Ionic Dissociation Constants Research Articles

Study of the absorption spectra and ionic equilibria of cationic-anionic polymethine dyes

The absorption spectra of a number of cationic-anionic polymethine dyes in polar, low-polarity, and nonpolar solvents were studied. It was established that in polar solvents the absorption spectra represent the sum of the absorption spectra of the cation and anion, whereas in nonpolar and low-polarity solvents, in many cases the appearance of a short-wave absorption band is observed, due to interaction of the chromophores of the cation and anion in ion pairs. In solvents of intermediate polarity (for example, in chloroform and ethyl acetate), a concentration dependence of the absorption spectra is observed, determined by the equilibria of ionic dissociation. The ionic dissociation constants of a number of cationic-anionic dyes in chloroform and ethyl acetate were determined by a conductometric method. The thermodynamic data obtained are compared with the spectral data.

Participation of the phenolic hydroxyl group of Tyr-8 in the catalytic mechanism of human glutathione transferase P1-1.

The coding region of cDNA corresponding to human class Pi glutathione transferase P1-1 was amplified by the PCR, subcloned into an expression vector, pKHP1, expressed in Escherichia coli, and characterized. The physicochemical and catalytic properties of recombinant glutathione transferase P1-1 were indistinguishable from those of the enzyme previously isolated from human placenta. The active-site residue Tyr-8 of the wild-type enzyme was converted into Phe by means of oligonucleotide-directed mutagenesis. The mutant enzyme Y8F displayed a 300-fold decrease in specific activity, ascribable mainly to a lowered k(cat.) (or V) value. Kinetic parameters reflecting binding affinity, S0.5 (substrate concn. giving 1/2V) and I50 (concn. of inhibitor giving 50% remaining activity), were only moderately elevated in the mutant enzyme. These results indicate that Tyr-8 contributes primarily to catalysis as such, rather than to binding of the substrates. The dependence of k(cat.)/Km on pH shows an optimum at pH 7.0, defined by acidic and basic ionic dissociation constants with pKa1 = 6.7 and pKa2 = 7.3 respectively. The mutant enzyme Y8F does not display the basic limb of the k(cat.)/Km versus pH profile, but shows a monotonic increase of k(cat.)/Km with an apparent pKa1 of 7.1. The results indicate that the phenolic hydroxyl group of Tyr-8 in un-ionized form, but not the phenolate of Tyr-8, contributes to catalysis by glutathione transferase P1-1.

Electrochemical and thermodynamic behaviour of oxygenated nitrogen compounds and aromatic hydrocarbons in nitromethane, and the nitration process with NO +2 (solvation phenomena)

The electrochemical properties of oxygenated nitrogen compounds in anhydrous deoxygenated nitromethane have been studied by means of linear and cyclic voltammetry at a platinum electrode. Taking into account previous works in sulpholane, the various electrochemical couples involving nitrogen oxides at oxidation states from II to V have been identified in nitromethane, and their standard potentials have been determined at 298 K. Furthermore we have induced electrochemically the molecular association: NO + 1 2 N 2O 4 → N 2O 3 (I) on a Pt electrode surface in order to reach the equilibrium constant of this reaction (I). All these findings have also allowed us to calculate the ionic dissociation constants of N 2O 3 and N 2O 4 according to N 2O 3 ⇆ NO + + NO − 2 (II) and N 2O 4 ⇆ NO + 2 + NO − 2 (III), respectively. It has been shown that thermodynamic data are comparable with those obtained in non-polar inert solvents and the standard potential of a couple hardly varies at all in various aprotic media if the nitrogen species involved in these chemical and electrochemical processes are solvated weakly (the solvent molecules barely intervene in these processes). On the other hand, we have undertaken the electrochemical study of aromatic hydrocarbons (ArH) in nitromethane in order to assess solvation effects on the aromatic radical cation (ArH + ) and hence to evaluate the standard potentials of ArH/ArH + couples. Finally, the survey of the redox process NO + 2 + ArH ⇆ NO 2 + ArH + (IV) in nitromethane has allowed us to obtain energetic and mechanistic information about the aromatic nitration with NO + 2 and to compare it with that found in other aprotic media by reference to solvation phenomena.

ChemInform Abstract: Electrochemical Study of the Molecular and Ionic Dissociation of NO2Cl in Aprotic Media.

AbstractThe ionic dissociation constant of NO2Cl (‐ (NO2)+ Cl‐) has been determined (potentiometrically, Ag ‐ AgCl indicator electrode) to be 4.0 · 10 exp‐18 in nitromethane and 2.0 · 10 exp ‐19 mol/l in propylene carbonate (298 K).

An electrochemical study of the molecular and ionic dissociation of NO 2Cl in aprotic media

The ionic dissociation constants of NO 2Cl: NO 2Cl ⇆: NO + 2 + Cl − in aprotic media, has been determined by means of the potentiometric technique (employing a silver chloride—silver indicator electrode). The electrochemical properties of NO 2Cl have been studied by a voltammetric technique at the platinum electrode. The mechanism of NO 2Cl reduction is complicated by water at trace levels in the medium. The NO 2Cl species undergoes three consecutive reduction steps: 2 NO 2Cl + ρ − ⇆ NCl + NO − 3 + 1 2 Cl 2 (I) 2 NO 2Cl + 2 ρ − ⇆ NOCl + NO − 3 + Cl − (II) and NOCl + ρ − ⇆ NO + Cl + (+)Cl 2 + 2 ρ − ⇆ 2 Cl − (III) An extensive study of the first reduction wave (reaction I) indicates that the electrochemical process is kinetically controlled by the molecular dissociation 2 NO 2Cl ⇆ N 2O 4 + Cl 2 (IV) The normal potential accounting for step (I), the ionic dissociation constant of NO 2Cl and the homolytic dissociation constant of N 2O 4 have allowed us to derive the constant values for reaction (IV) and the equilibrium 2 NO 2Cl ⇆ 2 NO 2 + Cl 2 (V) in sulfolane. These values are in good agreement with those found in nitromethane using the ESR technique.

Influence of the medium on the ionic dissociation of N 2O 4 and the kinetics of decomposition of N 2O 3 in aprotic solvents

Using the silver-silver chloride indicator electrode, the potentiometric technique has been employed to determine the ionic dissociation constant K i N 2O 4 corresponding to the reaction N 2O 4 ⇄ NO + + NO − 3 in aprotic media (nitromethane, propylene carbonate), taking into account the homolytic dissociation of N 2O 4 to NO 2 in our calculation. The extent of N 2O 4 solvation has been correlated with the donor character of the solvent. We have generalized this property to reducible oxygenated nitrogen compounds such as NO + and NO + 2. In order explain the relative stability of N 2O 3 in sulfolane (despite the low solubility of NO in aprotic media), we have evaluated the rate constants for the reactions ▪ and ▪ in this solvent. Furthermore, the determination of enthalpies H° N 2O 3 , H° N 2O 4 and H° NO 2 corresponding to the heats of solution of N 2O 3, N 2O 4 and NO 2, respectively, has allowed us to gain information concerning the effect of the medium on the molecular dissociations of N 2O 3.

Stabilite thermodynamique des phases dans le systeme CaOAl 2O 3CO 2H 2O

The paper presents a theoretical investigation on the nature of thermodynamical stable precipitating phases in the CaOAl 2O 3CO 2H 2O system when monocalcium aluminate (CA) with or without an excess of lime or alumina is mixed with water in equilibrium with gaseous carbon dioxide. Basing on the nature of ions in solution (A10 2 −, Ca ++, CO 3_ −−, OH − and H +). and supposing that only C C , AH 3, CH, CAH 10, C 2AH 8 and C 4ACH 11 are the possible stable phases, the system was mathematically described taking into account the values of solubility products, the equilibrium between atmospheric carbon dioxide and the solution, ionic dissociation constant of water, matter balance of calcium and aluminate ions and condition of the solution electroneutrality. For a given system of initial parameters, computer calculation has allowed to determine the only possible solid-liquid system among the 22 theoretically possible cases (zero, one or two solids in equilibrium with the solution). So, varying the initial parameters has led to draw isobaric and isosteric phase diagramms, what shows that, in the conditions of the study, CAH 10 is the only stable phase at 21°C. The result suggests that the precipitation of C 2AH 8 and C 4 A CH 11 can only be a consequence of saturations of the solution with respect to CAH 10.

Determination des constantes de dissociation ionique du chlorure de nitrosyle et de N 2O 4 dans le sulfolane

Determination of the ionic dissociation constants of nitrosyl chloride and dinitrogen tetroxide in sulpholane. A silver chloride—silver electrode is used to study electrochemical systems involving nitrosyl chloride and dinitrogen tetroxide in sulpholane: Ag↓ + NOCl ⇌ AgCl↓ + NO + + e - and Ag↓ + NOCl + NO - 3 ⇌ AgCl↓ + N 2O 4 + e - The dissociation constants of these two NO + donors determined in this way are: K (NOCl) = 10 -11.8 mol l -1 and K (N 2O 4) = 10 -7.2 mol l -1. A titration of N 2O 4 solutions alone or in the presence of nitric acid is described. This titration is possible in mixtures which contain up to 90% (w/w) nitric acid.

Preparation et etude des oligomeres du styrene

Dicarbanionic oligomers of styrene and α-methylstyrene, as well as the corresponding protonated species, have been prepared and studied in tetrahydrofuran. The molar extinction coefficient and the ionic dissociation constant of the dianionic dimers have been determined at 25°. The structure of the α-methylstyrene oligomers is identical to that observed by Richards and Williams; the styrene oligomers, isolated for the first time, show a symmetrical structure for the dimer and the tetramer, implying a higher reactivity for the styryl dianionic dimer than for the trimer dianionic species.

IONIC DISSOCIATION CONSTANT OF TRIETHYLAMINE IN METHANOL UNDER HIGH PRESSURE AT 25°C

Abstract The ionic dissociation constant of the ionization of triethylamine in methanol have been determined from the measurements of electrical conductivities at high pressures up to 2000 kg/cm2 at 25°C. The observed dissociation constant increased from 0.50×10−6 mol/kg at 1 atm to 5.78×10−6 mol/kg at 2000 kg/cm2, and its pressure dependence was quantitatively explained by the change of intrinsic volume and electrostrictive volume with pressure.

Kinetic study of anionic polymerization of butadiene in a polar solvent

The kinetics of the propagation reaction for the polymerization of butadiene initiated by cumyl potassium in tetrahydrofuran solution at several temperatures have been studied. Kinetic data and electrolytic behaviour indicate that polybutadienyl free ions assume the whole of the propagation reaction. At 0°C, the respective rate constants for ion-pairs and free ions are 1 l mol −1 s −1 and 4.8 × 10 4 l mol −1 s −1 respectively. The ionic dissociation constant is 7.8 × 10 −9 mol/l. The activation energy of the propagation reaction for free ions is 6.5 kcal/mol.

Ion and Ion-Pair Contributions to the Polymerization of Tetrahydrofuran

Abstract The cationic polymerization of tetrahydrofuran is generally considered to be effected by an oxonium chain end in the form of an ion pair. Consideration of expected ionic dissociation constants of salts in typical solvents for these reactions suggests that appreciable concentration of free oxonium ions would be present under polymerization conditions. The dissociation constants of such salts have been measured in methylene chloride solution and shown to be near 4 × 10−6, sufficiently high that a contribution to the polymerization by free ions could be expected. Rate constants for the propagation reaction of the polymerization initiated by triethylox-onium tetrafluoroborate have been measured, in both the presence and absence of the common-ion salt tetrabutylam-monium tetrafluoroborate. From the dependence of the rate constant on the initiator concentration and from its reduction by varying concentration of the free common ions, it is possible to calculate the ion-pair rate constant to be 1.4 × 10−...

Ion and Ion-Pair Contributions to the Polymerization of Tetrahydrofuran

Abstract The cationic polymerization of tetrahydrofuran is generally considered to be effected by an oxonium chain end in the form of an ion pair. Consideration of expected ionic dissociation constants of salts in typical solvents for these reactions suggests that appreciable concentration of free oxonium ions would be present under polymerization conditions. The dissociation constants of such salts have been measured in methylene chloride solution and shown to be near 4 × 10−6, sufficiently high that a contribution to the polymerization by free ions could be expected. Rate constants for the propagation reaction of the polymerization initiated by triethylox-onium tetrafluoroborate have been measured, in both the presence and absence of the common-ion salt tetrabutylam-monium tetrafluoroborate. From the dependence of the rate constant on the initiator concentration and from its reduction by varying concentration of the free common ions, it is possible to calculate the ion-pair rate constant to be 1.4 × 10−...

Anionic polymerization of isoprene. Ion and ion-pair contributions to polymerization in tetrahydrofuran

The kinetics of the propagation reaction in the polymerization of isoprene initiated by butyllithium in tetrahydrofuran solution have been studied. The kinetic behavior has indicated that the reactive species involves both the polyisoprenyllithium ion-pair and the free poly-isoprenyl carbanion. The conductances of solutions of polyisoprenyllithium have been measured and the ionic dissociation constant derived. From these the free ion rate constant has been evaluated. It has also been shown that the free ion reaction may be suppressed by the common ion effect when the salt lithium tetraphenylboron is added. The rate constants for the ion-pair and free ion are 0.20 M−1 s−1 and 2.8 × 103 M−l s−1 respectively; the ionic dissociation constant is 5.0 × 10−10. Nuclear magnetic resonance determinations of the structures of the polymers formed primarily by the free carbanion, and by the ion-pair only, show that the lithium ion has only a small directing effect.

Comparative strengths of four organic bases in benzene

For ditolylguanidine, the equilibrium constants K and K2 for the first and second reactions, respectively, are est imated to be 1.1 X10 6 and 6.4. These values are compared with values for Ki and K2 previously found for diand triphenylguanidine and the value of Ki found for t r ie thylamine. The values for K\, which measure the relative tendencies of the bases to form salts with the indicator acid in benzene, would be expected to parallel the ionic dissociation constants of the bases in water. However, the parallelism is not good. Diphenylguanidine and ditolylguariidine, which are presumed to be weaker bases in water than triethylamine, are much more reactive in benzene. The results demonstrate how misleading the aqueous dissociation constants may be as a gage of the relative reactivities of bases in a nonaqueous solvent such as benzene. Steric and solvation effects are discussed.