Acid-base Equilibrium Reactions Research Articles

Crystal and Molecular Structures and Acid–Base Equilibria of N-(4-Chlorophenyl)-2-Benzoylethylamine and Bis[3-(o-Chlorophenyl)-1-Phenylaminopropyl]diazene

N-(Phenyl)-2-benzoylethylamine (I), N-(4-chlorophenyl)-2-benzoylethylamine (II), and N-(2-chlorophenyl)-2-benzoylethylamine hydrazone (III) were synthesized in the crystalline state. The heating of ethanolic solutions of compound III at reflux gave dimeric bis[3-(о-chlorophenyl)-1-phenylaminopropyl]diazene (IV). The crystal and molecular structures of compounds II and IV were determined by X-ray diffraction. In the crystal of II, the molecules are packed in stacks in a head-to-head fashion stabilized by intermolecular hydrogen bonds involving carbonyl oxygen atoms and hydrogen atoms at nitrogen. Compound IV is composed of two independent bis[3-(о-chlorophenyl)-1-phenylaminopropyl]diazene molecules as structural units. Each structural unit is a centrosymmetric dimer. There are no hydrogen bonds in compound IV. Acid–base equilibrium reactions were studied by spectrophotometric titration. The protonation constants are 4.68 ± 0.14 for I and 4.11 ± 0.13 for II.

Computer Simulations of Static and Dynamical Properties of Weak Polyelectrolyte Nanogels in Salty Solutions.

We investigate the chemical equilibria of weak polyelectrolyte nanogels with reaction ensemble Monte Carlo simulations. With this method, the chemical identity of the nanogel monomers can change between neutral or charged following the acid-base equilibrium reaction HA ⇌ A− + H+. We investigate the effect of changing the chemical equilibria by modifying the dissociation constant . These simulations allow for the extraction of static properties like swelling equilibria and the way in which charge—both monomer and ionic—is distributed inside the nanogel. Our findings reveal that, depending on the value of , added salt can either increase or decrease the gel size. Using the calculated mean-charge configurations of the nanogel from the reaction ensemble simulation as a quenched input to coupled lattice-Boltzmann molecular dynamics simulations, we investigate dynamical nanogel properties such as the electrophoretic mobility and the diffusion coefficient D.

Theory of pH changes in water desalination by capacitive deionization

In electrochemical water desalination, a large difference in pH can develop between feed and effluent water. These pH changes can affect the long-term stability of membranes and electrodes. Often Faradaic reactions are implicated to explain these pH changes. However, quantitative theory has not been developed yet to underpin these considerations. We develop a theory for electrochemical water desalination which includes not only Faradaic reactions but also the fact that all ions in the water have different mobilities (diffusion coefficients). We quantify the latter effect by microscopic physics-based modeling of pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technology employing porous carbon electrodes and ion-exchange membranes. We derive a dynamic model and include the following phenomena: I) different mobilities of various ions, combined with acid-base equilibrium reactions; II) chemical surface charge groups in the micropores of the porous carbon electrodes, where electrical double layers are formed; and III) Faradaic reactions in the micropores. The theory predicts small pH changes during desalination cycles in MCDI if we only consider phenomena I) and II), but predicts that these pH changes can be much stronger if we consider phenomenon III) as well, which is in line with earlier statements in the literature on the relevance of Faradaic reactions to explain pH fluctuations.

Lignin oxidation mechanisms under oxygen delignification conditions. Part 3. Reaction pathways and modeling

AbstractA detailed kinetic model for the lignin oxidation chemistry is presented. It is based mainly on the mechanisms and kinetics presented in the literature. Parameters that could not be found in the literature were regressed against the experimental data obtained from oxidation experiments with softwood kraft lignin. In addition to the detailed model for the chemistry, acid-base equilibrium reactions and gas-liquid mass transfer were modeled. Most of the experimental observations could be reproduced with the developed model. The reasons behind the behavior of guaiacyl and condensed phenols are discussed. The reaction routes affecting lignin solubilization and chemical consumption are presented. Model-ing of acid-base equilibria proved to be important because acid-base pairs of reactants react differently. Carbon dioxide buffers the pH and in this way affects the chemical reactions through the pH. A similar model could also be developed for other lignin treatments; for example, waste water purification or chemicals production in new biorefinery concepts. The developed reaction scheme will be used as a part of oxygen delignification model.

Acid−Base Equilibria inside Amine-Functionalized Mesoporous Silica

Acid-base equilibria and effective proton concentration inside a silica mesopore modified with a trimethyl ammonium (TMAP) layer were studied by steady-state fluorescence experiments. The mesoporous silica with a dense TMAP layer (1.4 molecules/nm(2)) was prepared by a post grafting of N-trimethoxysilylpropyl-N,N,N-trimethylammonium at surfactant-templated mesoporous silica (diameter of silica framework =3.1 nm). The resulting TMAP-modified mesoporous silica strongly adsorbed of anionic fluorescence indicator dyes (8-hydroxypyrene-1,3,6-trisulfonate (pyranine), 8-aminopyrene-1,3,6-trisulfonate (APTS), 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid disulfuric acid (TPPS), 2-naphthol-3,6-disulfonate (2NT)) and fluorescence excitation spectra of these dyes within TMAP-modified mesoporous silica were measured by varying the solution pH. The fluorescence experiments revealed that the acid-base equilibrium reactions of all pH indicator dyes within the TMAP-modified silica mesopore were quite different from those in bulk water. From the analysis of the acid-base equilibrium of pyranine, the following relationships between solution pH (pH(bulk)) and the effective proton concentration inside the pore (pH(pore)) were obtained: (1) shift of pH(pore) was 1.8 (ΔpH(pore)=1.8) for the pH(bulk) change from 2.1 to 9.1 (ΔpH(bulk)=7.0); (2) pH(pore) was not simply proportional to pH(bulk); (3) the inside of the TMAP-modified silica mesopore was suggested to be in a weak acidic or neutral condition when pH(bulk) was changed from 2.0 to 9.1. Since these relationships between pH(bulk) and pH(pore) could explain the acid-base equilibria of other pH indicator dyes (APTS, TPPS, 2NT), these relationships were inferred to describe the effective proton concentration inside the TMAP-modified silica mesopore.

Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I: Acid–base equilibria and pH buffers

We review fundamental and applied acid-base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid-base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss "real world" buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid-base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems.

Quantitative interpretation of pH distributions in aquatic sediments: A reaction-transport modeling approach

Despite its status of master variable, there have been relatively few attempts to quantitatively predict the distributions of pH in biogeochemical reactive transport systems. Here, we propose a theoretical approach for calculating the vertical pore water profiles of pH and the rates of proton production and consumption in aquatic sediments. In this approach, the stoichiometric coefficients of species that participate in acid-base equilibrium reactions are treated as unknown variables in the biogeochemical reaction network. The mixed kinetic-equilibrium reaction system results in a set of coupled differential and algebraic equations and is solved using a new numerical solver. The diagnostic capabilities of the model are illustrated for depositional conditions representative of those encountered on the continental shelf. The early diagenetic reaction network includes the major microbial degradation pathways of organic matter and associated secondary redox reactions, mineral precipitation and dissolution processes, and homogeneous acid-base reactions. The resulting pH profile in this baseline simulation exhibits a sharp decrease below the sediment-water interface, followed by an increase with depth and again a decrease. The features of the pH profile are explained in terms of the production and consumption of protons by the various biogeochemical processes. Secondary oxygenation reactions are the principal proton producers within the oxic zone, while reduction of iron and manganese oxyhydroxides are primarily responsible for the reversal in the pH gradient in the suboxic zone. Proton production in the zone of sulfate reduction outweighs alkalinity production, maintaining the undersaturation of the pore waters with respect to calcite. Integrated over the entire depth of early diagenesis, dissolution of CaCO3 is the main sink for protons. Variations in the reaction rate order and rate constant for CaCO3 dissolution do not fundamentally alter the shape of the pH profile. An entirely different shape is obtained, however, when the pore waters are assumed to remain in thermodynamic equilibrium with calcite at all depths. Pore water (bio)irrigation decreases the amplitude of pH changes in the sediment and may modify the shape of the pH profile.

Kinetics and mechanism of the acid-base equilibrium of mexazolam analogues: A modification of the mechanism proposed previously for mexazolam

Oxazolidine ring-opening and ring-closing (acid-base equilibrium) reactions of 3-methyl-11b-hydrogen (1) and −11b-methyl (2) analogues of mexazolam (3) have been studied kinetically in solutions of various pH values and at 25°C. Compound 2 isomerizes to the cis/trans isomers (referring to substituents at the 3- and 11b-position) in methanol-d4 withh a half-life of baout 50 min, and at equilibrium the ratio of the cis to trans isomers is 6.9:1. The acid-base equilibrium reactions of 1 and 2 proceed via two steps due to the cis and trans isomers, the reactions of the trans isomer being faster than those of the cis isomer. The large difference between the cis and trans isomers of 2 is ascribed to their conformational differences (normal boat X1 and flat form Y11, respectively). These results required a modification of the mechanism previously proposed for mexazolam (Kurono et al., Chem. Pharm. Bull., 35 (1987) 3831–3837).

Kinetics and mechanism of oxidation of 2-mercaptopyrimidine by a tetraaza macrocyclic-silver(II) complex

In buffered solutions the kinetics of the oxidation of 2-mercaptopyrimidine (MP) by Ag(cyclam) 2+ follows a second-order rate law. At T = 25.0 °C, μ = 0.10 M, pH from 4.67 to 6.53, [Ag(cyclam) 2+] = 0.89 × 10 −4 M and [MP] from 1.25 × 10 −3 to 2.50 X 10 −3 M, the second-order rate constants for the oxidation of MP and its anion are 5 and 145 M −1 s −1, respectively. Rate-pH profile observed in the reaction can be accounted for by the acid-base equilibrium reaction of MP and the higher reactivity of the anion with the oxidant.

The behavior of 1,4-benzodiazepine drugs in acidic media. II. Kinetics and mechanism of the acid-base equilibrium reaction of oxazolam.

The oxazolidine ring-opening and ring-closing reactions (acid-base equilibrium reactions) of oxazolam were investigated by using a stopped-flow spectrophotometer. The cis and trans isomers of oxazolam were found to have distinct ring-opening rate constants and the constant of the cis isomer was 10 to 20 times larger than that of the trans isomer. The ring-opening is hydrogen ioncatalyzed at low pH region. The ring-closing occurs in a hydroxide ion-catalyzed reaction in the alkaline region and is affected by the deprotonation of the diazepinone ring. The individual intrinsic rate constants for the ring-opening and ring-closing reactions were determined from the rate-pH profile. These rate measurements are of value to distinguish between cis and trans isomers of oxazolam.

The acid-base equilibrium reaction of benzodiazepinooxazoles.

The acid-base equilibrium reactions of oxazolam (10-chloro-2, 3, 5, 6, 7, 11b-hexahydro-2-methyl-11b-phenylbenzo [6, 7]-1, 4-diazepino [5, 4-b] oxazol-6-one) and thirteen other derivatives of 1, 4-benzodiazepinooxazole (BDOZ) were studied. Ultraviolet absorption and fluorescence spectroscopies were employed to obtain the equilibrium constants. In most BDOZs the protonated species were promptly cleaved at the oxazolidine ring fused to the diazepine nucleus, and equilibrated with the original species. A few exceptional compounds required an appreciable time lag for the equilibration. The effects of substituents on the pKa value are discussed from the viewpoint of physical organic chemistry.

A spectral and polarographic study of the acid—base and complexing behaviour of bromazepam

A spectral and polarographic study of bromazepam yielded pK a values of 2.5, 5.2 and 11.8, corresponding to protonation at the azomethine and pyridine nitrogen atoms, and deprotonation at the nitrogen atom in position 1, respectively. These acid-base equilibrium reactions are discussed with reference to the hydrolysis and complexing properties of this compound. An indirect analytical method, based on the complexation of bromazepam with Cu 2+ ions, has been investigated by differential pulse anodic stripping voltammetry.