Showing papers on "Equilibrium constant published in 1988"

Interactions of metals and protons with algae.

Abstract: Proton uptake by intact algal cells was found to consist of two processes: (1) a fast (<4 s) surface reaction and (2) a slow (2h) diffusion of protons into cells. A pH titration technique measured only the rapid surface reaction that forms negative sites at higher pH. Adsorption of alkali, alkaline earth, and transition metal ions on algae was quantitatively represented by the Langmuir adsorption isotherm with its two parameters y/sub m/, the maximum amount of metal adsorbed, and K, the equilibrium constant taken as a measure of bond strength. Variations of these parameters with pH and type of metal indicate that metals adsorb to algal surfaces by electrostatic attraction to negative sites, such as carboxylate anions of poly(galaturonic acid) (pectin), as previously suggested.

Thermodynamics of aqueous association and ionization reactions at high temperatures and pressures

Abstract: Electrochemical and electrical conductance cells have been widely used at ORNL over the years to quantitatively determine equilibrium constants and their salt effects to 300°C (EMF) and 800°C (conductance) at the saturation pressure of water (EMF) and to 4000 bars (conductance). The most precise results to 300°C for a large number of weak acids and bases show very similar thermodynamic behavior, which will be discussed. Results for the ionization constants of water, NH3(aq), HCl(aq), and NaCl(aq), which extend well into the supercritical region, have been fitted in terms of a model with dependence on density and temperature. The entropy change is found to be the driving force for ion-association reactions and this tendency increases (as it must) with increasing temperature at a given pressure. Also, the variation of all thermodynamic properties is greatly reduced at high fixed densities. Considerable variation occurs at low densities. From this analysis, the dependence of the reaction thermodynamics on the P-V-T properties of the solvent is shown, and the implication of large changes in hydration for solutes in the vicinity of the critical temperature will be discussed. Finally, the change in the molar compressibility coefficient for all reactions in water is shown to be the same and dependent only on the compressibility of the solvent.

A tetrahedral intermediate in the EPSP synthase reaction observed by rapid quench kinetics

Abstract: Direct evidence for an enzyme-bound intermediate in the EPSP synthase reaction pathway has been obtained by rapid chemical quench-flow studies. The transient-state kinetic analysis has led to the following complete scheme: (formula; see text) Values for all 12 rate constants were obtained. Substrate trapping experiments in the forward and reverse reactions established the kinetically preferred order of binding and release of substrates and products and showed that shikimate 3-phosphate (S3P) and 5-enolpyruvoylshikimate 3-phosphate (EPSP) dissociate at rates greater than turnover in each direction. Pre-steady-state bursts of product formation were observed in the reaction in each direction indicating a rate-limiting step following catalysis. Single turnover experiments with enzyme in excess over substrate demonstrated the formation of a transient intermediate in both the forward and reverse reactions. In these experiments, the enzymatic reaction was observed by employing a radiolabel in the enol moiety of either phosphoenol pyruvate (PEP) or EPSP. The separation and quantitation of reaction products were accomplished by HPLC monitoring radioactivity. The intermediate was observed as the transient production of radiolabeled pyruvate, formed due to the breakdown of the intermediate in the acid quench used to stop the reaction. The intermediate was observed within 5-10 ms after the substrates were mixed with enzyme and decayed in a reaction paralleling the formation of product in each direction. Thus, the kinetics demonstrate directly the kinetic competence of the presumed intermediate. No pyruvate was formed, on a time scale which is relevant to catalysis, after incubation of the enzyme with dideoxy-S3P and PEP or with EPSP in the absence of phosphate; and so, the intermediate does not accumulate under these conditions. The intermediate broke down to form PEP and EPSP in addition to pyruvate when the reaction was quenched with base rather than acid; therefore, the intermediate must contain the elements of each product. Other experiments were designed to measure directly the phosphate binding rate and further constrain the PEP binding rate. The overall solution equilibrium constant in the forward direction was determined to be 180 by quantitation of radiolabeled reactants and products in equilibrium after incubation with a low enzyme concentration. The internal, active site equilibrium constant was obtained by incubation of radiolabeled S3P with excess enzyme and high concentrations of phosphate and PEP to provide the ratio of [EPSP]/[S3P] = 2.3, which is largely a measure of K4.(ABSTRACT TRUNCATED AT 250 WORDS)

Dimerization and trimerization of rhodamine 6G in aqueous solution. Effect on the fluorescence quantum yield

Abstract: The trimerization equilibrium constant of rhodamine 6 G in aqueous solution as determined by absorption spectroscopy is higher than that of the dimer previously reported. Similarities observed between the absorption spectra of both aggregates are explained by exciton theory considering a sandwich structure for the aggregates. The lack of emission from the aggregates implies an important decrease in the fluorescence quantum yield of the dye, and the rate constant of the quenching produced by the dimer and trimer are calculated separately. The dominant mechanism for the quenching seems to be long-range energy transfer from the monomer to the aggregates, being affected by molecular diffusion as well as excitation energy migration between monomers.

Analysis of equilibrium shift in isothermal reactors with a permselective wall

Abstract: The objective of this study is to obtain a fundamental understanding of the behavior of reactors with a permselective wall (membrane reactors) in terms of: design parameters (reactor length, membrane thickness); operating variables (pressure ratio, feed flow rate); physical properties (rate constant, permeability of fast gas, permselectivity, equilibrium constant); and flow patterns (recycle, cocurrent, countercurrent). Pure feed reacts on the high-pressure side of the membrane, and the product(s) formed are continuously removed to the low-pressure side so that thermodynamic equilibrium is never reached. It is shown by simulation that equilibrium shift can be enhanced by: recycling unconverted reactant; shifting feed location to separate products; and maintaining high permeation rates to reduce backreaction. It is also shown that the choice between cocurrent flow and countercurrent flow depends on the system parameters.

The statistical mechanics of the ionic equilibrium of water: A computer simulation study

Abstract: A theory is presented to study the ionic equilibrium of water. Its equilibrium constant is expressed in terms of the chemical potentials of H2O, H3O+, and HO− and is calculated by combining molecular dynamics and test particle methods. Intermolecular potentials are studied in detail. The theory predicts a pH of a correct order of magnitude. Polarization effects are surprisingly large; both the polarization of water molecules by the ion and the backpolarization of strong H bonds around the ion by water molecules play an important role and dominate the process under investigation.

Organic complexes in hydrothermal systems

Abstract: Electrochemical techniques (two different polarographic techniques and the ion specific electrode method) were used to measure the equilibrium constants of lead(II) organic complexes up to 90 degrees C. The data are extrapolated to 200 degrees C. The equilibrium speciation model EQBRM was used to calculate the significance of organic complexing in Na-Ca-Cl-bearing brines, from 50 degrees to 200 degrees C, by comparing total organic complexing to total chloro complexing with lead(II) and zinc(II). The significance of organic complexing in hydrothermal brines depends mostly on the solubility of the calcium(II) organic salts, the concentration of calcium in solution, the thermal stability of the organic species, and the acidity of the ligands' binding groups. The model showed that acetate (or other soluble carboxylates) complexes with lead(II) and zinc(II) can predominate over chloro complexes under neutral to slightly acidic pH conditions when present at concentrations which have been observed in hydrothermal brines. A minimum concentration of about 10 (super -3) m of organic ligand is necessary for species such as oxalate to complex base metals in Na-Ca-Cl-rich brines in a significant way. This concentration (10 (super -3) m) is one or more orders of magnitude supersaturated with respect to Ca oxalate. The low calcium salt solubility also prevents ligands, such as phthalate, anthranilate, picolinate, dipicolinate, 1,10 phenanthroline, and thiosalicylate, to be of potential importance for base metal transport in Ca-rich hydrothermal fluids.

Preferential solvation of ions in mixed solvents. Part 2.—The solvent composition near the ion

Abstract: The quasi-lattice quasi-chemical theory of preferential solvation that was related previously to the Gibbs free energy of transfer of an ion from a reference solvent to a mixture of solvents has now been extended to yield the actual amount of the preferential solvation of the ion. This is defined as δX1=xL1–x1, the excess (or deficiency) of the local mole fraction of the solvent S1 in the vicinity of the ion X over its mole fraction in the bulk, in a very dilute solution of X, in the mixture of solvents S1 and S2. The preferential solvation obtained for six cases as illustrations (sodium, silver and chloride ions in aqueous dimethyl sulphoxide and acetonitrile) is related to the equilibrium constants for solvent exchange.

The methyl(cyclam)nickel(III) dication in aqueous solutions: determination of the equilibrium constant of homolysis, kinetics of oxygen insertion, and methyl transfer to aquated chromium(2+)

Abstract: Le compose du titre est prepare par reaction du radical methyle avec le complexe LNi 2+ (L=cyclam=tetraaza-1,4,8,11cyclotetradecane)

Thermodynamics far from equilibrium: Reactions with multiple stationary states

Abstract: We present a thermodynamic analysis of global validity for effectively one‐variable, irreversible chemical systems with multiple steady states. A hypothetical reaction chamber is held at constant temperature and volume and is connected by selectively permeable membranes to reservoirs of reactant(s) and product(s), each at constant selected pressures. An appropriate free energy function, which yields criteria of evolution to equilibrium for the composite system of reaction chamber and reservoirs, is a hybrid of Gibbs and Helmholtz free energies. The one variable in the reaction chamber is the pressure of a chemical intermediate which varies in time according to a given reaction mechanism. With the hybrid free energy, the kinetics for a given mechanism, and a concept of instantaneous indistinguishability of systems with different mechanisms, we establish a thermodynamic driving force, or species‐specific affinity, for the intermediate. The species‐specific affinity vanishes at steady states, and upon its di...

Yeast flocculation: a dynamic equilibrium.

Abstract: The steady state in yeast flocculation is a dynamic equilibrium between flocculated and dispersed yeast cells. The free cell concentration is directly proportional to the total cell concentration and may be expressed as an equilibrium constant. Increased agitation decreases floc size and equilibrium constant whilst increasing floc-surface area and free-cell concentration. Values of equilibrium constant are influenced by agitation in a complex relationship probably involving the floc-surface area and floc momentum. Inhibition of flocculation by mannose and low pH is reversible and becomes greater with increased agitation. Both these inhibitions appear consistent with a weakening of flocculent bond strength by these inhibitors.

A polarimetric and 11B and 13C nuclear magnetic resonance study of the reaction of the tetrahydroxyborate ion with polyols and carbohydrates

Abstract: The reaction of the tetrahydroxyborate ion, B(OH)–4, with 31 polyols and carbohydrates in aqueous solution has been studied by complementary investigations involving polarimetry and 11B and 13C n.m.r. spectroscopy. The calculation of equilibrium constants for the complexation of the carbohydrates with B(OH)–4 from the polarimetry results, using a previously derived equation, was only partially successful owing to the presence of multiple equilibria in a number of cases. The presence of several complexed species in these reactions is demonstrated by 11B n.m.r. studies. A calibration method devised to relate peak area in 11B n.m.r. spectra with concentration has been used to calculate equilibrium constants for the various equilibria present, and an attempt has been made to rationalise the equilibrium constants with the molecular structures of the substrates. In the majority of cases the 13C n.m.r. spectra confirm the polarimetric and 11B n.m.r. studies, and in most cases allow a more specific identification of the reaction sites in the polyols/carbohydrates to be made.

Equilibrium studies on the binding of cadmium(II) to human serum transferrin.

Abstract: The binding of cadmium(II) to human serum transferrin in 0.01 M N-(2-hydroxyethyl)-piperazine-N'-2-ethanesulfonic acid with 5 mM bicarbonate at 25 degrees C has been evaluated by difference ultraviolet spectroscopy. Equilibrium constants were determined by competition versus three different low molecular weight chelating agents: nitrilotriacetic acid, ethylenediamine-N,N'-diacetic acid, and triethylenetetramine. Conditional equilibrium constants for the sequential binding of two cadmium ions to transferrin under the stated experimental conditions are log K1 = 5.95 +/- 0.10 and log K2 = 4.86 +/- 0.13. A linear free energy relationship for the complexation of cadmium and zinc has been prepared by using equilibrium data on 243 complexes of these metal ions with low molecular weight ligands. The transferrin binding constants for cadmium and zinc are in good agreement with this linear free energy relationship. This indicates that the larger size of the cadmium(II) ion does not significantly hinder its binding to the protein.

The kinetics and thermodynamics of the reaction H+NH3⇄NH2+H2 by the flash photolysis–shock tube technique: Determination of the equilibrium constant, the rate constant for the back reaction, and the enthalpy of formation of the amidogen radical

Abstract: Equilibrium constants K1 for the reaction H+NH3⇄NH2+H2 were measured over the temperature range 900 to 1600 K using a flash photolysis–shock tube apparatus. The experimental values of K1 ranged from 1.0 at 900 K to 2.2 at 1620 K with an estimated experimental error of about ±10%. The value obtained from the third law analysis for the enthalpy of formation of the amidogen radical, ΔH0f298 (NH2), is 45.3 kcal/mol (ΔH0f0 =46.0). The corresponding value for the bond dissociation energy, D0(NH2–H), is 107 kcal/mol. These values are in good agreement with the data tabulated in the revised JANAF tables (1982), with those derived from new measurements of the photoionization threshold for the amidogen radical, and with those from independent measurements of the rate constants of the forward and back reactions. The Arrhenius rate expression derived for reaction (−1), NH2+H2→NH3+H, is k−1(T)=5.38×10−11 exp(−6492/T) cm3 molecule−1 s−1 for the temperature range 900–1620 K. The estimated error in k−1(T) is about ±25%.

Influence of tert -butyl alcohol on cyclodextrin inclusion complexes of pyrene

Abstract: The effect oftert-butyl alcohol on complexes of pyrene and various cyclodextrins is investigated. The equilibrium constant for the complexation is derived from the fluorescence decay parameters. A greater than twofold enhancement of pyrene lifetime is observed in the presence oftert-butyl alcohol and β-cyclodextrin or γ-cyclodextrin. As the number of hydroxyl groups decreases, substituted β-cyclodextrins show smaller enhancements to both the fluorescence lifetime and the formation constant. These observations are explained by proposing that alcohol molecules are associated with the inclusion complex. This association increases the apparent hydrophobicity of the cyclodextrin cavity, protects the molecule from collisional quenching and deactivation, and provides additional rigidity to the system.

Kinetics of the reaction of C3H3 with molecular oxygen from 293–900 K

Abstract: The kinetics and mechanism of the reaction of C3H3 with O2 were investigated from 293–900 K using a tubular reactor coupled to a photoionization mass spectrometer. From 293–333 K, the reaction proceeds by a simple reversible addition reaction. Between 380 and 430 K the equilibrium C3H3+O2 ahC3H3O2 was clearly observable, and equilibrium constants were measured as a function of temperature. These measurements yielded the values of ΔH298o (−79±6 kJ mole−1) and ΔS298o (−131±12 J mole−1 K−1) for this reaction. A mechanism change was observed as temperature rises. Above 350 K, H2C2O is observed as a new product whose importance increases rapidly with increasing temperature. Rate constants were measured as a function of density and temperature for the low temperature addition reaction and the high temperature reaction (from 500–900 K). An Arrhenius expression is recommended for the high temperature process (represented as C3H3+O2→H2C2O+HCO), 5×10−14exp(−12 kJmole−1/RT) cm3molecule−1 s−1, which is based on the results of this study and the kinetic model offered to explain the findings. A mechanism of the C3H3+O2 reaction is proposed which involves initial formation of a C3H3O2 adduct. At temperatures above 350 K, a cyclic rearrangement of the adduct, which leads to the formation of H2C2O+HCO, competes with its redissociation to C3H3+O2.

The equilibrium constant for N2O5⇄NO2+NO3: Absolute determination by direct measurement from 243 to 397 K

Abstract: Direct determinations of the equilibrium constant for the reaction N2O5 = NO2 + NO3 were carried out by measuring NO2, NO3, and N2O5 using long-path visible and infrared absorption spectroscopy as a function of temperature from 243 to 397 K The first-order decay rate constant of N2O5 was experimentally measured as a function of temperature These results are in turn used to derive a value for the rate coefficient for the NO-forming channel in the reaction of NO3 with NO2 The implications of the results for atmospheric chemistry, the thermodynamics of NO3, and for laboratory kinetics studies are discussed

Fundamental equation for systems in chemical equilibrium

Abstract: At constant temperature and pressure, the differential of the Gibbs energy can be written in terms of species, components, extents of reaction, isomer groups, and homologous series groups under specified conditions. The applicability of each of these options in deriving expressions for equilibrium constants and devising other methods for calculating equilibrium compositions is discussed. If one or more reactants are available to a system at a constant chemical potential, their terms can be removed from the summation by use of a Legendre transform to obtain a new thermodynamic potential that is a minimum at equilibrium. This pemits the definition of homologous series and other groups that can be used as single terms in the fundamental equation for the purpose of equilibrium calculations. By use of these techniques, calculations can be made on organic systems with indefinitely large numbers of species.