Showing papers on "Equilibrium constant published in 1996"

Adsorption of EDTA and Metal-EDTA Complexes onto Goethite

Abstract: The adsorption characteristics of a variety of divalent and trivalent metal-EDTA complexes onto goethite (alpha-FeOOH) were examined in aqueous solution. Uncomplexed EDTA is adsorbed as a binuclear complex at low pH and as a mononuclear complex at high pH. Adsorption is ligand-like with a high extent of adsorption at low pH. The process can be described by formation of inner-sphere complexes by the surface complexation model with constant capacitance. The EDTA complexes of the divalent metals Ca, Zn, Ni, Cu, Co(II), and Pb, which are quinquedentate in solution (free donor atoms bound to the metal ion), all showed the same ligand-like adsorption behavior. Their adsorption as a function of pH and concentration can be described by the formation of one type of ternary surface complex and can be fitted with the same equilibrium constant. Pd(II)EDTA, which is bidentate or quadridentate in solution, is adsorbed more strongly, but also in a ligand-like manner. The EDTA complexes of the trivalent metals LaEDTA and BiEDTA are adsorbed very strongly over the whole pH range. The sexidentate complex of Co(III) is weakly adsorbed at low pH outer-spherically, i.e., by electrostatic interaction only. Fe(III)EDTA is weakly adsorbed over the whole pH range with a predominant nonspecific surface complex at low pH and a specific complex at high pH.

Nonreversible Adsorption of Divalent Metal Ions (MnII, CoII, NiII, CuII, and PbII) onto Goethite: Effects of Acidification, FeII Addition, and Picolinic Acid Addition.

Abstract: Differences in the coordination chemistry of divalent metal ions affect their adsorption onto goethite (a-FeOOH) surfaces and their subsequent release by acidification, Fe II addition, and picolinic acid addition. Equilibrium constants for adsorption onto FeOOH (goethite) decrease in the order : Cu II > Pb II > Ni II Co II > Mn II . Equilibrium constants for picolinic acid complexation follow the Irving-Williams series ; complex formation with Pb II and Mn II is nearly negligible. Divalent metal ion release 24 h following acidification or picolinic acid addition is typically less than the amount predicted using an equilibrium speciation model, indicating adsorption/desorption hysteresis. Experiments performed under strict exclusion of O 2 reveal a slight increase in Co II , Ni II , and Cu II adsorption upon the addition of 1000 μM Fe II . Fe II interferes with the ability of picolinic acid to cause divalent metal ion release.

Ion exchange equilibria in zeolite minerals

Abstract: Cation exchange equilibria of the most common sedimentary zeolites are reviewed. Selected exchange isotherms and thermodynamic equilibrium constants are reported and interpreted in terms of selectivity of one cation over another. Selectivity sequences of various zeolites are compared with each other in the light of Eisenman's theory and utilized to predict or explain specific practical performances.

Heats of Formation of Organic Free Radicals by Kinetic Methods

Abstract: It has long been recognized that the key intermediates for the majority of the reactions of organic molecules in the gas phase, and frequently in the liquid phase as well, are free radicals. An important prerequisite for the description of the behavior of such systems is their thermodynamic properties. At first glance it may be surprising that thermodynamic properties should have any applicability to transient species that are present in trace quantities and disappear in short times. Indeed, if the kinetic properties of such systems are known, then thermodynamic properties are not really necessary and can in fact be derived from the kinetics. In reality, however, kinetic properties may be unavailable or difficult to measure. Thermodynamic properties serve as limits for kinetics and more generally as a basis for the estimation and evaluation of kinetic information [1]. More directly, through the equilibrium constant, rate constants for the reverse direction can be directly calculated from that in the forward direction. There are other physical situations where local thermodynamic equilibrium turns out to be a satisfactory approximation and kinetic information is not important.

Simulating equilibrium within aerosols and nonequilibrium between gases and aerosols

Abstract: A numerical method to solve chemical equilibrium equations and a method of coupling the equilibrium calculations to nonequilibrium growth and evaporation are discussed. The equilibrium program solves any number of equations for gas, aqueous, ionic, and solid equilibrium concentrations over large spatial grids and particle size grids. It also simultaneously computes electrolyte mean mixed activity coefficients and aerosol liquid water content. Mean mixed activity coefficient calculations require mean binary activity coefficient information. Temperature-dependent mean binary activity coefficient polynomials were constructed using mean binary activity coefficient data at 298 K, apparent molal enthalpy data, and apparent molal heat capacity data. The equilibrium solver is mole and charge conserving, requires iteration, but always converges. Solutions to the equilibrium equations are used for two purposes. The first is to estimate surface vapor pressures over particles containing a solution and/or a solid phase. Such vapor pressures are then applied in gas-aerosol transfer equations. The second is to estimate intraparticle composition and size immediately after gas-aerosol transfer.

Lewis acid catalysis of a Diels-Alder reaction in water

Abstract: Here we report the first detailed study of a Diels-Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co2+, Ni2+, Cu2+ and Zn2+ ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-1-(2-pyridyl)-2-propen-1-ones (1a-e) and cyclo-pentadiene (2) in water has been studied. Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M CU(NO3)(2) in water accelerates the reaction by a factor of 79 300. The kinetics of the catalyzed reaction were analyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a-e and rate constants for the reaction of the resulting complexes with 2. The rate enhancement imposed upon the uncatalyzed DA reaction of substrates 1 with 2 by water is much more pronounced than that for the catalyzed reaction. The increase of the endo-exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. The modest solvent and substituent effects observed for the catalyzed reaction indicate that the change in charge separation during the activation process is not larger than the corresponding change For the uncatalyzed reaction.

Quantitative Equilibrium Constants between CO2 and Lewis Bases from FTIR Spectroscopy

Abstract: Equilibrium constants measured from the ν2 bending mode of CO2 by FTIR spectroscopy are reported for the electron donor−acceptor interactions of CO2 with three Lewis bases: triethylamine (TEA), pyridine (PYR), and tributyl phosphate (TBP). The average Kc values are 0.046 (CO2−TEA), 0.133 (CO2−PYR), and 1.29 (CO2−TBP) L/mol at 25 °C in the solvent pentane. For the CO2−TBP system, the average enthalpy of association, ΔH°, is −4.7 kcal/mol. Ab initio calculations indicate that steric repulsion of the ethyl groups in TEA cause the binding energy of the CO2−TEA complex to be weaker than that of the CO2−PYR complex by 1.34 kcal/mol, a trend that is in agreement with our spectroscopic data. The lattice fluid hydrogen bonding model was used in conjunction with the spectroscopically determined Kc values to predict bubble points for the CO2−TEA and CO2−TBP systems and CO2 sorption in a hypothetical polymer. These calculations indicate that these relatively weak specific interactions have a measurable effect on pha...

Modeling Multi-Metal Ion Exchange in Biosorption

Abstract: Some types of dead microbial or seaweed biomass are capable of effectively binding heavy metals often through ion exchange. This biosorption can be used for purification of metal-bearing effluents. A recently developed two-site model for the ion exchange between divalent metal ions and protons in biosorption was extended to describe multi-site and multi-ion system behavior. A model case for two ion exchange sites and three ions was compared to the experimental data for the sorption of Cd, Cu, and Zn by protonated Sargassum fluitans biomass in systems containing two metal ions and protons at different pH values. The equilibrium constants were comparable to those obtained for systems where, in addition to protons, only one metal ion was present. This opens the possiblity of predicting the sorption performance for three-cation systems from equilibrium constants determined in two-cation systems. There was no interference between the sorbed species except for competition for the same sorption sites. The model ...

Uptake of Metals on Peat Moss: An Ion-Exchange Process†

Abstract: Various biomaterials have shown promise as sorbants to remove heavy metals from water. Several advantages of peat moss for such applications include its abundance, low cost, and high metal capacity. A Ca-loaded column of peat moss was therefore studied with mixtures of metals. Metals bind to anionic sites by displacing protons from acidic groups ( ) or existing metals from anionic sites at high pH (Kex). These ion-exchange equilibrium constants were determined in batch experiments by direct measurement of species in solution and sorbed on the solid phase. The same Kex values of Mg 0.342, Mn 0.862, Ca 1.00, Ni 1.42, Zn 1.88, Cd 2.82, Cu 9.97, and Pb 26.7 relative to Ca were found for a given metal alone or in the presence of a mixture, thus showing that the metals function independently. Under conditions employed for a Mg/Mn mixture, it was found that ion-exchange equilibria were maintained along the column due to very fast rates for metal−metal exchange as measured in a separate kinetic study. A linear re...

Reaction of BrONO2 with H2O on submicron sulfuric acid aerosol and the implications for the lower stratosphere

Abstract: Uptake of BrONO2 onto submicron sulfuric acid aerosol was studied in a cylindrical flow reactor coupled to a chemical ionization mass spectrometer. On aerosol particles of ∼0.1 μm radius, the reaction of BrONO2 with H2O to produce HOBr and HNO3 is very efficient, with the reaction probability γ ≈ 0.8 for H2SO4-content ≤ 70 wt%. The equilibrium constant for the gas phase process, BrONO2 + H2O ⇄ HOBr + HNO3, was measured and the heats of formation of the bromine compounds were deduced. The conversion of HOBr back into BrONO2 via reaction with HNO3 can be neglected in the stratosphere. The heterogeneous hydrolysis of BrONO2, followed by photolysis of the HOBr product, is a major source of HOx in the lower stratosphere. The role of bromine as an activator of chlorine is also discussed.

Equilibrium and Kinetics of Bromine Hydrolysis.

Abstract: Equilibrium constants for bromine hydrolysis, K1 = [HOBr][H+][Br-]/[Br2(aq)], are determined as a function of ionic strength (μ) at 25.0 °C and as a function of temperature at μ ≃ 0 M. At μ ≃ 0 M and 25.0 °C, K1 = (3.5 ± 0.1) × 10-9 M2 and ΔH° = 62 ± 1 kJ mol-1. At μ = 0.50 M and 25.0 °C, K1 = (6.1 ± 0.1) × 10-9 M2 and the rate constant (k-1) for the reverse reaction of HOBr + H+ + Br- equals (1.6 ± 0.2) × 1010 M-2 s-1. This reaction is general-acid-assisted with a Bronsted α value of 0.2. The corresponding Br2(aq) hydrolysis rate constant, k1, equals 97 s-1, and the reaction is general-base-assisted (β = 0.8).

High-Pressure Mass Spectrometric Investigations of the Potential Energy Surfaces of Gas-Phase SN2 Reactions

Abstract: High-Pressure Mass Spectrometric (HPMS) experiments have been carried out to probe the details of the double minimum potential energy surface for gas-phase SN2 reactions. The well depths and entropy changes associated with the formation of entrance and exit channel electrostatic complexes for the chloride and bromide adducts of methyl, ethyl, isopropyl, and tert-butyl chlorides and bromides have been determined from the temperature dependence of the equilibrium constants for adduct formation. In the cases of “symmetric” complexes associated with identity SN2 reactions, there is an increase in well depth as the size and, therefore, polarizability of the alkyl group increases. Concomitant with this is an increase in the magnitude of the negative entropy change for complex formation which is the result of an increase in the frequency of the intermolecular mode(s) of the complex arising from the increased bond strength. The data for the unsymmetrical adducts for the non-identity SN2 reactions show the same pa...

Deactivation of Methylrhenium Trioxide−Peroxide Catalysts by Diverse and Competing Pathways

Abstract: The peroxides from methylrhenium trioxide (MTO) and hydrogen peroxide, CH3ReO2(η2-O2), A, and CH3Re(O)(η2-O2)2(H2O), B, have been fully characterized in both organic and aqueous media by spectroscopic means (NMR and UV−vis). In aqueous solution, the equilibrium constants for their formation are K1 = 16.1 ± 0.2 L mol-1 and K2 = 132 ± 2 L mol-1 at pH 0, μ = 2.0 M, and 25 °C. In the presence of hydrogen peroxide the catalyst decomposes to methanol and perrhenate ions with a rate that is dependent on [H2O2] and [H3O+]. The complex peroxide and pH dependences could be explained by one of two possible pathways: attack of either hydroxide on A or HO2- on MTO. The respective second-order rate constants for these reactions which were deduced from comprehensive kinetic treatments are kA = (6.2 ± 0.3) × 109 and kMTO = (4.1 ± 0.2) × 108 L mol-1 s-1 at μ = 0.01 M and 25 °C. The plot of log kψ versus pH for the decomposition reaction is linear with a unit slope in the pH range 1.77−6.50. The diperoxide B decomposes mu...

Surface activity and premicellar aggregation of some novel diquaternary gemini surfactants

Abstract: A series of novel cationic gemini surfactants, C n H 2n+1 N+(CH3)2CH2CHOHCHOHCH2N+(CH3)2C n H 2n+1 ·2Br−, have been synthesized, and their surface properties were investigated in water, 0.1 N NaCl, and 0.1 N NaBr at 25°C. From surface tension-log molar concentration plots, the pC20, critical micelle concentration (CMC), and γCMC values have been determined, and the area/molecule at the aqueous solution/air interface was calculated. When the number of carbon atoms in the alkyl (hydrophobic) chains is above a certain number, which depends upon the molecular environment, the surface activity of the compounds is less than expected. This appears to be due to formation of small, soluble aggregates below the CMC. Equilibrium constants calculated for this aggregation indicate that a series of oligomers are formed.

An improved potential energy surface for the h2cl system and its use for calculations of rate coefficients and kinetic isotope effects

Abstract: We present a new potential energy surface (called G3) for the chemical reaction Cl + H2 → HCl + H. The new surface is based on a previous potential surface called GQQ, and it incorporates an improved bending potential that is fit to the results of ab initio electronic structure calculations. Calculations based on variational transition state theory with semiclassical transmission coefficients corresponding to an optimized multidimensional tunneling treatment (VTST/OMT, in particular improved canonical variational theory with least-action ground-state transmission coefficients) are carried out for nine different isotopomeric versions of the abstraction reaction and six different isotopomeric versions of the exchange reaction involving the H, D, and T isotopes of hydrogen, and the new surface is tested by comparing these calculations to available experimental data. The theoretical data are also used to investigate the equilibrium constant and the branching ratio for the reverse reaction, and calculations of...

Alcohol Effect on Equilibrium Constants and Dissociation Dynamics of Xanthone−Cyclodextrin Complexes

Abstract: The effect of alcohol addition on the ground state complexation of xanthone with cyclodextrins (CDs) and on the dissociation rate constants of triplet xanthone from these complexes was studied by fluorescence and laser flash photolysis experiments. In the case of β- and Hp-β-CD, the addition of alcohol led to the formation of weaker ternary complexes when compared to the xanthone CD binary complexes. In contrast, for γ-CD a slight increase of the complexation strength was observed for the ternary complexes. Addition of alcohols decreased the dissociation rate constant of triplet xanthone from β- and γ-CD by at least a factor of 5. The fact that the dissociation processes was slowed down for both CDs suggests that the effect of ternary complexation agents on the dynamics of complexation was not related to the strength of the ternary complexes formed.

Mechanistic Studies of Dichloro(1,4,7-triazacyclononane)copper(II)-Catalyzed Phosphate Diester Hydrolysis†

Abstract: In order to explore the role of labile metal complexes in promoting phosphodiester hydrolysis, detailed kinetic and mechanistic studies with Cu[9]aneN3Cl2 (1) were performed. The hydrolysis of bis(4-nitrophenyl) phosphate (2) by 1 is catalytic, as determined by the observation of both rate enhancement and turnover. The catalyst is selective for phosphodiesters; the rate of hydrolysis for 4-nitrophenyl phosphate (4) is 50 times slower than the hydrolysis rate of 2. A rate enhancement of 2000 is observed relative to hydroxide ion mediated hydrolysis, and the rates reported herein are comparable to the rates reported for phosphodiester hydrolysis by other labile metal complexes. The reaction of 1 with 2 showed a half-order dependence on the catalyst, implicating a monomer−dimer equilibrium with the monomer as the catalyst. The kinetically determined equilibrium constant is 1220 M-1, favoring the dimer. The reaction of 1 with ethyl 4-nitrophenyl phosphate (3) follows Michaelis−Menten kinetics (KM = 62.3 mM an...

Spectroscopy of Hydrothermal Reactions. 1. The CO2−H2O System and Kinetics of Urea Decomposition in an FTIR Spectroscopy Flow Reactor Cell Operable to 725 K and 335 bar

Abstract: A combined microflow reactor and short-path-length spectroscopy cell along with the accompanying process controls are described to obtain real-time, in situ transmission IR spectra of reaction components of aqueous solutions up to 725 K and 335 bar Quantitation of the spectra was required to obtain kinetics and equilibrium constants The extinction coefficient of CO2 in H2O at 275 bar was found to increase monotonically from 152 × 106 at 298 K to 226 × 106 cm2 mol-1 at 573 K Also, CO2 dissolved in H2O was rotationally quenched on the IR time scale below 375 K but progressed into rotational diffusion around 625 K and finally essentially free rotation above 700 K The kinetics and pathway of hydrothermolysis of urea to CO2 and NH3 were determined directly from spectral data at 473−573 K Good agreement was obtained between experimental and calculated concentration−time data by using a reaction model consisting of (NH2)2CO → NH4+ + OCN- and NH4+ + OCN- + H2O → CO2 + 2NH3 The Arrhenius parameters for the

Reaction of OH with Dimethyl Sulfide (DMS). 1. Equilibrium Constant for OH + DMS Reaction and the Kinetics of the OH·DMS + O2 Reaction

Abstract: The formation of a weakly bound adduct in the reaction of OH with DMS-d6 was observed between 217 and 240 K using the technique of pulsed laser photolysis/pulsed laser-induced fluorescence. The equilibrium constant for this process, OH + DMS-d6 ↔ OH·DMS-d6, was measured as a function of temperature. The bond strength of this adduct was determined to be 10.7 ± 2.5 kcal mol-1. The weakly bound adduct was observed to react rapidly with O2. The rate constant for the reaction OH·DMS-d6 + O2 → products was determined to be (1.00 ± 0.33) × 10-12 cm3 molecules-1 s-1, independent of pressure and temperature. The atmospheric implications of the formation of this adduct and its reaction with O2 to the mechanism of DMS oxidation in the atmosphere are discussed.

Modeling the Effect of Temperature on Adsorption of Lead(II) and Zinc(II) onto Goethite at Constant pH

Abstract: The influence of temperature and adsorbate concentration on the sorption of Zn(II) and Pb(II) by goethite was studied at fixed pH. Proton stoichiometry, measured by direct titration, was found to depend on the identity of the metal ion, the pH, and the temperature. For Zn(II) the proton stoichiometry ranged from 1.55 at pH 6.5 and 10°C to 1.95 at pH 7.5 and 70°C, while for Pb(II) at pH 5.50 the value varied from 1.05 at 10°C to 1.4 at 70°C. Three adsorption models—the Langmuir two-site model, the surface precipitation model of Farley, Dzombak, and Morel (FDM), and the BET model—were applied to the data, and the fitted parameters were used to determine the enthalpy and entropy of adsorption. Although the models assume rather different sorption reactions, and therefore generate correspondingly different equilibrium constants, the estimated enthalpies of adsorption were all positive, between 4 and 30 kJ/mol. There was also fair agreement between the various estimates of the adsorption capacities. However, the models yield very different estimates of entropies of adsorption, these being positive (and generally in the range 100 to 160 J K −1 mol −1 ) for the Langmuir and BET models but negative (in the range −80 to −160 J K −1 mol −1 ) for the FDM model.

Structure, binding energy, and equilibrium constant of the nitric acid‐Water complex

Abstract: Author Institution: Harvard University, Cambridge, MA 02138.; Aerodyne Research Inc., 45 Manning Rd., Billerica, MA 01821.

One- and Two-Metal Ion Catalysis of the Hydrolysis of Adenosine 3‘-Alkyl Phosphate Esters. Models for One- and Two-Metal Ion Catalysis of RNA Hydrolysis

Abstract: Adenosine 3‘-O(PO2-)OCH2R phosphate esters have been synthesized with R = 8-hydroxyquinol-2-yl (1a) and 8-(hydroxyquinolyl)-2-methylene (1b). The adenosine 3‘-O(PO2-)OCH2R structure has the essential features of an RNA dinucleotide. Equilibrium binding studies with metal ions Mg2+, Zn2+, Cu2+, and La3+ have been carried out with 1a, 1b, HOCH2R (7a and 7b), and 8-hydroxyquinoline (8), and equilibrium constants (Kas) have been determined for the formation of 1:1 (L)Mn+ complexes. The hydrolysis of 1a and 1b as well as (1a)Mn+ and (1b)Mn+ species are first order in HO-. The rate enhancement for hydrolysis of 1a by complexation with metal ions is as follows: ∼105 with Zn2+, ∼103 with Mg2+, ∼105 with Cu2+, and ∼109 with La3+. Molecular modeling indicates that metal ions ligated to the 8-hydroxyquinoline moiety in the complexes (1a)Mn+ and (1b)Mn+ catalyze 1a and 1b hydrolysis by interacting as Lewis acid catalysts with the negatively charged oxygen atom of the phosphate group. In the instance of La3+ complexe...