Protonation Equilibrium Constants Research Articles

Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water

Surface protonation is the most fundamental adsorption process of geochemical interest. Yet remarkably little is known about protonation of mineral surfaces at temperatures greater than 25°C. Experimentally derived standard enthalpies of surface protonation, ΔH r,1 °, ΔH r,2 °, and ΔH r,ZPC °, correspond to the reactions >SOH+H +=>SOH 2 + >SO −+H +=>SOH >SO −+2H +=>SOH 2 + respectively, and provide a starting point for evaluating the role of surface protonation in geochemical processes at elevated temperatures. However, the experimental data for oxides do not have a theoretical explanation, and data are completely lacking for silicates other than SiO 2. In the present study, the combination of crystal chemical and Born solvation theory provides a theoretical basis for explaining the variation of the enthalpies of protonation of oxides. Experimental values of ΔH r,1 °, ΔH r,2 °, and ΔH r,ZPC ° consistent with the triple layer model can be expressed in terms of the inverse of the dielectric constant (1/ε) and the Pauling bond strength per angstrom (s/r M-OH) of each mineral by equations such as ΔH r,ZPC °=ΔΩ r,Z[(1/ε)−(T/ε) 2(∂ε/∂T)]−B ′ Z(s/r M-OH)+H ′ Z. The Born solvation coefficient ΔΩ r,Z was taken from a prior analysis of surface equilibrium constants. The coefficients B Z ′ and H Z ′ were derived by regression of experimental enthalpies for rutile, γ-alumina, magnetite, hematite, and silica. This approach permits widespread prediction of the enthalpies of surface protonation. Predicted standard enthalpies of surface protonation for oxides and silicates extend over the ranges (in kcal.mole −1): ΔH r,1 ° ≈ −3 to −15; ΔH r,2 ° ≈ −0.5 to −18; ΔH r,ZPC ° ≈ −4 to −33. Minerals with the largest values of s/r M-OH (e.g., quartz and kaolinite) are predicted to have weakly negative enthalpies and a weak temperature dependence for their protonation equilibrium constants. Conversely, minerals with the smallest values of s/r M-OH (e.g., garnets and olivines) should have strong negative enthalpies and a strong temperature dependence for their protonation equilibrium constants.

Electrode processes of chloramines in aqueous solutions1Dedicated to Professors Z. Kublik and S. Rubel on the occasion of their 75th birthdays.1

Abstract Electroreduction of chloramines (H2NCl, HNCl2 and NCl3) at the platinum electrode was investigated in aqueous solutions of varying pH. The two former compounds are reduced irreversibly and their cathodic peak potentials are pH dependent (Epc (H2NCl)=−0.25 to −0.15 V (pH 7–4), Epc (HNCl2)=0.35–0.45 V (pH 4.5–2.5), at scan rate ν=0.05 V s−1). Trichloramine is reduced at the same potential at which Cl2 is formed, and the whole system is reversible. From the dependence of cathodic peak potentials on pH the protonation equilibrium constants of H2NCl and HNCl2 were determined. They were found to be (pK) 6.4±0.1 and 3.41±0.05, respectively. The extinction coefficients of protonated species are lower than that of non-protonated ones. The formal potential of the electrode reaction: NCl3+4H++6e−=NH4++3Cl− extrapolated to pH 0 equals 1.44 V (pH 0, SHE), thus the Gibbs energy of formation of NCl3, ΔGf=362.1 kJ mol−1.

Ionic Strength Dependence of Transition and Heavy Metal Adsorption onto Metal Oxides

Transition and heavy metal adsorption onto metal oxides influences the global cycling of metals, plays a role in the formation of enriched metal deposits, and filters natural and waste waters in the subsurface environment. The ionic strength dependence of metal adsorption is important to geochemists because natural waters are composed of different electrolyte types and vary widely in ionic strength. It is generally accepted that transition and heavy metal adsorption onto metal oxides does not exhibit an ionic strength dependence and that this behaviour indicates that metals bind to solid surfaces as inner-sphere complexes. However, Fig. 1 illustrates that while Cd adsorption from NaNO3 solutions onto goethite (Hayes and Leckie, 1987) does not exhibit major variations with ionic strength, Cd adsorption from NaCI solutions onto y-alumina (Kosmulski, 1996) progressively decreases with increasing ionic strength, and Cd adsorption from NaCIO4 solutions onto 7-alumina (Kosmulski, 1996) may increase with increasing ionic strength. An analysis of these datasets and many others reported in the literature indicates that transition and heavy metal adsorption is very strongly dependent on the nature of the electrolyte, and may increase, decrease, or exhibit little variation with increases in the ionic strength of the solution. In this study, experimental data for the adsorption of numerous transition and heavy metals onto quartz, silica, alumina, corundum, goethite, anatase, magnetite and manganese dioxide, from NaC104, NaNO3, KNO3, and NaC1 solutions, over a wide range of ionic strengths, have been fit using the triple layer model of Sahai and Sverjensky (1997), and at most two metal surface complexes. This model accounts for the ionic strength dependence of aqueous ion activity coefficients through the use of the extended Debye-Huckel expression. It is accompanied by a database of surface site densities, capacitances, and equilibrium constants for surface protonation, deprotonation, and electrolyte adsorption, which was modified to accommodate reported surface titration data and experimentally-determined points of zero charge. Recent applications of the triple layer model have suggested that divalent transition and heavy metals (M 2+) form simple surface species such as >SOM + in nitrate solutions (Hayes and Leckie, 1987; Katz and Hayes, 1995). However, using the triple-layer model of Sahai and Sverjensky (1997), surface complexes of the form >SOHM2+_NO3, where M 2+ is bound to a neutral surface site (>SOH) on the 0-plane of the triple layer model and NO3 is on the ]3-plane, are found to best fit data collected over a wide range of ionic strengths for Cd and Pb adsorption onto goethite (Hayes and Leckie, 1987) and Co adsorption onto corundum (Katz and Hayes, 1995). A similar type of surface complex >SOM+_NO3 fits the adsorption data for UO2 onto hydrous ferric oxide at pH values where the system can be assumed to be depleted in CO2. A combination of two complexes, including >SOHM2+_NO3 was found to fit Zn adsorption onto anatase (James and MacNaughton, 1977). In addition to sharing a common electrolyte, the systems described above have several other common characteristics: (a) the solids have intermediate-range dielectric constants between 10 and 22, (2) metal adsorption occurs on positively-charged surfaces, and (3) MNOfis the predominant aqueous complex over the pH range of adsorption and the range of ionic strengths studied. The one exception to (3) is Zn adsorption onto anatase, which required a second surface complex, >SOZnOH, to fit the adsorption data at low ionic strengths, where ZnOH-is the predominant aqueous complex. Adsorption of the predominant aqueous complex rather than the more abundant bare metal ion may be favoured because the free energy of solvation is smaller for a monovalent cationic complex (e.g. MNO3 + or MOH § than for a divalent cation (i.e. M 2+) (James and Healy, 1972). Metal-ligand complexation is also favoured in low dielectric constant environments like the interface. Metal adsorption data over a wide range of ionic strengths in NaC1 and NaC104 solutions are more limited. Calculations for Cd adsorption from NaC1 and NaC104 solutions onto 7-alumina are best matched by calculations using the surface complexes,

Synthesis and characterisation of two new catechol-based iron(III) ion-sequestering agents

The new compounds 1,4,10,13-tetrakis(2,3-dihydroxybenzoyl)-7,16-dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane (H8L1) and 1,4,7,10,13,16-hexakis(2,3-dihydroxybenzoyl)-1,4,7,10,13,16-hexaazacyclooctadecane (H12L2), having macrocyclic skeletons bearing four and six catechol hanging groups, respectively, have been synthesized and characterised. The crystal structure of H8L1·2dmso·2H2O (dmso = dimethyl sulfoxide), has been solved by single-crystal X-ray analysis. The equilibrium constants for protonation of the macrocycles and complexation of Fe3+ have been studied by potentiometric procedures in water–dmso (50∶50 v/v), 0.1 mol dm–3 NMe4Cl, at 298.1 ± 0.1 K. Both compounds are able to form mono- and di-nuclear iron(III) complexes of very high stability. These results indicate that the two compounds are thermodynamically able to scavenge Fe3+ from iron(III) transferrin. Their effectiveness as scavengers has been demonstrated by spectrophotometric measurements on the transmetallation reactions occurring in the presence of diiron(III) transferrin in aqueous solution. For comparison, the drug desferrioxamine B (DFB) has been also considered under the same experimental conditions. In 0.1 mol dm–3 phosphate buffer, at pH 7.4, both H8L1 and H12L2 remove iron from diiron(III) transferrin, the transmetallation reactions being much faster for H12L2 than for DFB which is slightly more effective than H8L1.

A Predictive Model of Surface Charge on Oxides in Aqueous Electrolyte Solutions: The Extended Triple Layer Model (ETLM)

Although experimental studies of the surface protonation of oxides in electrolyte solutions have been carried out for many years (Davis and Kent, 1990), few attempts have been made to place the experimental measurements into a coherent framework that permits prediction for systems that have yet to be studied experimentally. The enormous range of different natural and synthetic oxides of interest, and the correspondingly large range of electrolyte types and ionic strengths of relevence to industrial and natural processes, accentuate the need for such a predictive model. The purpose of the present study is to review progress made towards a comprehensive model for the prediction of surface charge on oxides in electrolyte solutions. Experimental data clearly show that surface charge depends on pH, ionic strength, and on the specific electrolyte in solution. Consequently, the triple layer model is used to analyse existing experimental data. It is extended with crystal chemical and Born solvation theory to make predictions of surface charge for an oxide in any specific 1:1 electrolyte over a wide range of ionic strengths. In order to do this, the key parameters of the triple layer model (Westall and Hohl, 1980) must be estimated in a consistent way. These include a site density for each mineral; the surface protonation equilibrium constants Kt and K2; the electrolyte adsorption constants KM+ and KL-; and the inner layer integral capacitance Cv

Theoretical prediction of single-site surface-protonation equilibrium constants for oxides and silicates in water

The equilibrium constants for surface protonation of solid oxides and silicates can be estimated from theoretical considerations and known properties of the solids for use in the constant capacitance, diffuse double layer or triple layer models of surface complexation. The theoretical considerations take into account Born solvation theory for the adsorbing proton, electrostatic interactions of the adsorbing proton with a surface oxygen and an underlying metal, and an intrinsic binding of the proton to the surface. As a consequence, the equilibrium constants for the νth ( ν = 1 or 2) surface protonation reaction on the kth solid can be expressed in terms of the inverse of the dielectric constant of the solid (1/ ε k ) and an average Pauling bond strength per angstrom (s/r M-OH) for the solid according to log K v = M v ( 1 / ∈ k ) − B v ( s / r M − OH ) + log K ii , v " , where the coefficients M ν , B ν , and K ii, ν ″ are constants characteristic of all oxides and silicates for each surface complexation model. Evaluation of these constants using experimental data for TiO 2, γ-alumina, Al 2O 3, FeOOH, Fe(OH) 3, silica, quartz, and kaolinite permits widespread prediction of surface protonation equilibrium constants from the known bulk structure properties 1/ ε κ and s/r M-OH. Such predictions should replace attempts to estimate surface protonation equilibrium constants for solids from empirical correlations with aqueous equilibrium constants. Surface protonation constants should also not be estimated from correlations with only the Pauling bond strength because these neglect specific treatment of solvation. Each individual oxide or silicate is expected to have its own unique surface characteristics, depending on the bonding in the bulk crystal structure connected to the surface. It follows that the bonding in surface protonated species is probably far more analogous to that in the bulk crystal structure than to the bonding in protonated aqueous species.

Influence of solution acidity on the adsorption and charge transfer kinetics of parabanic acid reduction

Parabanic acid reduction has been studied in the − 1.4 ⩽ H0 ⩽ 0.9 acidity range by the ac impedance technique. Changes in the adsorption of reactant and products with solution acidity lead to protonation equilibrium constants that differ from their bulk values. The observed pKa shifts were consistent with interfacial dielectric saturation. Variation of the charge transfer parameters with potential and proton activity allows a detailed analysis of the reduction mechanism to be performed in terms of the scheme of squares.

Complexation chemistry of ionophores A23187 and ionomycin

Abstract

Kinetics and mechanism of the oxidation of iminodiacetate, nitrilotriacetate and ethylenediaminetetraacetate by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III) in aqueous media

Kinetic studies of the oxidation of iminodiacetate (ida), nitrilotriacetate (nta) and ethylene-diaminetetraacetate (edta) by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [MnIII(cdta)]–, have been made in aqueous solution in the range pH 3.0–10.0 with varying reductant concentrations at constant ionic strength, I= 0.50 mol dm–3(NaClO4), and temperature, 30°C. All the reactions are first order both in complex and reductant concentration, and follow the general rate law –d[MnIII]/dt=kobs[MnIII]=(kd+ks[R])[MnIII], where kd denotes the autodecomposition rate of the complex, ks the electron-transfer rate and R is the reductant irrespective of the nature and type of the reacting species. The complex [MnIII(cdta)]– showed an interesting behaviour in acidic and alkaline media. For ida oxidation both the aqua- and hydroxo-forms of the complex are reactive and an inner-sphere mechanism has been proposed. However, for nta and edta oxidations, the aqua form is the sole reacting species and an outer-sphere mechanism has been proposed. The rate parameters and proton equilibrium constants of the complex and reductants were obtained by fitting of the experimental data by appropriate rate equations using computer-fit programs. Thus the reactivities of all the species of the polycarboxylates available in the reaction solutions have been evaluated individually. The reactivity orders are Hida– < ida2–, H2nta– < Hnta2– < nta3– and H3edta– < H2edta2– < Hedta3– < edta4–.

Metallation of porphyrins in lipid monolayers at the air/water interface

Protonation and metallation equilibrium constants of two amphiphilic porphyrins in different lipid matrices have been determined at the air-water interface by means of reflection spectroscopy. Evidence for a dissociation mechanism of porphyrin metal complex formation is obtained from shifts of the protonation pK a and metallation pK c values upon variation of matrixand porphyrin.

The mechanism of imine–enamine tautomerism of 2- and 4-phenacylquinolines

Isomerisations of 2- and 4-phenacylquinolines to their enaminone tautomers via 1,3 or 1,5 carbon–nitrogen hydrogen shifts occur by stepwise acid- or base-catalysed pathways similar to those for the enolisation of ketones. The reactions are observed as relaxations of the unstable to stable tautomers by stopped-flow spectrophotometry or, where the aromatic imine is the stable form, by trapping the enaminone with iodine in the reverse reaction. Evidence of mechanism comes from observations of general acid and general base catalysis, agreement between kinetically determined pKa values and independently measured values, and comparisons between rate and equilibrium constants for protonation of the enaminone tautomers and their N-methyl derivatives. The reactions show a primary isotope effect and yield normal Brönsted plots with αca. 0.5. The kinetically determined pKa values indicate N- rather than O-protonation of phenacylquinolines but for the enaminones O-protonation competes kinetically with the thermodynamically preferred C-protonation. Combination of pKa values for C-, N-, and O-protonation leads to equilibrium constants KT for enamine–imine and (protonated) keto–enol tautomerisation. The effect of 2- and 4-N-protonation (proton activating factors) upon rates and equilibria for ionisation of hydrogen from the methylene carbons is discussed and evidence of ‘imbalance’ in charge development on the carbon base in the transition state is noted. A concerted intramolecular 1,3-proton transfer is predicted but not observed.

Myeloperoxidase-dependent fluorescein chlorination by stimulated neutrophils.

Hypochlorous acid (HOC1) rapidly chlorinates fluorescein compounds forming, sequentially, the corresponding 4'-chlorofluorescein and 4',5'-dichlorofluoresceins. Chlorination by cell-free myeloperoxidase-catalyzed chloride peroxidation systems gives rise to these compounds as well as variable amounts of isomeric compounds chlorinated in the 2'- and 2',7'-positions. The fluorescence intensity of the dianionic form of the dye is partially quenched upon chlorination, and its proton equilibrium constants are shifted to more acidic values. Fluorescein covalently bound to zymosan (5-isothiocyanatofluorescein-zymosan) also formed these products when the unopsonized particles were incubated with phorbol myristate acetate- or N-formyl-methionyl-leucyl-phenylalanine-stimulated human neutrophils. This reaction was associated with a fall in fluorescence intensity, which was not observed when cells from individuals with chronic granulomatous disease or myeloperoxidase deficiency were used or when azide or catalase were added to the reaction medium. Fluorescent changes accompanying phagocytosis of serum-opsonized 5-isothiocyanatofluorescein-zymosan were also consistent with chlorination of the label; the changes were shown to be myeloperoxidase-dependent by use of myeloperoxidase-deficient or azide-treated cells. Oxidative bleaching of the structurally similar sulfonphthalein dyes by HOCl also occurs at rates which parallel the dye basicities. Results are discussed in relation to the use of fluoresceinated particles and sulfonphthalein dyes in the measurement of intraphagosomal acidification.

Kinetics of the decomposition and formation of ethylenedinitrilotetra-acetatovanadate(III) in dilute aqueous perchloric acid

The kinetics of the title reaction V(Y)⇌ V3++ HY have been studied spectrophotometrically in the forward direction at [H+]= 0.6–4.0 mol dm–3 and in the reverse direction at [H+]= 0.1–1.0 mol dm–3. V(Y) represents [V(edta)]– together with [V(Hedta)] and [V(H2edta)]+; the corresponding protonation equilibrium constants have been measured by using an e.m.f. cell under the conditions of the kinetic experiments, viz. ionic strength I= 4.0 mol dm–3(NaClO4) and 25.0 °C. HY represents H4edta together with H5edta+ and H6edta2+. Two reaction paths have been identified for both the forward and the reverse reaction. The equilibrium constant for the reaction has been measured directly at [ H+]= 0.5–1.0 mol dm–3 and its value is similar to that obtained from kinetic parameters.

ChemInform Abstract: STUDIES IN NICKEL(IV) CHEMISTRY. PART 2. KINETICS OF PROTON‐ASSISTED DECOMPOSITION OF TRIS(DIMETHYLGLYOXIMATO)NICKELATE(IV) WITH CONCOMITANT INTRAMOLECULAR ELECTRON TRANSFER IN AQUEOUS MEDIA

AbstractStudies in NickeNIV) Chemistry.

Studies in nickel(IV) chemistry. Part 2. Kinetics of proton-assisted decomposition of tris(dimethylglyoximato)nickelate(IV) with concomitant intramolecular electron transfer in aqueous media

The kinetics of proton-assisted decomposition of tris(dimethylglyoximato)nickelate(IV), [Ni(dmg)3]2–, with concomitant Intramolecular electron transfer forming bis(dimethylglyoximato)nickel(II) and/or hexa-aquanickel (II) have been studied. The values of pseudo-first-order rate constants (kad) corresponding to the solvent-assisted and proton-assisted decomposition satisfy the expression (1)kad=kod+k1dK1H[H+]+k2dK1HK2H[H+]2//1 +K1H[H+]+K1HK2H[H+]2(1) at I= 0.57 and 0.25 mol dm–3, where kod, k1d, and k2d, respectively, refer to the pseudo-first-order rate constants for the solvent-assisted, first protonation-assisted, and second protonation-assisted decomposition paths, and K1H and K2H are the first and second protonation equilibrium constants of [Ni(dmg)3]2–. Results are interpreted in terms of a mechanism involving formation of protonated nickel(IV) species prior to axial ring-opening and concomitant Intramolecular electron transfer.

Oxidation of (1-hydroxybenzyl)ferrocene and its derivatives substituted in phenyl ring with bis(triphenylsilyl) chromate

Kinetics have been studied of oxidation of (1-hydroxybenzyl)ferrocenes substituted in phenyl ring with bis(triphenylsilyl) chromate in benzene solutions as well as protonation of these alcohols in sulphuric acid medium. Logarithms of the oxidation rate constants (kobs, 20-40 °C) and those of the protonation equilibrium constants (KR+, 25 °C) show linear dependence on the Hammett σ constants, the ρ constant values being -0.86 to -0.40 and -2.50, respectively. These negative values suggest that the both processes are influenced by the same effects and confirm the mechanism proposed earlier for oxidation of alcohols with ferrocenyl substituent by action of bis(triphenylsilyl) chromate in aprotic solvents.

Fluorescence study of the divalent cation-transport mechanism of ionophore A23187 in phospholipid membranes

The mechanism for transport of divalent cations across phospholipid bilayers by the ionophore A23187 was investigated. The intrinsic fluorescence of the ionophore was used in equilibrium and rapid-mixing experiments as an indicator of ionophore environment and complexation with divalent cations. The neutral (protonated) form of the ionophore binds strongly to the membrane, with a high quantum yield relative to that in the aqueous phase. The negatively charged form of the ionophore binds somewhat less strongly, with a lower quantum yield, and does not move across the membrane. Complexation of the negatively charged form with divalent cations was measured by the decrease in fluorescence. An apparent rate constant (kapp) for transport of the ionophore across the membrane was determined from the rate of fluorescence changes observed in stopped-flow rapid kinetic experiments. The variation of kapp was studied as a function of pH, temperature, ionophore concentration, membrane lipid composition, and divalent cation concentration and type. Analysis and comparison with equilibrium constants for protonation and complexation show that A23187 and its metal:ionophore complexes bind near the membrane-water interface in the lipid polar-head region. The interfacial reactions occur rapidly, compared with the transmembrane reactions, and are thus in equilibrium during transport. The transport cycle can be described as follows: a 1:1 complex is formed between the membrane bound A23187-(Am-) and the aqueous divalent cation with dissociation constant K1 approximately 4.6 x 10(-4) M. This is in equilibrium with a 1:2 (metal:ionophore) complex (K2 approximately 3.0 x 10(-4) [ionophore/lipid]) that is responsible for transporting the divalent cations across the membrane. The rate constant for translocation of the 1:2 complex is 0.1-0.3 s-1. Dissociation of the complex of the trans side and protonation occur rapidly. The rate constant for translocation of H+ . A23187- is 28 s-1. A theory is presented that is capable of reproducing the kinetic data at any calcium concentration. The cation specificity for ionophore complex transport (kapp), determined at low ionophore concentration for a series of divalent cations, was found to be proportional to the equilibrium constant for 1:1 complexation. The order of ion specificity for these processes was found to be Ca2+ greater than Mg2+ greater Sr2+ greater than Ba2+. Interactions with Na+ were not observed. Maximal values of kapp were observed for vesicles prepared from pure dimyristoyl phosphatidylcholine. Inclusion of phosphatidyl ethanolamine, phosphatidic acid, or dipalmatoyl phosphatidylcholine resulted in lower values of kapp. Calcium transport by A23187 is compared with that of X537A, and it is shown that the former is 67-fold faster. The difference in rates is due to differences in the ability of each ionophore to form a 1:2 complex from a 1:1 complex.

General acid catalysis of alcohol heterolysis

The conversion of diferrocenylphenylmethanol (5) into the carbocation is general acid catalyzed in H/sub 2/O:CH/sub 3/CN, 1:1 w/w, and ..cap alpha.. = 0.88. The corresponding reaction of tropyl alcohol is also general acid catalyzed, and ..cap alpha.. = 0.71 and 0.90 in water and H/sub 2/O:CH/sub 3/CN, 1:1 w/w, respectively. However, the hydrogen ion catalyzed reactions have inverse solvent kinetic hydrogen isotope effects, and k/sub H20//k/sub D20/ = 0.56 for reaction of 5 in H/sub 2/O(D/sub 2/O):CH/sub 3/CN. Consideration of the equilibrium constants for alcohol protonation suggests that acid-catalyzed conversion of alcohol into carbocation should go from stepwise to concerted as the stability of the carbocation is increased.

Protonation equilibria of 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes

The spectroscopic behaviour of substituted 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes has been investigated in a solvent system consisting of 20% v/v ethanol and 80% v/v sulphuric acid–water in order to measure the protonation equilibrium constants. Some naphthols (I; X = H, o-OCH3, p-OCH3, o-CH3, p-Cl) and all the methoxynaphthalenes (II) show two absorption bands due to the free base and the conjugate acid, with a characteristic isosbestic point. For these compounds, pkBH+ values have been calculated using Jaffé's acidity function. Other naphthols (I; X =m-OCH3, m-CH3, p-CH3, o-Cl, m-Cl) show a single absorption band which shifts on changing the acidity of the medium. The lack of an isosbestic point has been ascribed to the overlap of the azo–hydrazone equilibrium with the protonation equilibrium of the azo-group. For methoxynaphthalenes, separate correlations of pKBH+ values with σm and σp of the substituents are found, owing to the presence in the molecules of two distinct basic centres.

The catalytic effect of acetate and its α-chloroderivatives on the acid hydrolysis of Cr(CN)(H 2O) 4NO +

The hydrolysis of Cr(CN)(H 2O 4)NO + in carboxylate buffer solutions was studied. Acetate and its α-chloroderivatives catalyze the aquation reaction of the complex. The observed rate of the aquation of the CN ligand is the sum of the contributions of acid hydrolysis and the catalytic factor. The reactions are pseudo-first order with respect to the Cr complex. The catalytic contribution to the hydrolysis depends on the concentration of carboxylic acid anion and of H + ions. The rate law also contains a term independent of the acid. The kinetic parameters of the catalyzed aquation were found and the reaction scheme proposed. The influence of carboxylate anions depends on their effect on the protonation equilibrium constant for the CN ligand and on the effect on the limiting rate of substitution of HCN and is explained in terms of ion-pair association. This mechanism is discussed and compared with other possible catalytic effects of anions on substitution reactions of the Cr(III) complexes.