Magda S. Saleh

Spectrophotometric study of the complexation equilibria of zirconium(IV) with 1-amino-4-hydroxyanthraquinone and the determination of zirconium

The spectral absorption and acid-base characteristics of 1-amino-4-hydroxyanthraquinone (AMHA) were studied in water-ethanol media. The composition, molar absorptivities, equilibrium constants and stability constants of the chelates of this reagent with zirconium(IV) have been determined spectrophotometrically in 40%V/V ethanol at 20 °C and an ionic strength of 0.1 M(NaClO4). Graphical logarithmic analysis of the absorbance graphs was used to demonstrate and characterise the complexation equilibria in solution. A simple, rapid, selective and sensitive method for the spectrophotometric determination of trace amounts of zirconium is proposed based on the formation of the Zr(AMHA)2 complex at pH 3.5 (λmax.= 600 nm, Iµ= 1.621 × 104 l mol–1 cm–1). Interference caused by a number of ions was masked by the addition of cyanide ions.

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.

Solution equilibria and stability of the complexes of pyridinecarboxylic acids: Complexation reaction of mercury(II) with 2-hydroxynicotinic acid

SummaryThe solution equilibria of 2-hydroxynicotinic acid (hyna) complexes with mercury(II) have been studied spectrophotometrically in 50% (v/v) ethanol at 20°C and an ionic strength of 0.1mol dm−3 (NaClO4). Three mercuric complexes are formed in solution in dependence on the acidity of the medium. The basic characteristics of the different complexes are determined and the analytical aspects of the complexation reaction are demonstrated. A critical investigation has also been presented of the solution equilibria and stability of the mixed complex of mercury(II) withhyna and thiosalicylic acid (tsa). The various complex transitions leading to the formation of the 1 : 1 : 1 Hg(tsa)(hyna) ternary complex in solution are investigated. The non-charged mono-ligand complex Hg(hyna) is used for UV-spectrophotometric determination of mercury atpH 4.5–5 (λmax=325nm, ɛ=0.8·104lmol−1cm−1). The system obeyed Beers law up to 36.1 µg ml−1 of Hg(II). The optimum concentration range (Ringbom) is between 6 and 28.5µg ml−1. Interference caused by a number of ions was masked by the addition of fluoride ions.ZusammenfassungDie Lösungsgleichgewichte von 2-Hydroxynikotinsäure (hyna) mit Hg(II) wurde spektrophotometrisch in 50% (v/v) Ethanol bei 20°C und einer Ionenstärke von 0.1 mol dm−3 (NaClO4) untersucht. In Abhängigkeit von der Acidität des Mediums werden drei Quecksilberkomplexe gebildet. Die grundlegenden Charakteristika der Komplexe wurden bestimmt und die analytischen Aspekte aufgezeigt. Die gemischten Komplexe von Hg(II) mithyna und Thiosalicylsäure (tsa), insbesondere die verschiedenen Komplexübergänge zum ternären 1 : 1 : 1 Hg(tsa)(hyna)-Komplex, wurden ebenfalls untersucht. Der ungeladene Monoligandenkomplex Hg(hyna) kann beipH 4.5–5 zur UV-spektroskopischen Quecksilberbestimmung eingesetzt werden (λmax=325nm, ɛ=0.8·104lmol−1cm−1). Das System gehorcht bis zu einer Hg(II)-Konzentration von 36.1µgml−1 dem Beerschen Gesetz. Der optimale Konzentrationsbereich (Ringbom) liegt zwischen 6 und 28.5µgml−1. Interferenzen mit einer Reihe anderer Ionen konnten durch Maskierung mit Fluoridionen umgangen werden.

Reaction of mercury(II)-thiosalicylate complex with picolinic acid and ultraviolet spectrophotometric determination of mercury(II)

The monothiosalicylate complex of mercury(II) reacts with picolinic acid (PIC) to form a ternary complex which has an absorption maximum at 295 nm in 50%V/V ethanol-water in the pH range 6.3–7.5. Beers law is obeyed up to a 20.15 µg ml–1 concentration of HgII. The molar absorptivity of the ternary complex is 0.86 m2 mol–1 at 295 nm and the sensitivity for the HgII ion is 5.2 × 10–3µg cm–2 per 0.001 absorbance unit. The composition of the ternary complex is estimated to be Hg(TSA)(PIC). A number of ions which interfere with the determination could be masked by the addition of EDTA or fluoride ions as masking agents. Further information on the complex-forming equilibria and stability of the HgII ternary complex was obtained by potentiometric titrations at 20 ± 0.1 °C and 0.1 M(NaClO4) ionic strength.

Synthesis, Characterization, and Photochemical Studies of Some Copper Complexes of Schiff Bases Derived from 3‐Hydrazino‐6‐methyl[1,2,4]triazin‐5(4H)one

Abstract Cu(II) complexes of new bidentate and tridentate Schiff bases derived from the condensation of 3‐hydrazino‐6‐methyl[1,2,4]triazin‐5(4H)one and aromatic aldehyde derivatives were synthesized and characterized. The structure of the complexes proposed according to elemental analyses, molar conductance, IR UV‐Visible absorption spectra is square‐planar. The thermodecomposition kinetics of the complexes were investigated under non‐isothermal condition. TG and DTG curves indicated that the complexes decompose in three to six steps. The photochemical behavior of the complexes depends on the substituted group on the aromatic aldehyde. A mechanism has been suggested to account for the recorded results.

Acid–base characteristics of naphthazarin and solution equilibria of yttrium(III) chelates

The spectral absorption and acid–base characteristics of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone; NAZA) in ethanol–water medium containing 50% v/v ethanol have been studied, and pKa values have been determined. Two equilibria are established in solution at pH 2.5–11.5, based on the acid dissociation of the non-ionized form of the reagent. The existence of these equilibria is confirmed by potentiometric pH titrations, and the precision of the equilibrium study is improved. The composition, molar absorptivities and stability constants of the chelates of NAZA with yttrium(III) have been determined by spectrophotometry under the conditions encountered in this study. The complexation equilibria existing in solution are demonstrated and characterized. The Y(NAZA) mono-ligand chelate formed at pH 5.5 (Iµ= 1.15 × 104 l mol–1 cm–1 at 595 nm) allows the determination of 1.95–8.86 µg ml–1 of yttrium. Interfering species and their elimination have been studied.

The visible absorbance maximum of 2-hydroxy-1,4-naphthoquinone as a novel probe for the hydrogen bond donor abilities of solvents and solvent mixtures

SummaryThe electronic absorption spectra of 2-hydroxy-1,4-naphthoquinone (HNQ) in one- and two-component solvents are discussed. The visible absorption of this reagent has been shown to arise from a charge-transfer transition of the zwitterionic tautomer stabilized by a hydrogen bond donating solvent. The formation constant of the 1:1 hydrogen bonded complex with methanol, ethanol, andn-propanol has been determined at 25 °C from the spectral behaviour in mixed solvents. The tautomerization equilibrium ofHNQ in aqueous ethanol is demonstrated and characterized. The dependence of the absorbance maximum ofHNQ on the hydrogen bond donor ability of the solvent in the visible spectrum, as measured by theTaft-Kamlet α parameter, has been established. Experimentally derived evidence is supplied to justify the recommendation ofHNQ as a novel probe for hydrogen bond donor ability in pure solvents and in mixed aqueous solvents.ZusammenfassungDie Absorptionsspektren von 2-Hydroxy-1,4-naphthochion (HNQ) in Ein- und Zweikomponentensystemen werden diskutiert. Die Absorption im sichtbaren Bereich stammt von einemcharge-transfer — Übergang des zwitterionischen Tautomers, das über Wasserstoffbrückenbindungen zum Lösungsmittel stabilisiert wird. Die Bildungskonstanten der binären Komplexe mit Methanol, Ethanol undn-Propanol wurden aus spektroskopischen Daten in Lösungsmittelgemischen bei 25 °C emittelt. Das Tautomeriegleichgewicht vonHNQ in wäßrigem Ethanol wird iskutiert. Die Abhängigkeit des Absorptionsmaximums vonHNQ vond der Wasserstoffbrückenbindungsbildungsfähigkeit des Lösungsmittels wird mittels desT aft-Kamlet — Parameters α beschrieben. Experimentelle Ergebnisse ermutigen zur Verwendung vonHNQ als neue Testsubstanzfür die Wasserstoffbrückenbindungsbildungsfähigkeit von reinen und gemischten Lösungsmitteln.

Solution equilibria of pyridinecarboxylic acids : complexation reaction of 2-mercaptonicotinic acid-copper(II) complex with thiosalicylic acid

The reaction of 2-mercaptonicotinic-copper (II) complex (Cu2+-MENA, 1 3n 1) with thiosalicylic acid (TSA) has been investigated spectrophotometrically in ethanol-water solution (50%, v/v) at I=O.IM (NaClO4) and 25°C. Under the acidic conditions encountered in this study, there are competing equilibria between protonation of the Cu2+-MENA binary chelate, the formation of the Cu(MENA)(TSA) ternary complex and protonation of the second ligand (TSA). The equilibrium constants for the complexation reactions and the stability of the mixed-ligand complex are determined. The optimum conditions for the predominance of the ternary complex are established and the enhancement of this complex over binary complex formation is evaluated. Complex-forming equilibria have also been examined by potentiometric-pH titrations and the experimental data are discussed in relation to various equilibria existing in solution. Structural and bonding features of the mixed-ligand complex are illustrated from considerations of the IR spectral data.

Potentiometric determination of the second stage dissociation constant of N-[Tris-(hydroxymethyl)-methyl]-2-aminoethane-sulphonic acid (TES) in different solvent mixtures

SummaryThe second stage dissociation constants of N-[Tris-(hydroxymethyl)methyl]-2-aminoethane sulphonic acid (TES) were determined at 25±0.1 °C by potentiometricpH-titration in different mixed solvent mixtures. The organic solvents used were methanol, ethanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetone, and dioxane. The ionization ofTES depends on both the proportion and the nature of the organic co-solvent used. It was observed that thepKa2 value is slightly influenced as the solvent is enriched in methanol, ethanol and remains practically constant in presence of different amounts ofDMF andDMSO. A pronounced change in thepKa2 is observed as the solvent is enriched in acetone or dioxane. These results are discussed in terms of various solvent characteristics. It is concluded that the electrostatic effect has only a relatively small influence on the dissociation equilibrium of the cationic group -