal cryptates in binary acetonitrile-dimethylsulfoxide mixtures

A conductance study of the interaction between Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Zn²⁺ and Pb²⁺ ions with cryptands C211, C221 and C222 in different acetonitrile-dimethylsulfoxide mixtures has been carried out at various temperatures. The formation constants of the resulting metal cryptates were determined from the molar conductance-mole ratio data. It was found that the stability of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ cryptates vary in the order C211>C221>C222, while for Cd²⁺ cryptates the stability order is C221>C222>C211. A linear relationship is observed between logK _f of different metal cryptates and the mole fraction of acetonitrile in the solvent mixture. The enthalpy and entropy of cryptate formation reactions were determined from the temperature dependence of the formation constants. The enthalpy and entropy changes are quite sensitive to the solvent composition and the resultingTS ⁰–H ⁰ plot shows a fairly good linear correlation, indicating the existence of an entropy-enthalpy compensation in the cryptate formation reactions. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82181 (30 pages).     

Thermodynamic Properties of Peptide Solutions. Part 17. Partial Molar Volumes and Heat Capacities of the Tripeptides GlyAspGly and GlyGluGly,and Their Salts K[GlyAspGly] and Na[GlyGluGly] in Aqueous Solution at 25 °C

The partial molar volumes, V^o₂, and partial molar heat capacities, C_p,2^o, at infinite dilution have been determined for the two tripeptides glycylaspartylglycine (glyaspgly) and glycylglutamylglycine (glyglugly), and also for their salts K[glyaspgly] and Na[glyglugly], in aqueous solution at 25 °C. The ionization constants at 25 °C for the aspartyl and glutamyl side-chains have also been determined. These new thermodynamic results have been combined with literature data for electrolytes to obtain the volume and heat capacity changes upon ionization of the acidic side-chains of the peptides. The results are compared with those for other carboxylic acid systems. The partial molar heat capacities and volumes have also been used to calculate the contributions of the acidic amino acid side-chains to the thermodynamic properties.     

Ionization enthalpy of benzoic acid in water—dimethylsulfoxide mixtures

The ionization enthalpy of benzoic acid has been measured calorimetrically at 25°C in H₂ODMSO mixtures ranging from pure water to a maximum DMSO molar ratio X_DMSO = 0.80. With the increase of DMSO content, the ionization becomes more and more endothermic, and for X_DMSO = 0.8 the ionization enthalpy is about 6 kcal mol^?1 higher than in water. By also measuring the solution enthalpy of crystalline benzoic acid in the mixtures, it has been shown that the solvation of the undissociated molecule is the main cause for the increase of the dissociation enthalpy. A comparison has been made between the relative enthalpies of benzoic and hydroxide ions in H₂ODMSO mixtures.     

Aqueous iodic acid: Conductance and thermodynamics

Precision conductance measurements are reported on aqueous solutions of iodic acid for 16 concentrations between 17 and 0.7 mM and for 20 temperatures between 5° and 100°C. RlnK _a (m) and _o were calculated at each temperature and the data expressed by suitable temperature functions. From RlnK _a (m) as a function of temperature changes in standard enthalpy, entropy, and heat capacity were calculated. C _p proved to be independent of temperature so that H⁰ was a linear function of temperature. Comparisons have been made with other published data for iodic acid. The pattern of variation of Walden products with temperature was similar to that found earlier for substituted benzoic acids.     

Thermodynamic dissociation constants of salicylic and monochloroacetic acids in mixed solvent systems from conductance measurements at 25°C

The molar conductances of dilute solutions of salicylic and monochloroacetic acids in binary mixtures of methanol, ethanol, 1-propanol, N,N-dimethylformamide, acetone, acetonitrile, tetrahydrofuran and dioxane with water have been measured at 25°C. The Lee-Wheaton conductance equation was fitted to the data in order to derive thermodynamic dissociation constants and limiting molar conductances. The results were compared with those in the literature pertaining to analogous media, mostly derived potentiometrically. The findings are interpreted in terms of a solvent effect on the ionization of these acids in mixed solvent systems.This paper is dedicated to the late Professor Raymond M. Fuoss on his 3^rd anniversary.     

The association of aromatic anions in water

Molar conductances of dilute solutions of sodium 2-naphthalenesulfonate at 25°C and of sodium 2-anthraquinonesulfonate have been measured at 25 and 37°C. These data are interpreted to show that the anthraquinonesulfonate anion is dimerized in solution. Dimerization constants and the enthalpy and entropy of dimerization are calculated. Spectrophotometric absorbancies have been measured at 25, 30, 35 and 40°C for a series of aqueous solutions containing both sodium 4-dimethylaminobenzenesulfonate and sodium 3-nitrobenzenesulfonate. Equilibrium constants and enthalpy and entropy changes for formation of the 1 1 complex between these two ions have been calculated from these data. The formation of these complexes between like-charged ions, and of dimers of other aromatic solutes in water is discussed in terms of a two-state model of hydrophobic hydration.