Khalid M. Tawarah

A spectrophotometric study of the tautomeric and acid- base equilibria of methyl orange and methyl yellow in aqueous acidic solutions

Abstract The UV-visible spectra of methyl orange and methyl yellow were examined in alkaline acidic and strongly acidic aqueous solutions at 25° C. In the alkaline medium methyl orange is present in the monomer form for concentrations ⩽1.2 × 10−4 moldm−3 while methyl yellow shows significant deviations from Beers law in the alkaline medium and time has a drastic effect on the spectra. In 0-032–0-46 mol dm−3 HCl both azo dyes are present as a tautomeric mixture. The position of the tautomeric equilibrium shifts to the side of the azonium tautomer for acidities > 0.5 mol dm−3 HCl. The tautomeric equilibrium constant (Kt) is calculated at several HCl concentrations. The pKa values for methyl orange and methyl yellow are 3.37 ± 0.01 and 3.01 ± 0.01 respectively. The microscopic acid dissociation constants of both azo dyes were calculated. Methyl orange was not extractable into CCl4 at any acidity while methyl yellow was completely extractable into CCl4 at acidities ⩽ 0.3 mol dm−3 HCl.

A spectrophotometric study of the acid-base equilibria of o-methyl red in aqueous solutions

Abstract The UV-visible spectra of o-methyl red were examined at 25°C in water at various acidities and the variations in the spectra were correlated with the structures of the several forms of the compound. The two acid-base equilibria that involve the monoprotonated form of o-methyl red were treated as overlapping equilibria. The thermodynamic acid dissociation constants (Ka2 and Ka3) of the diprotonated and the monoprotonated forms were found to be (4.16 ± 0.14) × 10−3 and (1.40 ± 0.02 ) × 10−5 mol dm−3, respectively. Complete extraction of o-methyl red from the aqueous phase into the CCl4 phase was observed in the pH range 1.25–5.6. The significance of this result with respect to the structure of the monoprotonated form is discussed.

Determination of the stability and stoichiometry of p-methyl red inclusion complexes with γ-cyclodextrin

The eAect of cyclomaltooctaose (g-cyclodextrin, gCD) on the UV‐visible spectrum of p-methyl red {4-[4-(dimethylamino) phenylazo] benzoic acid} in alkaline and acidic aqueous solutions was analyzed according to a 2:1 (dye:gCD) inclusion process. The inclusion process in acidic solutions was studied at diAerent ionic strengths. The values of the Gibbs free energy, G 0 , obtained in alkaline and acidic solutions indicate that the 2:1 inclusion complex of the cation form of the dye is more stable than that of the anion form. The values of the thermodynamic parameters H 0 and S 0 of the inclusion process are given for both media. The formation of the 2:1 inclusion complexes is exothermic and entropy stabilized. # 2000 Elsevier Science Ltd. All rights reserved.

Conductance study of the binding of methyl orange, o-methyl red and p-methyl red anions by α-cyclodextrin in water

A conductance study concerning the binding of methyl orange (MO, 4-[4-(dimethylamino)phenylaza]benzene-sulfonic acid), o-methyl red (o-MR, 2-[4-(dimethylamino)phenylaza]benzoic acid) and p-methyl red (p-MR) anions by α-cyclodextrin (α-CD) in water has been carried out at 25 °C. The observed molar conductances (Λs) were found to decrease significantly for mole ratios less than unity. A model involving 1 : 1 stoichiometry has been used to analyse the conductance data. The stability constant (K) and the molar conductance (Λc) for each 1 : 1 inclusion complex were determined from the conductance data by using a non-linear least-squares curve-fitting procedure. The binding sequence, based on the value of K at 25 °C, is found to be p-MR > MO > o-MR. The values of the ionic molar conductances of the complexed anions at infinite dilution are reported and the differences between such values and those of uncomplexed anions are in accord with the above binding sequence.

A thermodynamic study of the inclusion processes of α- andβ-cyclodextrins with the acid forms of methyl orange and methyl yellow

The effect of temperature on the UV-visible spectra of the acid forms of methyl orange and methyl yellow in the presence of α- andβ-cyclodextrins was analysed in terms of 1 : 1 inclusion processes. Three inclusion processes were considered for each azo dye/cyclodextrin system. The processes are the apparent inclusion process, the ammonium inclusion process, and the azonium inclusion process. The values of ΔH0 and ΔS0 are reported for each process. The inclusion complexes of the ammonium tautomer are more stable than those of the azonium tautomer and α-cyclodextrin inclusion complexes are more stable than those ofβ-cyclodextrin. The inclusion processes resulted in a shift in the position of the tautomeric equilibrium of an azo dye. A significant color fading was observed in α-cyclodextrin/methyl yellow solutions. No measurable changes in the absorbances were observed in the case of azo dye solutions containing γ-cyclodextrin.

An equilibrium study of p-Methyl Red inclusion complexes with α- and β-cyclodextrins

Abstract The effects of cyclomaltohexaose (α-cyclodextrin, αCD) and cylomaltoheptaose (β-cyclodextrin, βCD) on the UV-visible spectra of the anion and cation of p -Methyl Red 4-[4-(dimethylamino)phenylazo]benzoic acid in aqueous solution were analyzed according to 1:1 inclusion processes. The values of the thermodynamic parameters Δ H °, Δ S °, and Δ° were calculated from the effect of temperature on the thermodynamic stability constants of the inclusion complexes of the anion and cation with both cyclodextrins. The stability constants of the inclusion complexes of the tautomers of p -Methyl Red were evaluated at 25°C. The addition of a cyclodextrin shifts the position of the tautomeric equilibrium towards the side of the ammonium tautomer. Both α- and β-cyclodextrin bind the ammonium tautomer more strongly than the azonium tautomer. The most stable inclusion complex is that of the anion with αCD; the other complexes are of comparable stability.

A thermodynamic study of the association of the acid form of methyl orange with cyclodextrins

Abstract The UV-visible spectrum of the first conjugate acid of methyl orange was investigated at pH of 1.1 and at several temperatures in the presence of α-, β- and γ-cyclodextrins in aqueous solutions. The presence of γ-cyclodextrin has no measurable effect on the spectrum while α- and β-cyclodextrins induced relatively larger changes in the absorbances of the visible region. These changes were analyzed according to a 1:1 inclusion process and the equilibrium constants were determined at four temperatures. The values of the thermodynamic quantities ΔH°, ΔS° and ΔG° were obtained for the inclusion processes of α- and β-cyclodextrins. The inclusion complex of α-cyclodextrin is more stable than that of β-cyclodextrin. The findings were rationalized on the basis of variations in the size of a cyclodextrin cavity.

A spectrophotometric determination of the formation constants of the inclusion complexes ofα- andβ-cyclodextrins with the azonium and ammonium tautomers of methyl orange and methyl yellow

The color fading caused by the addition ofα-cyclodextrin orβ-cyclodextrin to an aqueous solution of a tautomeric mixture of methyl orange or methyl yellow is studied spectrophotometrically at pH 1.1 and 25.0°C. A model involving 1 : 1 stoichiometry has been used to analyze the spectrophotometric data. The addition of a cyclodextrin shifts the tautomeric mixture towards the side of the ammonium tautomer. An expression allowing the calculation of the tautomeric equilibrium constant of the inclusion complexes is derived. The formation constants of the inclusion complexes of the individual tautomers are determined. Bothα- andβ-cyclodextrins bind the ammonium tautomer stronger than the azonium tautomer. The inclusion complexes ofα-cyclodextrin are more stable than the corresponding ones ofβ-cyclodextrin.

Kinetics and mechanism of the photolytic CO-substitutions of cyclopentadienyl(dicarbonyl)iron thiocarboxylate complexes, Cp′Fe(CO)2SCOR (Cp′ = C5H5, ButC5H4,1,3-Bu2tC5H3; R = alkyl, aryl)

The kinetics of the photolytic CO-substitution of Cp′Fe(CO)2SCOR [Cp′ = C5H5, ButC5H4, 1,3-Bu2tC5H3; R = Me, But, Ph, 2-(O2N)C6H4, 3-(O2N)C6H4, 4-(O2N)C6H4, 3,5-(O2N)2C6H3] with PPh3 were studied in CH2Cl2 at 0 °C by i.r. spectroscopy. The reactions yielded exclusively the mono-CO-substituted derivatives, Cp′Fe-(CO)(PPh3) SCOR, and were found to follow second order kinetics with first order dependence on the concentration of each reactant. The differences in rates are discussed in terms of current knowledge pertaining to such reactions. An associative mechanism is proposed to account for the kinetic data of the reactions described.

A conductance study of the binding of benzo-15-crown-5 with alkali cations in acetonitrile

The decrease in the molar electrolytic conductance of Na+, K+, Rb+, and Cs+ tetraphenylborates, caused by the addition of benzo-15-crown-5 in acetonitrile at constant ionic strength, is analyzed according to a model involving 1:1 stoichiometry. The stability constant,K, and the limiting molar conductivity, Λc, for each 1:1 complex are determined from the conductance measurements by using a nonlinear least squares curve fitting procedure. The stability sequence of the 1:1 complexes, as deduced from data at 288, 293, 298, 303, and 308 K, has the order Na+>K+>Rb+>Cs+. Values of ΔH0, ΔS0, and Λc at 298 K are reported and their significance is discussed.