M.S. Abu-Bakr

Acid dissociation and solution equilibria of some pyridinecarboxylic acids

The acid–base equilibria of 2-hydroxynicotinic acid (hyna), 2-mercaptonicotinic acid (mena), 3-hydroxypicolinic acid (hypa) and pyridine-2,3-dicarboxylic acid (quinolinic acid, quina) have been studied spectrophotometrically in a water–ethanol medium containing 23.5 mol-% ethanol at 20 °C. The solution spectra of these compounds have been recorded at different pH values and the spectral changes are explained in terms of shifts in equilibria between the different species that exist in solution. Two equilibria have been established in solutions at pH values of approximately 3–10.5, based on the acid dissociation of the non-ionized form (LH2) of the reagents. The existence of these equilibria is confirmed by potentiometric pH titrations. The release of the proton from the cationic form (LH3+) of substituted nicotinic acids was verified by measurements of the spectra at pH 0.5–2. The pKa values corresponding to various acid-base equilibria have been determined and are discussed in relation to molecular structure. Tautomeric equilibria of the type: neutral species ⇌ zwitterionic species, in solutions of hyna and mena, have also been characterized using the ionization constants of the two acids.

Complexation equilibria and spectrophotometric determination of iron(III) with 1-amino-4-hydroxyanthraquinone

The complex equilibria of iron(III) with 1-amino-4-hydroxyanthraquinone (AMHA) were studied spectrophotometrically in 40% (v/v) ethanol and an ionic strength of 0.1M (NaClO(4)). The complexation reactions were demonstrated and characterized using graphical logarithmic analysis of the absorbance-pH graphs. A simple, rapid, selective and sensitive method for the spectrophotometric determination of trace amounts of Fe(III) is developed based on the formation of Fe(AMHA) complex at pH 2.5 (lambda(max) = 640 nm, epsilon approximately = 2.1 x 10(4) L. mol(-1) . cm(-1)) in the presence of a large number of foreign ions. Interferences caused by palladium(II) was masked by the addition of cyanide ions. The method has been applied to the determination of iron in some synthetic samples and polymetallic iron ores.

Complex formation equilibria of iron(III) with 2-hydroxy-3- pyridinol and 2-mercapto-3-pyridinol. Spectrophotometric determination of iron(III) in multivitamins

ABSTRACT The complex equilibria of iron(III) with 2-hydroxy-3-pyridinol (HHP), and 2-mercapto-3-pyridinol (MHP) were studied spectrophotometrically in 40% (v/v) ethanol and an ionic strength of 0.1M (NaCIO4). The complexation reactions were demonstrated and characterized using graphical logarithmic analysis of the absorbance pH-graphs. After considering all the different parameters a simple, rapid, sensitive and selective method for spectrophotometric determination of trace levels of iron(III) was proposed based on the formation of (Fe -MHP) complex at pH 2.5 (λmax = 640 nm, ɛ = l×104 L mol−1 cm−). The interference of a large number of foreign ions was investigated. The method has been applied successfully for the determination of iron content in some multivitamins with mineral preparations and infant milk products.

Spectrophotometric study of acid-base equilibria of 4-(2-benzothiazolylazo)resorcinol and 4-(2-benzothiazolylazo)salicylic acid in water-organic solvent media

The visible absorption spectra of 4-(2-benzothiazolylazo)resorcinol (BTAR) and 4-(2-benzothiazolylazo)salicylic acid (BTAS) have been recorded in water-organic solvent mixtures in the pH range 0.5–12.0. The organic solvents used are methanol, ethanol,n-propanol, iso-propanol, acetone, dioxane and dimethyl formamide. The spectral changes have been explained in terms of shifts in equilibria among different molecular and ionic species of BTAR and BTAS existing in solution. The pKa values corresponding to the different ionization steps have been determined at 25°C and I=0.1M (KNO3) by graphic analysis of the absorbance-pH curves. The results obtained are discussed in terms of the molecular structure of the reagents and the nature of the organic co-solvent used.

Spectral properties and analytical application of the ternary complex of lanthanum(III) with 1,10-phenanthroline and eosin

Abstract The pink lanthanum—(1,10-phenanthroline)2—(eosin)2 complex is used to determine 0.5–10 × 10-5 M lanthanum, either in aqueous solution or chloroform. In the presence of EDTA, only aluminium and cyanide interfere.

Spectrophotometric study of the complexation equilibria of uranium(VI) with 1,4-bis(4'-methylanilino) anthraquinone and determination of uranium(VI)

Abstract The reaction of U(VI) with 1,4-bis(4′-methylanilino)anthraquinone (quinizarin green) in water-dimethylformamide medium was investigated spectrophotometrically. The complexation equilibria in solution were demonstrated. The study of the reaction in presence of equimolar concentrations or in solutions containing metal or ligand excess gave evidence for the formation of complexes with stoichiometric ratios of UO2:L = 1:1 and 1:2 in dependence on the pH of the medium. Their thermodynamic stabilities and the values of their molar absorption coefficients were determined. The optimum conditions for spectrophotometric determination of U(VI) with this reagent were found.

Facile Spectrophotometric Determination of Aluminum(III) and Copper(II) Simultaneously with Alizarin Yellow R in Aluminum-Copper Alloys

A facile, highly sensitive, and selective spectrophotometric method has been developed for the determination of aluminum and copper in different alloys. The method is based on the formation of the binary complexes of aluminum and copper with Alizarin yellow R (AYR) [5-(4-nitrophenylazo) salicylic acid] at pH 2.0 and 6.2, respectively, at wavelengths 422 and 445 nm with molar absorbtivity 1.1 × 104 l mol−1 cm−1 for each one. Aluminum and copper were thus determined in the ranges 0.4–2.0 μg ml−1 and 1.9–3.6 μg ml−1, respectively, in the presence of one to another. The detection limits were found to be 1.4 ng ml−1 of aluminum and 0.015 μg ml−1 for copper. The relative standard deviations were in all instances less than 1.3%. The proposed method was successfully applied for the simultaneous determination of aluminum and copper in certified reference aluminum-copper alloys.

Molecular complex formation of thiosalicylic acid with ethanol

Abstract The formation of a 1:1 molecular complex of thiosalicylic acid (TSA) with ethanol was studied spectrophotometrically. Ethanol is postulated to form an intermolecular hydrogen bond with the carboxyl OH group of TSA, the intramolecular hydrogen bond of the SH group remaining intact. Results of SCF-CI molecular orbital calculations support the spectral interpretation. The formation constant ( K ⨍ ), free energy change (Δ G ) and molar absorptivity of the TSA-ethanol molecular complex with cyclohexane, CCl 4 or CHCl 3 as solvents were determined. K ⨍ values vary with solvent in the order cyclohexane > CCl 4 > CHCl 3 . The electronic absorption spectra of TSA in one- and two-component solvents are discussed.

An analytical study of mixed-ligand selenium(IV) complexes

SummarySpectrophotometric and potentiometric studies have been made to investigate the reaction of selenium(IV) with eosin and 1,10-phen-anthroline in aqueous solution. An ion association ternary complex with a stoichiometric ratio Se-(phenanthroline)2-(eosin)2 is formed. The stability constant of the complex is determined and the optimum conditions for the spectrophotometric determination of Se(IV) have been established.ZusammenfassungDie Reaktion von Selen(IV) mit Eosin und 1,10-Phenanthrolin in wäßriger Lösung wurde spektrophotometrisch und potentiometrisch untersucht. Ein ternärer Assoziationskomplex der stöchiometrischen Zusammensetzung Se-(Phenanthrolin)2-(Eosin)2 wird dabei gebildet. Die Stabilitätskonstante wurde bestimmt und die optimalen Bedingungen für die spektro-photometrische Bestimmung des Selens wurden ermittelt.

Ternary complexes in the spectrophotometric determination of trace amounts of silver(I) and cadmium(II)

The reaction of silver(I) and cadmium(II) with 1,10-phenanthroline (PHEN) and Eosin (2,4,5,7-tetrabromofluorescein) gives coloured association complexes. The solution spectra of the mixed-ligand complexes are characterised by high intensity (∈ ≈ 104) metal-to-ligand charge-transfer bands at 540–555nm. The optimum conditions for the spectrophotometric determination of Ag(I) or Cd(II) have been established. A simple, sensitive and selective method was proposed for the determination of traces of the metal ions either in aqueous or organic media. In the presence of EDTA only aluminium and cyanide interfere.