Complete Resolution and Thermodynamic Parameters of the Ionization Equilibria of Pyridoxal in Water—Dioxane Mixtures

A potentiometric method has been used to determine the thermodynamicequilibrium constants for the macroscopic ionization processes of pyridoxal inwater—1,4-dioxane mixtures (0–70% weight fraction dioxane)at temperatures ranging from 10 to 50°C. These data, combined with the equilibrium constants for the tautomericand hemiacetalization processes, allow complete resolution of the microconstantsystem and calculation of the microscopic ionization equilibrium constants underall our experimental conditions. The standard thermodynamic function changesfor the macroscopic and microscopic ionization processes were obtained in variouswater—1,4-dioxane mixtures at 25°C. The values of any given microscopic pKfor the different solvents and temperatures fit very well to a single equation. Thefree energy, enthalpy, and entropy obtained for the different ionization processesin water—dioxane mixtures correlate with Kamlet and Taft's solvatochromicparameters ^* and , which are a measure of the dipolarity/polarizability andhydrogen-bonding capacity of the solvent, respectively. These correlations explainmore fully the mutual compensation between the contributions of enthalpy andentropy and the origin of the solvent effect on the pK.     

Solvent and Structural Effects on the Photoreduction of Cobalt(III)-Aryl Amine Complexes in Water–Organic Solvent Media*

Photoreduction of [Co(En)₂Cl(RC₆H₄NH₂)]²⁺ ions (where R = p-OMe, p-OEt, p-Me, m-Me, H, p-F, and m-OMe) in varying compositions of water–methanol and water–1,4-dioxane mixtures containing 15–40% (vol.) of organic co-solvent is carried out. Ultraviolet excitation of the above complexes in air-equilibrated solutions causes bleaching of its intense LMCT excited states with concurrent production of Co²⁺ion. As seen from the quantum yield data, _Co(II) increases smoothly with increasing content of organic co-solvent in the binary mixtures. The observed values indicate that the metal center is reduced by both ligands and solvent. The quantum yield _Co(II) is considerably affected by the substituent R of the amine, RC₆H₄NH₂ ligand. The effects of solvent and substituted ligands on the _Co(II) are estimated quantitatively using linear regression and multiple correlation methods. The former analysis was carried out using Grunwald–Winstein (Y) Gutmann donor number (DN ^N) and Krygowski–Fawcett E ^N _T (solvent empirical parameters). In addition, Kamlet–Taft's , , and * solvatochromic parameters were also used to study the effect of solute–solvent interaction. The effect of substitution on the aromatic amine ligand affects the quantum yield values, which was established using Hammett's substituent constant . Extensive tabulations of percentage contributions of these parameters, calculated using methods reported earlier, provide suitable values which are presumed to explain the quantitative effects of solvent and structural changes in the aromatic ligand on photoreduction of the cobalt(III) complexes.     

Correlation analysis of reactivity in the reduction of trans-chloro(RC6H4NH2)bis(1,2-diaminoethane)cobalt(III) complexes by hexacyanoferrate(II) in aqueous methanol

The structural and solvent effects on the reduction of trans-chloro(RC₆H₄NH₂)bis(1,2-diaminoethane)cobalt(III) ion (R=H, p-Me, p-OMe, p-OEt, p-F, m-Me and m-OMe) by the hexacyanoferrate(II) complex was investigated in aqueous MeOH. The second order rate constants for these reactions were determined spectrophotometrically at three different temperatures and activation parameters have been computed. Reduction proceeding through ion-pair formation is proposed on the basis of the Laidler–Eyring and Grunwald–Winstein equations. The influence of added co-solvent on the reactivity has been analysed using multiple regression equations viz. Kamlet–Taft and Swain. The reduction rates were correlated with Hammett's substituent constants yielding a negative reaction constant.     

Thermodynamics of the dissociations of glycine in 50 mass % aqueous monoglyme from 5 to 55°C

Electromotive-force measurements on cells without liquid junction have been used to determine the pK ₁ and pK ₂ values of glycine in 50 mass % aqueous monoglyme at 11 temperatures from 5 to 55°C. The change in the first dissociation constant is given as a function of the thermodynamic temperatureT by the equation pK ₁=–2058.6/T+15.421–0.019169T, whereas that for the second dissociation constant is given by the equation pK ₂=1200.5/T+6.7211–0.0042897T. At 25°C, the pK ₁ is 2.806 in the mixed solvent, as compared with 2.350 in water; hence, protonated glycine becomes a weaker acid in the mixed solvent. The pK ₂ is 9.453 in the mixed solvent, whereas that in water is 9.780, suggesting that the second dissociation process becomes stronger in terms of acidity. The thermodynamic quantities G ^o, H ^o, S ^o, and C _p ^o have been calculated, and the results have been discussed with respect to preferential solvation and also compared with similar data for the same two processes in 50 mass % methanol.     

Vibrational spectral studies of ion-ion and ion-solvent interactions. II. Zinc nitrate in water/dioxane mixtures

Raman and infrared spectral data have been collected forp-dioxane and solvated and bound nitrate ions for Zn(NO₃)₂/p-dioxane/water systems. It is concluded that Zn²⁺ is preferentially solvated by water and that this aquation is also responsible for a lower concentration of ion pairs than is found for methanol/Zn(NO₃)₂ solutions for which the dielectric constant of the bulk solvent is similar. Values of K₁ (M^–1), the association constant for Zn(NO₃)⁺, are 0.22±0.02 (2/1 solvent,D=12.6) and 0.071±0.01 (4/1 solvent,D=33.0). The log K₁ against 1/D plot is not linear.     

Dissociation constants of the ampholyte glycine in 50 mass % methanol-water from 5 to 55°C,with related thermodynamic quantities

The two thermodynamic dissociation constants of glycine at 11 temperatures from 5 to 55°C in 50 mass % methanol-water mixed solvent have been determined from precise emf measurements with hydrogen-silver bromide electrodes in cells without liquid junction. The first acidic dissociation constant (K ₁)for the process HG⁺H⁺+G^± is expressed as a function ofT(^oK) by the equation pK ₁ = 2043.5/T – 9.6504 + 0.019308T. At 25°C, pK ₁is 2.961 in the mixed solvent, as compared with 2.350 in water, with H°=1497 cal-mole^–1, G°=4038 cal-mole^–1, S°=–8.52 cal-°K^–1-mole^–1, and C _p ^o =–53 cal-°K^–1-mole^–1. The second acidic dissociation constant (K ₂)for the process G^±H⁺+G^– over the temperature range studied is given by the equation pK ₂ = 3627.1/T – 7.2371 + 0.015587T. At 25°C, pK ₂is 9.578 in MeOH–H₂O as compared with 9.780 in water, whereas H° is 10,257 cal-mole^–1, G° is 13,063 cal-mole^–1, S° is –9.41 cal-°K^–1-mole^–1, and C _p ^o is –43 cal-°K^–1-mole^–1. The protonated glycine becomes weaker in 50 mass % methanol-water, whereas the second dissociation process becomes stronger despite the lower dielectric constant of the mixed solvent (=56.3 at 25°C).     

Solute-solvent effects in the acidic dissociation of the ampholyte N-tris(hydroxymethyl)methylglycine (“tricine”) in 50 mass % methanol-water solvent

The pK values for the two acidic dissociation steps of the ampholyte N-tris-(hydroxymethyl)methylglycine (tricine) in 50 mass % methanol-water solvent have been determined by emf measurements of cells of the type Pt|H₂(g, 1 atm), tricine buffer, Br^–, AgBr|Ag over the range 5 to 50°C (pK ₁)and 5 to 60°C (pK ₂).The standard thermodynamic quantities H^o, S^o, and C _p ^o for the two dissociation processes have been derived and are compared with the corresponding values for tricine and the parent glycine in water and with those for other acids in 50 mass % methanol-water solvent. Both tricine and protonated tricine become weaker acids when methanol is added to the aqueous solvent. It appears that a strong stabilization of the zwitterion in water is responsible for this behavior. This conclusion is supported by comparing the changes in entropy and heat capacity for the dissociation of tricine with the values of these quantities for the dissociation of model acids of simple structure, such as ammonium ion and acetic acid.On leave 1971–1973 from Drury College, Springfield, Missouri     

Electrochemical study of 18-crown-6-Tl+ complexes in binary solvent mixtures

Summary The formation constants,K _S, of the 18-crown-6 complex with thallium(I) ion were studied by polarographic measurements in binary mixtures of acetonitrile, acetone, tetrahydrofuran, and dimethylsulfoxide with water, as a function of the solvent mole fraction. In all the cases, the variation of the stability constant can be described by the empirical relation logK _S=a[(–1)/(2+1)]+b where stands for relative permittivity of a given mixture anda andb mark the regression coefficients. The values ofa calculated for four series of binary mixtures showed correlation with the Gutmann donor numbers of the neat organic solvents which form the mixture.On leave from the Department of Chemistry, Jingzhou Teacher's College, Jingzhou, Hubei, China     

NMR investigation of the micellar properties of monoalkylphoshpates

The micellization of several monoalkylphoshpates with chain lenghts from 6 to 10 carbons have been investigated by³¹P and¹³C NMR spectroscopy with some additional electrical conductivity, density and small angle neutron scattering measurements.The major part of this study was performed on sodium monooctylphosphate. The critical concentration of micellization is 0.14 M, the partial molar volume is 170 cm³/mole in the micellar state. Small angle neutron scattering measurements on a 0.5 molar solution lead to an aggregation number of 43. NMR studies over the whole concentration range show that the pseudophase model holds. The geometry around the polar head is found almost unchanged upon micellization. The frequency denpendence of the³¹P longitudinal relaxation is interpreted by an isotropic fas motion ( ^R=6.5 × 10^–11 for the free monomers and by a strongly anisotropic motion withD /D 10 and ₀=(6D)^–1 =1.25×10^–9 s in the micellar state.     

Thermodynamic Properties of Alanylpeptide Buffer Systems and Their Potential as Standards in Biological Applications

The second dissociation constants pK ₂of the NH₃ ⁺charge center of the alanylpeptides, alanylglutamine (Ala–Gln), alanylleucine (Ala–Leu), alanylglycine (Ala–Gly), and DL-alanyl–DL-methionine (DL-Ala–DL-Met) were determined at ten temperatures in the range, 5–50°C. These pK ₂values were calculated from the emf of cells containing buffer solutions of these dipeptides. A cell of the type described by Harned and Ehlers,⁽¹⁾utilizing hydrogen and silver–silver bromide electrodes was used. The thermodynamic quantities, H^o, S^o, and C_p ^owere derived from the temperature coefficients of the dissociation constants. The pK ₂values at 25°C, 8.2105 ( Ala–Gln), 8.2668 ( Ala–Leu), 8.2940 ( Ala–Gly), and 8.3054 ( DL-Ala–DL-Met). These values show that different substituent groups on the -carbon atom (which include polar and nonpolar groups), have a small effect on the dissociation of the NH₃ ⁺charge center. These compounds were also found to be suitable as buffers in the pH range(7–9). The thermodynamics of the solute–solvent interaction is interpreted in terms of the mixture model.⁽²⁾     

Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C

The dissociation constant of NH ₄ ⁺ and the associated thermodynamic functions H° and S° in synthetic seawaters of salinity 20.31, 35.00, and 44.55 have been determined by emf measurements of cells without liquid junction over the temperature range 5 to 40°C. Cells with hydrogen electrodes and silver-silver chloride electrodes, whose standard potentials in seawater media were determined in an earlier investigation, were used. At a given temperaturet (in °C), pK _a varies linearly with the formal ionic strengthI _f (uncorrected for ion pairing) according to pK _a=pK _a ^w +(0.1552–0.0003142t)I _f where pK _a ^w is the pK in pure water. The medium effect of seawater on H° amounts to less than 200 cal-mole^–1 for a change inI _f from 0 to 0.72 mole-kg^–1, and S° is less than 1 cal-°K^–1-mole^–1 in all of the media studied, as it is in water. The observed salt effect on pK _a is in close agreement with values predicted from theories of ion-ion interactions.     

A conductance investigation of HCl in aqueous solutions of tetrahydrofuran,dimethoxyethane and dioxane

Conductance of dilute solutions of HCl in water-tetrahydrofuran (THF), water-dioxane and water-dimethoxyethane (DME) were measured and the data processed using the Justice-Ebeling equation to evaluate the limiting conductance _O, the association constant K_A and the apparent distance of closest approach a'. The Friedman-Rasaiah Gurney cosphere overlap term d_+–/kT was calculated from a' values and illustrates the different solvation pattern of dioxane compared to that found for THF and DME. It would appear that dioxane can solvate cations more strongly due to its ability to form a boat configuration around a cation. The association constants are reasonably reproduced by the Bjerrum equation using realistic molecular dimensions of the ions involved for the distance of closest approach. The limiting conductance in the mixtures illustrates clearly the different mechanism for proton conductance compared to that for the cesium ion. The dielectric constants, viscosities and densities of the solvent mixtures were measured and are reported. The lowest dielectric constant for each type of solvent mixture was about 10.     

Spectrophotometric and potentiometric determination of acidic constants of oxo-phenyl pyridinium monoxime and dioxime

The UV absorption spectra of 1-(1-hydroxyimino-2-oxo-2-phenyl) pyridinium chloride (compound I) and 1-(1-hydroxyimino-2-oxo-2-phenyl)-4-hydroxyiminomethyl pyridinium chloride (compound II) in water solution at differentpH values have been measured. The spectral changes, with changingpH, in aqueous solutions are attributed to the dissociation of individual functional groups of the compounds. The mixed acidic constants (pKa) of the investigated monoxime and dioxime, have been determined spectrophotometrically in the series ofBritton-Robinson's buffer solutions in thepH range 3.0–5.19 and 7.70–9.90 (t=25±0.5°C,I=0.2). The followingpKa values have been obtained for monoximepKa ₁=4.30 and for dioximepKa ₁=4.28,pKa ₂=8.36.Thermodynamic acidic constants (pKa) have been determined on the basis of potentiometric titrations and they have been found to bepKa ₁=4.32 for compound I andpKa ₁=4.27,pKa ₂=8.51 for compound II. The values obtained by transferringpKa intopKa are in good agreement with the values obtained potentiometrically.

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Spektrophotometrische und potentiometrische Bestimmung der Aciditätskonstanten von Oxo-Phenyl-Pyridinium-Monooxim und -Dioxim

Zusammenfassung Die UV-Absorptionsspektren von 1-(1-Hydroxyimino-2-oxo-2-phenyl)-pyridiniumchlorid (Verbindung I) und 1-(1-Hydroxyimino-2-oxo-2-phenyl)-4-hydroxyiminomethylpyridiniumchlorid (Verbindung II) wurden in wäßrigen Lösungen bei verschiedenenpH-Werten aufgenommen. Die Änderungen in den Spektren, die in wäßrigen Lösungen mit derpH-Änderung entstehen, können der Dissoziation der einzelnen funktionellen Gruppen der untersuchten Verbindungen zugeschrieben werden. Die Mischaciditätskonstanten (pKa) des untersuchten Monooxims und Dioxims wurden spektrophotometrisch in einer Reihe vonBritton-Robinson-Pufferlösungen inpH-Intervallen 3.0–5.19 und 7.70–9.90 (t=25±0.5°C;I=0.2) bestimmt: für das MonooximpKa ₁=4.30 und für DioximpKa ₁=4.28 undpKa ₂=8.36. Die thermodynamischen Aciditätskonstanten (pKa) wurden aufgrund der potentiometrischen Titration berechnet:pKa ₁=4.32 für die Verbindung I undpKa ₁=4.27 undpKa ₂=8.51 für die Verbindung II. Die durch ÜbertragungpKa inpKa erhaltenen Werte sind mit den über die potentiometrische Methode erhaltenen Werten in guter Übereinstimmung.

     

Thermodynamic Constants of Glycyl Peptides: Solvation and Buffers for the Physiological pH Range

The pK ₂ values for the dissociation of the NH ₃ ⁺ charge center of the glycyl peptides, e.g., glycyl-D-asparagine, glycyl-DL-serine, glycyl-L-leucine, and glycyl-DL-methionine have been determined at 10 temperatures in the range 5–50°C by measurements of the emf of cells without liquid junction, utilizing hydrogen electrodes and silver–silver bromide electrodes. The thermodynamic quantities, H ^o, C _p ^o and S ^o were calculated from the temperature coefficients of the dissociation constants. The pK ₂ values at 25°C are 8.268 (glycyl-D-asparagine), 8.277 (glycyl-DL-serine), 8.323 (glycyl-L-leucine), and 8.408 (gly-cyl-DL-methionine). These values show that changes in the substituents on the -carbon atom have very little effect on the dissociation of the NH ₃ ⁺ , with the possible exception of glycyl-DL-methionine. The suitability of these compounds as buffers in the physiologically important pH range 7–9 is of interest. The thermodynamics of the solute–solvent interaction is interpreted in terms of a mixture model. The peptides chosen for study include both polar and nonpolar substituents.     

A New Relation Between the Maxima Conductivities of Nonaqueous Concentrated Electrolytes and Chemical Hardness of Solvents and Salts

For nonaqueous electrolytes, using the HSAB principle, we tried to correlate the conductivity maxima _MAX, vs. only two intrinsic parameters: chemical hardness of the solvent and that of the salt. Thus, not only the nature of the solvent but also that of the salt were taken into account. We were able to predict for a given solvent the variation of _MAX as a function of the hardness of the salt and that of the solvent: _MAX = K(1 – ||/_SOLVENT) with || = |_SOLVENT – _SALT| and K a constant in S-cm^–1 independent of the salt, but not of the solvent.     

Study of Solvent Composition Effects on the Protonation Equilibria of Various Anilines by Multiple Linear Regression and Factor Analysis Applied to the Correlation Between Protonation Constants and Solvatochromic Parameters in Ethanol–Water Mixed Solvents

The stoichiometric protonation constants (log) of aniline derivatives were determined potentiometrically over a wide range of solvent composition (0–0.74 mole fraction of ethanol). To explain the variation of the log values obtained over the whole composition range studied, the quasi-lattice quasi-chemical theory of preferential solvation was applied. The results were discussed in terms of macroscopic properties of the mixed solvent and different microscopic parameters, such as the Kamlet–Taft solvatochromic parameters to identify the solvent characteristics affecting the log values. Kamlet and Taft's general equation was reduced to two terms by using both multiple-linear regression analysis and combined factor analysis and target factor analysis in these mixtures: the independent term and the hydrogen-bond donating ability (HBD), which is a solvatochromic parameter. Hammett's reaction constant for the protonation of anilines has been obtained for all the solvent mixtures and correlates well with (HBD) of the solvent.     

Anion dependence of crown ether selectivities for alkali picrates and sulfonates in dioxane and toluene. A synergistic effect

The binding constants,K _N, of sodium and potassium 8-anilinonaphthalene-1-sulfonate (ANS) and of sodium 5-dimethylamino-1-naphthalenesulfonate (DNS) to benzo-18-crown-6 bound to a 2% cross-linked polystyrene network (RN18C6) were measured spectrophotometrically in dioxane and the results compared with those obtained for picrate salts. The network RN18C6 was then used to measure in dioxane and toluene by a competition method the equilibrium constant,K, of the reaction A^–M⁺N+CrA^–M⁺Cr+N.A^–M⁺N denotes the ionic solute (ANS, DNS, methyl orange or picrate salt) bound to the network RN18C6 (N) and A^–M⁺Cr is the solute bound to a soluble ligand Cr, where Cr represents a series of 18-crown-6 and 15-crown-5 compounds. Combining theK _N andK values the formation constants,K _L, of the crown ether complexes of the respective salts were obtained in dioxane. The data show a reversal in the complexation strength of the 18-crown-6 compounds in dioxane when sodium picrate is replaced by sodium ANS. The results were rationalized in terms of a synergistic effect exerted by dioxane, with dioxane forming a 1:1 dioxanate with the crown ion pair complex. This effect is especially strong with ANS and with a rigid planar crown ether like dibenzo-18-crown-6. The binding constants,K _N, of NaANS and NaDNS to RN18C6 in dioxane are nearly three times larger than for sodium picrate, and the same holds for the potassium salts. Differences in anion interactions with the network appear to be a plausible cause for the anion dependence ofK _N.     

Molecular Association and Solvent Effects in the Electronic and NMR Spectra of the Tricyanoterpyridineruthenate(II) Complex

Solvent effects on the electronic structure and NMR spectra of the[Ru(terpy)(CN)₃] complex(terpy = 2,2,6,2-terpyridine) have beeninvestigated and interpreted using quantum mechanical semiempirical methods.A systematic splitting of the electronic bands in the visible is observed, in addition to theirbathochromic shifts depending on the acceptor number of the solvent. The solvent-inducedsplittings are rationalized in terms of the involvement of several charge—transfertransitions exhibiting distinct participation of the ruthenium (II), terpy, and CN^–orbitals. Direct evidence of solvent interaction at the cyanide ligands is observedin the [¹³C]NMR spectra. The most enhanced solvent effects are at the middle¹³C-4 and ¹H-4 atoms of terpy, in complete agreement with the theoreticalcalculations. In D₂O, stacking interactions involving the terpy ligands are detectedfrom the [¹H]NMR spectra, leading to a bimolecular association constant of90 mol^–1-dm³. In acetone—D₂O mixed solvents, the spectroscopic changes areindicative of preferential solvation departing from the ideal linear behavior.     

Solvent effects on acid-base behavior: Five uncharged acids in water-sulfolane solvents

An electrometric titration method utilizing glass electrodes and silver-silver bromide electrodes in a cell without liquid junction has been applied to a determination of the dissociation constants of five uncharged weak acids (HA) in four mixtures of water and sulfolane (tetramethylene sulfone) at 25°C. The acids studied were monochloroacetic, formic, benzoic, and acetic acids andp-nitrophenol, and the mole fractions (x ₂) of sulfolane in the mixed solvents were 0.2, 0.4, 0.6, and 0.8. All measurements were made at a constant ionic strength of 0.02 mole-kg^–1 in a solution containing NaBr at a molality of 0.01 mole-kg^–1. The cell was standardized by measurements of HClO₄ at molalities from 0.002 to 0.01 mole-kg^–1 and, in subsequent measurements, a solution of NaA was titrated with HClO₄. To obtain the thermodynamic pK, an activity correction derived from the Debye-Hückel theory was applied. The pK of all five acids was found to vary linearly withx ₂ in the range 0 to 0.8. By comparison of data for acetic acid in water-methanol, water-dioxane, and water-sulfolane solvents, it was shown that the results are consistent with the known properties of sulfolane (low acidity, basicity, and solvating power), but the linear variation of pK with solvent composition remains unexplained.On leave 1973–1975 from the University of Gdansk, Poland.     

Conductance of Selected Alkali Metal Salts in Aqueous Binary Mixtures of 2-Methoxyethanol at 25°C

Precise conductance measurements have been performed for lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, sodium perchlorate, and sodium tetraphenylborate in 2-methoxyethanol–water mixtures at four different mole fractions, i.e., 0.056, 0.136, 0.262, and 0.486 of 2-methoxyethanol (69.73 D 26.55) at 25°C in the concentration range 0.0004–0.0642 mol-dm^–3. The limiting molar conductivity °, the association constant K _A, and the association distance R for the solvent mixtures have been evaluated from the conductance concentration data using the 1978 Fuoss conductance equation. The single-ion conductances have been estimated using the reference electrolyte tetrabutylam-monium tetraphynylborate(Bu₄NBPh₄). The analysis of the data indicates that for most salts ion association is appreciable in the solvent mixtures with a mole fraction of the cosolvent of 0.262 or higher. The results have been interpreted in terms of ion-solvent interactions and structural changes in the mixed solvent media.