Spectrophotometric Determination of Dissociation Constants of Thiovioluric Acid and the Stability Constant of its Fe(II) Complex

Spectrophotometric studies of Fe(II)-thiovioluric acid system indicates the presence of a 1:2 complex in the pH range 4.0–6.5. The apparent equilibrium constant (K′₁) has been determined by general absorbance-extinction concentration scheme. The first and second dissociation constants of the acid have been studied at an ionic strength of 0.1 M while the third dissociation constant has been determined at I = 1 M. The effects of sudden pH change and buffer compositions on the dissociations have also been investigated. The stability constant of the complex has been calculated from the apparent equilibrium constant at ionic strength of 0.1 M. The stability constant works out to be 15.67 at pH 5.5.     

Role of the activity coefficient in the dissemination of pH: comparison of primary (Harned cell) and secondary (glass electrode) measurements on phosphate buffer considering activity and concentration scales

Despite recent efforts devoted to assessing both the theoretical rationale and the experimental strategy for assignment of primary pH values, these have not yet been accomplished satisfactorily. Traceability and comparability of pH values are achieved only within the constraints of internationally accepted conventions and predefined conditions that cannot account for all possible situations when pH is measured. Critical parameters to be defined are, in particular, the activity coefficients (γ _i) of the ionic species involved in the equilibrium with the hydrogen ions in the solution, which are usually estimated with the approximation typical of the Debye–Hückel theoretical model. For this paper, primary (Harned cell) measurements (traceable to the SI system) of the pH of a phosphate buffer have been considered and the results have been compared with secondary (glass electrode) measurements obtained by considering either the activity (paH) or concentration (pcH) scale of the hydrogen ions. With conventional approaches based on measurements related to activity or concentration scale, discrepancies emerge which have been assigned to incomplete inferences of γ _i arising from chemical features of the solution. It is shown that fitting and comparable paH and pcH results are attainable if evaluation of γ _i is performed using better estimates of the ionic strength, according to an enhanced application of the Debye–Hückel theory.     

Electrode Kinetics and Activation Energy of Uranyl-Ferron Chelate by Polarography

The uranyl chelate of ferron was investigated polarographically over the pH range 1.98–10.00 and ligand concentration 0.005–0.060.M. A reversible and diffusion controlled reduction wave was obtained, however, when ferron concentration below 0.02M and pH below 5.0, it became irreversible. The chelate species identified were UO₂(HA) ₂ at pH range 2.5–7.1 and UO₂(OH) (A) ₂^?3 over pH 7.1. The electron-transfer coefficient, rate constant, diffusion coefficient and activation energy of the reduction process were determined.     

Spectrophotometric Determination of Dioxovanadium (V) Ion Using Some Halogen Derivatives of Phenylazo-Chromotropic Acid

A comparative study of the reaction of o-chloro-, m-chloro-, p-chloro-, p-bromo- and p-iodophenylazo-chromotropic acids with dioxovanadium (V) ion have shown that 1:1 and 1:2 violet water soluble complexes were formed at pH 4.5. The p-halogeno-derivatives were found to be very suitable reagents for spectrophotometric determination of VO⁺₂ up to 8 ppm. The application of these dyes as indicators in the spectrophotometric titration of VO⁺₂ with EDTA and the interference of a various cations and anions were reported.     

Reactions of Copper-Free Hemocyanin from Taiwan Snails with Metal Salts

Copper-free hemocyanin (apohemocyanin, apoHc) from Taiwan snails was prepared by cyanide treatment and the reconstitution of hemocyanin was carried out by treating apoHc with Cu(CH₃CN)₄ClO₄. At pH below 5, the apoHc solution is cloudy. When reconstituted at pH near 6, the ratio of absorbances, A₃₄₅/ a₂₈₀, has the normal value 0.21, and rapidly decreases with increase in pH. This indicates that reconstitution is best carried out near pH 6. Addition of fluoride at pH 5.3 and cyanide at pH 5.7 causes a relatively rapid drop in absorbance at 345 nm, a primary reaction followed by a slower secondary process. Comparison of the reactions of native and reconstituted hemocyanins with oxygen, fluoride, and cyanide shows that the reconstitution has been successful. Cadmium and manganese derivatives of hemocyanin have been prepared. The order of affinity of metals to the active site of hemocyanin is: Cu>Mn>Cd.     

Polarography of Molybdenum (VI)—Gallic Acid Complexes in Perchlorate Media

Polarographic and spectrophotometric studies were carried out on Mo (VI)—gallic acid (G) complexes in aqueous solution of perchlorate media. It was found that the number of waves of Mo (VI) depends on gallic acid concentration as well as on the pH of solution. Only a single well-defined irreversible wave is obtained at pH 8.0 for the reduction of Mo (VI) to Mo (III), proportional to Mo (VI) concentration in presence of 20 mM gallic acid with E_1/2=?1.22 V vs SCE. The composition of the formed complexes depends on the pH of solution as revealed from spectrophotometric studies, a 1:1 Mo/G complex is formed at pH 3.0 and 1:2 complex at higher pH′s-NMR study of the isolated 1:2 complex proves that the complexation occurs through—OH groups of gallic acid.     

The Electrochemical Behavior of Complexes of Cobalt and Nickel with δ-Valerolactam at the Dropping Mercury Electrode

The kinetics of the polarographic reduction of Co(II) and Ni(II) has been investigated in the presence of an increasing concentration of the complex forming agent δ-valerolactam at pH 6.0 and at constant ionic strength (0.1 M KNO₃,). In both cases the waves were found to be diffusion controlled and irreversible. The values of the kinetic para-meters such as the forward rate constant K°_{f, k} and the charge transfer coefficient αn have been calculated as a function of ligand concentration. The diffusion current was precisely proportional to the concentration up to a metal ion concentration of 3.5 mM. Hence, a straight line is useful as a calibration curve in the quantitative analysis of these two metal ions. The method has also been applied to the determination of these metals in certain alloys.     

Spectrophotometric Studies of Cobalt(II) and Copper(II) Chelates of 2-Picolylamine

Cobalt (II) and copper (II) chelates of 2-picolylamine have been studied spectrophotometrically in aqueous solution. Formation of 2-picolylamine chelates was pH dependent, and the optimum pH range of the cobalt (II) chelate was at pH 8.8-9.6, and copper(II) chelate at 4.7-6.8, respectively. The mole ratio of 2-picolylamine to both of metal(II) ions was found to be 2 to 1 stoichiometry. The spectrophotometric sensitivities are 0.23γ of Co/Cm² and 8.9γ of Cu/Cm³, at 373 mμ, pH 9.0 for Co(II) chelate and 588 mμ, pH 6.0 for Cu (II) chelate. The stability constants (logK) of the chelates CoL₂²⁺ and CuL₂²⁺ have been evaluated at a constant ionic strength of 0.10 at 20, 25, 30, 35 and 40°C. Enthalpy and entropy changes characterizing the formation of the chelates have been calculated at 25°C The results are collected as follows:      

Spectrophotometry Determination of Cobalt after Extraction of its Ternary Complex with 3-(4-Phenyl-2-Pyridyl)-5,6-Diphenyl-1,2,4-Triazine and Tetraphenylborate into Molten Naphthalene

The reaction of 3-(4-phenyl-2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PPDT) and tetraphenylborate (TPB) with cobalt (II) has been studied to determine the optimum conditions for the extraction and quantitative spectrophotometry determination of this metal. The ternary complex is extracted into molten naphthalene at pH 3.6–7.4. The solid naphthalene containing the cobalt associated complex is separated by filtration and dissolved in acetonitrile. Beer's law is obeyed in the concentration range 8–140 μg cobalt in 10 ml of acetonitrile solution. The molar absorptivity and sensitivity are 4.2×10³ l·mol^?1·cm^?1 and 0.01408 μg/cm², respectively. The other factors such as pH, amounts of reagents and naphthalene, shaking and standing times, and the effect of diverse ions are studied. The method has been applied to the determination of cobalt in iron steel alloys.     

13C NMR investigation of tautomeric and acid-base equilibria of pyrazolone T and three analogs

Pyrazolone T and three derivatives have been characterized by ¹³C and, in part, ¹⁵N nmr at several pH values. The ¹³C chemical shifts have been assigned at, or near, the equivalence points and pK_a values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3-bond C-H couplings. The ¹³C chemical shift data indicate the existence of both tautomeric and acid-base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values.     

Carbon–13 NMR studies of tautomerism in some 2-substituted imidazoles and benzimidazoles

The ¹³C NMR spectra of several 2-substituted imidazoles and benzimidazoles have been measured. The substituent was CH₃, COOH and CONHR, where R = H, n-Bu, p-tolyl or m-chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N-1 to N-3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2-substituent (rate CH₃?CONH-phenyl > CONH-p-tolyl ? CONH-m-chlorophenyl ? CONH-n-butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.     

Spectrophotometric Studies of Nickel(II) and Copper(II) Chelates of p-(2-Hydroxy-1-Naphthylazo)-Benzenesulphonate

The ionization constant of p-(2-hydroxy-1-naphthylazo)benzene-sulphonate (Orange II) and the formation constants of the metal chelates of this reagent with Ni(II) and Cu(II) have been determined spectrophotometrically in aqueous solution at 25° and at an ionic strength of 0.10M. The ionization constant of orange II was found to be pK_a=10.95. Formation of orange II chelates with Ni(II) and Cu(II) was pH dependent, and the optimum pH range of the Ni(II) Chelate was at pH 9.2-9.4, and Cu(II) chelate at 9.5-9.7, respectively. The mole ratio of orange II to both of metal ions was found to be 2 to 1 stoichiometry. The formation constants (logK) of the Ni(II) and Cu(II) chelates were 12.50 and 16.11, respectively. The molar extinction coefficients and the photometric sensitivities of these chelates were determined.     

Solvent and substituent effects on the photoreduction of cobalt(III)-aryl amine complexes in an aquo-organic solvent medium

The photoreduction of a series of Co^III complex ions, [Coen₂Cl(RC₆H₄NH₂)]²⁺ where R = p-OMe, p-OEt, p-Me, m-Me, H, and p-F, has been studied using a low pressure Hg vapour lamp as the light source (254 nm) in an aquo-organic solvent [15–40% (v/v) 2-methylpropan-2-ol] medium. Quantum yields for Co^II production by redox decomposition have been determined in all cases. The quantum yield increases considerably with the increase in concentration of the organic co-solvent in the binary solvent mixture under investigation. The Hammett correlation is linear, affording negative reaction constants, which indicate that the excited state is electron deficient. Correlation of the experimental results with Kamlet–Taft's solvatochromic parameters indicates that the solvent hydrogen bond donor acidity plays a dominant role in governing the reactivity.     

Determination of pKa values of some novel benzimidazole salts by using a new approach with 1H NMR spectroscopy

Benzimidazoles and their derivatives including imidazole are studied widely because they exist in the structure of natural products and different drugs. pK_a values are extremely important for drug discovery and improvement in order to determine pharmacokinetic and pharmacodynamic features such as permeation through biological barriers, interactions with the target area or side effects. Acid–base features (pK_a) have great importance not only for physiological characteristics but also for being used as a ligand or changing physico‐chemical features by turning benzimidazoles into salts. Within the scope of this study, a variety of new benzimidazole salts were synthesized, and their characterizations were made by NMR spectroscopy, FTIR spectroscopy and element analysis techniques. The pK_a values of synthesized benzimidazole salts were determined by inflection point approach using integration values obtained with ¹H NMR spectroscopy and Henderson–Hasselbalch analysis. pK_a values of some benzimidazole salts were also determined by potentiometric methods in order to compare those of NMR spectroscopy results. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation of thermodynamic functions for complexation reactions involving bivalent metal ions and ethyl-2,3-dioxobutyrate-2 p -bromophenylhydrazone

Complexation reactions of bivalent metal ions and ethyl-2,3-dioxobutyrate-2p-bromophenylhydrazone (EDOB-2p-BPH) have been studied potentiometrically in 50% (v/v) ethanol/water medium at different ionic strengths with respect to NaClO₄ and at different temperatures and their stability constants determined. The method of Bjerrum and Calvin as modified by Irving and Rossotti has been used to determine then andpL values. S_min values which have the same significance as ‘T²’ have also been calculated. The thermodynamic stability constants and standard free energy change (ΔG) have also been calculated. ΔG values are negative in all cases indicating that the reactions are spontaneous. The ligand field stabilization energy (δH) has also been calculated for the 3d transition metals.     

Influence de la structure sur l'hydrolyse des N-pyrrolylmethylene-2 anilines

Hydrolysis of the imine function of a series of Schiff bases derived from pyrrole-2-carboxaldehyde and substituted anilines (X = H, p-OCH₃, p-OC₂H₅, p-CH₃, p-Cl, p-Br, m-CN, m-NO₂, p-NO₂) was studied in all of the pH ranges. The hydrolysis curves log k_abs (mn^?1) = f(pH) were established in buffered aqueous methanol by polarography or amperometry. The shapes of the curves obtained for pH > 5 indicates that N-pyrrolylmethylene-2 anilines hydrolyse according to the same mechanism as N-benzylidene anilines. The particular stability of these products for pH < 5 permits one to obtain complete hydrolysis curves in acid media. A very good Hammett correlation (k_obs = + 1,73) has been established from the maxima which appear for strong acidity. This stability is interpreted as due to the specific electron-donating effect of the pyrrole nucleus. The influence of the structural parameter X on the morphology of the curves log k_abs = f(pH) and on the hydrolysis mechanism of the imine function is discussed in all of the pH ranges.     

Determination of weak (2.0–2.5) dissociation constants of non-UV absorbing solutes by capillary electrophoresis

Summary Capillary electrophoresis (CE) has proved to be a fast and convenient method for the determination of the dissociation constants of non-UV absorbing solutes in the acidic pK _A range (2.0–2.5). The electroosmotic flow was reversed by washing the capillary with 0.2% polybren aqueous solution. A series of background electrolytes was prepared with phenylphosphonic acid (pK _A=1.29) and β-alanine (pK _A=3.55) with the same ionic strength and a high buffer capacity in order to improve the repeatability (0.1–0.2 %) of the electrophoretic mobility and to determine the values of pK _A accurately. This procedure was applied to the determination of the dissociation constants of several alkyl-alkylphosphonic acids whose pK _A values have not yet been published in the literature. In this work, their dissociation constants have been found to vary between 1.91 and 2.34 for alkyl-methylphosphonic acids and between 2.10 and 2.38 for alkyl-ethylphosphonic acids.     

Hydrolysis of carboxylate and phosphate esters using monopyridinium oximes in cationic micellar media

The reactions of p‐nitrophenyl acetate (PNPA) with a series of monopyridinium oximes, viz. 2‐PAM (2‐hydroxyiminomethyl‐1‐methylpyridinium iodide), 3‐PAM (3‐hydroxyiminomethyl‐1‐methylpyridinium iodide), and 4‐PAM (4‐hydroxyiminomethyl‐1‐methylpyridinium iodide) have been studied in the presence of cationic surfactants of same hydrophobic chain length (C₁₆) within the concentration range of 0.5–6.0 mM at pH 8.0 under the pseudo‐first‐order condition. The observed rate constant (k_obs) increases with increasing surfactant concentration culminating into a maximum, and this has been analyzed in detail following the concepts of micellar catalysis. The structure–activity relationship of the investigated oximes has been discussed, and 2‐PAM was found to be the most reactive among all the three investigated oximes for the cleavage of PNPA. Esterolytic decomposition of p‐nitrophenyldiphenyl phosphate with oximate ions (? CH?NO^?) was followed in cetyltrimethylammonium bromide micelles at pH 9.0, and 4‐PAM was the most reactive oxime for the micellar hydrolysis of phosphate ester. The apparent acid dissociation constants (pK_a) of the investigated oximes have been determined spectrophotometrically. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 569–578, 2011     

Grafting onto poly(vinyl alcohol). II. Graft copolymerization of methyl acrylate and vinyl acetate and their binary mixture using γ-rays as initiator

Methyl acrylate (MA), vinyl acetate (VAc) and their binary mixture (MA + VAc) have been graft copolymerized onto poly(vinyl alcohol) using γ-rays as initiator by mutual radiation method in aqueous medium. The optimum conditions for affording maximum grafting have been evaluated. The percentage of grafting has been determined as a function of total dose, concentrations of poly(vinyl alcohol), MA, VAc, and their binary mixture. Rate of grafting (R_p) and induction period (I_p) have been determined as a function of total initial mixed monomer concentration and concentration of poly(vinyl alcohol). The graft copolymer has been characterized by thermogravimetric method. The effect of donor monomer (vinyl acetate) on percent grafting of acceptor monomer (methyl acrylate) has been explained.     

Propagation Kinetics of Free-Radical Methacrylic Acid Polymerization in Aqueous Solution. The Effect of Concentration and Degree of Ionization

Propagation rate coefficients, k_p, of free-radical methacrylic acid (MAA) polymerization in aqueous solution are presented and discussed. The data has been obtained via the pulsed laser polymerization – size-exclusion chromatography (PLP-SEC) technique within extended ranges of both monomer concentration, from dilute solution up to bulk MAA polymerization, and of degree of ionic dissociation, from non-ionized to fully ionized MAA. A significant decrease of k_p, by about one order of magnitude, has been observed upon increasing monomer concentration in the polymerization of non-ionized MAA. Approximately the same decrease of k_p occurs upon varying the degree of MAA ionization, α, at low MAA concentration from α = 0 to α = 1. With partially ionized MAA, the decrease of k_p upon increasing MAA concentration is distinctly weaker. For fully ionized MAA, the propagation rate coefficient even increases toward higher MAA concentration. The changes of k_p measured as a function of monomer concentration and degree of ionization may be consistently interpreted via transition state theory. The effects on k_p are essentially changes of the Arrhenius pre-exponential factor, which reflects internal rotational mobility of the transition state (TS) structure for propagation. Friction of internal rotation of the TS structure is induced by ionic and/or hydrogen-bonded intermolecular interaction of the activated state with the molecular environment.