Spectkophotometric determination of the dissociation constants of n,n'-bis(3-hydroxypropyl)dithiooxamide and n,n'-bis(2-hydroxypropyl)dithiooxamide

The dissociation constants of N,N'-bis(3-hydroxypropyl)dithiooxamide and N,N'-bis(2-hydroxypropyl)dithiooxamide were determined by a spectrophotometric method using a weighted least squares technique for the calculations. For N,N'-bis(3-hydroxypropyl) dithiooxamide a thermodynamic constant pK₁^T of 11.37 was found. At ionic strength μ = 1, pK₁ = 11.27 and pK₂ = 14.29. For N,N'-bis-(2-hydroxypropyl) dithiooxamide, these values were respectively: pK₁^T = 11.11; pK₁ = 10.99 and pK₂ = 13.75.     

Autoprotolysis constants and standardization of the glass electrode in acetonitrile-water mixtures. Effect of solvent composition

Standard e.m.f.s for the cell GE/HCl/AgCl/Ag/Pt (GE=glass electrode) in acetonitrile-water mixtures containing 0–70% (w/w) of acetonitrile were obtained. Values of the autoprotolysis constant, K_ap, of these mixed solvents were also determined from e.m.f. measurements of the cell GE/KCl + KS/AgCl/Ag/Pt. The influence of variations in the solvent composition on pK_ap values was evaluated. Over the whole of the composition range studied the pK_ap values were linearly correlated with the mole fraction of acetonitrile and with the reciprocal of the relative permittivity of solvent mixtures. Linear relationships were also obtained for pK_ap values vs. the Kamlet-Taft π^★ polarizability/dipolarity parameter in the range 0–50% (w/w).     

The acidity functions of trifluoroethanol and hexafluoroisopropanol,and their mixtures with water

Acidity measurements in trifluoroethanol and hexafluoroisopropanol as solvents, and in their mixtures with water, are reported. The hydrogen electrode and the glass electrode were used for pH measurements. The “experimental” autoprotolysis constants were determined from measurements in strongly acidic (trifluoromethanesulfonic) media and in strongly basic (alcoholate) media: for trifluoroethanol, pK = -log K/mol² l^-2 = 15 and for hexafluoroisopropanol, pK = 14.8. Evaluation of the pH-indicator potential systems with reference to the ferrocene/ferricinium couple gives the acidity function R₀(H). The values obtained are compared to the H₀ values evaluated in the same conditions.     

The synthesis and analytical capabilities of chromogenic aza-12-crown-4 as a selective reagent for lithium ion

The synthesis and analytical capabilities of a new chromogenic crown ether, 1-(2-hydroxy-5-nitrobenzyl)-1-aza-4,7,10-trioxacyclododecane, chromogenic aza-12-crown-4. Constants found for this aza-12-crown-4 were pK_a1 = 5.77, pK_a2 = 10.31, pK_ex (Li) = 10.18, and pK_ex (Na) = 12.50. The calibration curve obtained for the extraction-spectrophotometric procedure was linear from 0.3 to 2 μg ml^?1. The imprecision of the measurement was calculated from the mean value of the standard deviations of triplicate samples over the linear range and was less than 5%. Results for the determination of lithium in blood serum and urine exhibited good agreement with atomic absorption data.     

Ligand effects in the substitution chemistry of cis-bis(piperidine)tetracarbonylmolybdenum(O). A molybdenum-95 NMR study

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₄(pip)_2−nL_n(n = 1,2; L = substituted pyridine ligands). The σ(⁹⁵Mo) values correlate well with the pK_a values for the substituted pyridines; for the n = 1 series, σ (⁹⁵Mo) ranges from − 1053 ppm (pK_a = 1.86 for 4-CN) to − 1120 ppm (pK_a = 9.61 for 4-NMe₂). The effects of solvent polarity and some in situ reactivity studies are described and the nature of the MoL bond compared to that with piperidine and some other ligands is discussed.     

Thermodynamics of the second-stage dissociation of n-(2-acetamido)iminodiacetic acid in water from 5 to 55°c

The thermodynamic second dissociation constants of the protonated form of N-(2-acetamido)iminodiacetic acid were determined at 12 temperatures from 5–55°C by measurement of the electromotive force using a cell without liquid junction, with hydrogen and silver—silver bromide electrodes. At 25°C, pK₂is 6.844. The standard changes in Gibbs energy, enthalpy, entropy and heat capacity were derived from the change of the pK₂ values with temperature. At 25°C, ΔG° = 9335 cal mol^-1, ΔH° = 2928 cal mol^-1, ΔS^o = -21.5 cal K^-1 mol^-1, and ΔC°_p = -34 cal K^-1 mol^-1. The results are interpreted and compared with those of structurally related compounds.     

Hydrogen bonding and the protonation site in 3-methyl-4-pyrimidone

The interaction between 3-methyl-4-pyrimidone and phenol derivatives or HBr has been studied by IR spectrometry in solution and in the solid state. For pK_a values ranging from 10.3 to 4.5, normal OH··· OC hydrogen bonds are formed. With stronger acids (PK_a = 0.4 to ?6) protonation occurs at the N(1) nitrogen atom of the ring. For phenols of intermediate pK_a values (3.5), there is no preferred site of interaction, both OH···OC and NH⁺···O^? bonds being formed in solution.     

Ionization of some derivatives of benzamide, oxamide and malonamide in DMF-water mixture

The ionization of six compounds of bis-phenolic amides was studied spectrophotochemically in DMF-water mixture. The compounds showed two pK_a values in the range of 5.97-7.32 for pK_a1 and 7.61-8.44 for pK_a2. The obtained values of K_a were normalized using the distribution diagrams of the different species and found to be in the range of 5.81-7.42 for pK_a1 and 7.48-8.27 for pK_a2.     

13C NMR measurements of acid-base equilibrium constants in alkaline aqueous solutions of D-glucose anomers

The concentration dissociation constants pK _c of the α and β D-glucose anomers were measured at pH 12–14 by the ¹³C NMR method. The pK _c values for L ? H_?1L + H equilibria at 25°C and an ionic strength of 1.0 mol/l (NaCl) were pK _c ^α = 12.31 ± 0.03 and pK _c ^β = 12.03 ± 0.01. Dissociation was found to shift all the spectrum lines downfield. The most probable hydroxyl group responsible for the dissociation of β-D-glucose was determined.     

Determination of pK a of benzoic acid- and p-aminobenzoic acid-modified platinum surfaces by electrochemical and contact angle measurements

Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)₆]^3?/4? redox probe solutions at different pH; this allowed us to estimate the surface pK _a values. Acidity constants for Pt-BA surface were found to be pK _a (3.09 ± 0.25), (4.89 ± 0.11), and (3.91 ± 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK _a (3.16 ± 0.45), (4.24 ± 0.40), and (5.64 ± 0.12). The Pt-BA surface pK _a values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK _a values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK _a of bulk pABA.     

Infrared spectrometric study of the interaction between 2-dimethylamino-3,3-dimethyl-1 azirine and some phenol derivatives

The H-bonded complexes between 2-dimethylamino-3,3-dimethyl-1-azirine (TMAAZ) and some phenol derivatives have been studied by IR spectrometry in carbon tetrachloride. The equilibrium constants at 300, 315 and 328 K and the ? ΔH°, ? Δν_OH and Δν_CN values have been determined. The comparison with previously studied OH … N complexes shows that TMAAZ forms stronger complexes than would be expected from the pK_a value. These facts are discussed in terms of steric, hybridization, solvation and charge transfer effects. From a pK_a of 8, the IR spectrum shows the appearance of ion pairs N⁺-H …^?O. A predominance of protonated species can be calculated for a pK_a of 6. This can explain why 1,2-ring cleavage whose first step is probably the formation of an azirinium cation, occurs for acids characterized by pK_a values lower than 5 but not for enolizable ketones having pK_a values higher than 9.     

Microscopic Protonation/Deprotonation Equilibria of the Anti-Inflammatory Agent Piroxicam

The microscopic ionization behavior of piroxicam was investigated using two different approaches, i.e., direct UV spectroscopy and an indirect analogue approach (deductive method). The best microscopic pK_a values (pK_a12 = 4.60, pK_a21 = 5.40, pK_a22 = 2.72, and pK_a11 = 1.92) were obtained by the deductive method using as pK_a22 the pK_a of the enolic O-methylated piroxicam 2 . The results show remarkable electrostatic effects in the protonation/deprotonation equilibria, a marked increase in the acidity of the enolic function (2.68 pK_a units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on ¹H-NMR chemical shifts at various ionization states, indicating an extended electron conjugation through the molecule. The partition measurements in octan-1-ol/H₂O of zwitterionic compound 3 (the pyridyl N-methyl derivative of piroxicam ( 1 )) suggest that the two opposite charges in zwitterionic piroxicam are indeed in a close intramolecular proximity.     

Determination of pKa values of some hydroxylated benzoic acids in methanol-water binary mixtures by LC methodology and potentiometry

An accurate estimation of pK_a values in methanol-water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the pK_a values of 11 polyphenolic acids have been determined in methanol-water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry, liquid chromatography (LC) and LC-DAD methodology.The results show a similar trend for the pK_a values of all the studied compounds, as they increase with increasing concentration of organic modifier, which allows a linear relationship between pK_a values and mole fraction of methanol to be obtained. The pK_a values obtained in aqueous medium have been compared with those given in the literature, and also with the values predicted by the SPARC on-line pK_a calculator. The data obtained have been used to test the feasibility of an estimation of dissociation constants in a methanol-water medium from the relationship between pK_a values and the organic cosolvent fraction in the mixtures.     

Acid-base chemistry of omeprazole in aqueous solutions

Omeprazole is a potent anti-acid drug. Its absorption and mode of action are closely related to its prototropic behavior. In the present study, omeprazole samples from different sources and in different forms were studied spectrophotometrically to obtain pK_a values. In the neutral to alkaline pH region, two consistent pK_a values of 7.1 and 14.7 were obtained from various samples. The assignment of these pK_a values was realized by comparison with the prototropic properties of N(1)-methylated omeprazole substituted on the nitrogen at the 1-position of the benzimidazole ring, which was found to have a pK_a of 7.5. The omeprazole pK_a of 14.7 is assigned to the dissociation of the hydrogen from the 1-position of the benzimidazole ring and the pK_a of 7.1 is assigned to the dissociation from the protonated pyridine nitrogen of omeprazole. The results presented are at variance with those of earlier work.     

Specific heats of guayule and natural rubbers above the glass transition temperature

Heat capacities of guayule and natural rubbers were measured between 228 and 333 K using a DuPont 990 Differential Scanning Calorimeter. Data obtained were fitted to a straight line. We obtained the following equations where C_p is given in cal g^?1 K^?1. For guayule rubber, C_p = 22.6152 × 10^?4T + 0.7731 (correlation factor = 0.99). For natural rubber. C_p = 16.9195 × 10^?4T + 0.9209 (correlation factor = 0.98). Furthermore, some theoretical considerations and instrumental conditions were analyzed so that the determinations of heat capacities could be improved.     

Correlations of 95Mo NMR chemical shifts with ligand basicity in substituted pyridine pentacarbonylmolybdenum(O) complexes

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₅L (L = pyridine derivatives). A good correlation is found between the δ(⁹⁵Mo) values and the Hammett sigma constant of the pyridine substituent or the pK_a of the substituted pyridine. The chemical shift values, which range from −1366 ppm (3-CN, σ = 0.62, pK_a = 1.35) to −1433 ppm (4-NMe₂, σ = −0.83, pK_a = 9.61), directly reflect the electronic properties of the pyridine derivatives even though the substituent is four or five bonds away from the molybdenum atom.     

Thermodynamics and kinetics of complexation of iron(III) ion by picolinic and dipicolinic acids

The complexation reactions of iron(III) with 2-pyridine carboxylic acia (picolinic acid) and 2,6-pyridine dicarboxylic acid (dipicolinic acid) in aqueous solutions have been studied by spectrophotometric and stopped flow techniques. Equilibrium constants were determined for the 1 : 1 complexes at temperatures between 25 and 80°C. The values obtained are: Picolinic Acid (HL): Fe³⁺+ H₂L⁺? FeHL³⁺+H+(K₁ = 2.8,ΔH = 2 kcal mole^?1 at 25°C, μ = 2.67 M) Dipicolinic Acid (H₂D): Fe³⁺+H₂D? FeD⁺+2H⁺(K₁K_1A= 227 M, ΔH = 3.4 kcal mole^?1 at 25°C,μ = 1.0 M). The rate constants for the formation of these complexes are also given. The results are used to evaluate the effects of these two acids upon the rate of dissolution of iron(III) from its oxides.     

Thermodynamic dissociation constants of risedronate using spectrophotometric and potentiometric pH-titration

Risedronate inhibits bone resorption in diseases like osteoporosis, Paget’s disease, tumor bone diseases or the malfunction of phosphocalcium metabolism. The acid-base properties of risedronate in an aqueous solution have been studied in a pH range from 2 to 12 and can be described in terms of four dissociation steps: pK _a,2, pK _a,4, pK _a,5 (related to the dissociation of POH groups) and pK _a,3 related to the dissociation of protonated amino group NH₃ ⁺. The mixed dissociation constants were determined at different ionic strengths I = 0.02 to 0.20 mol dm⁻³ KCl and of 25°C and 37°C using pH-spectrophotometric and pH-potentiometric titration methods. Determination of group parameters L ₀, H _T might lead to false estimates of common parameters p K _a;therefore, the computational strategy employed is important. A comparison between the two programs ESAB and HYPERQUAD demonstrated that the ESAB program provides a better fit of potentiometric titration curve. The thermodynamic dissociation constants pK _a^T were estimated by a nonlinear regression of (pK _a, I) data and a Debye-Hückel equation at 25°C and 37°C, pK _a,2^T = 2.37(1) and 2.44(1), pK _a,3^T = 6.29(3) and 6.26(1), pK _a,4^T = 7.48(1) and 7.46(2) and pK _a,5^T = 9.31(7) and 8.70(3) at 25°C and 37°C using pH-spectroscopic data and pK _a,2^T = 2.48(3) and 2.43(1), pK _a,3^T= 6.12(2) and 6.10(2), pK _a,4^T = 7.25(2) and 7.23(1) and pK _a,5^T = 12.04(5) and 11.81(2) at 25°C and 37°C. The ascertained estimates of three dissociation constants pK _a,3, pK _a,4, pK _a,5 are in agreement with the predicted values obtained using PALLAS      

Photometric determination of copper(II) in nickel alloys using azoderivatives of ethyl acetoacetate

1-Ethoxy-2-(2-hydroxy-3-sulfo-5-chlorophenylazo)butane-1,3-dion (L₁), 1-ethoxy-2-(2-hydroxy-3,5-disulfophenylazo)butane-1,3-dion (L₂), 1-ethoxy-2-(2-hydroxy-4-nitrophenylazo)butane-1,3-dion (L₃), and 1-ethoxy-2-(2-hydroxyphenylazo)butane-1,3-dion (L₄) were synthesized on the basis of ethyl acetoacetate, and their analytical characteristics were studied. The following dissociation constants of the reagents were determined by potentiometric titration: pK ₁ = 5.94 ± 0.04 and pK ₂ = 9.75 ± 0.04 (L₁), pK ₁ = 5.78 ± 0.02 and pK ₂ = 9.55 ± 0.03 (L₂), pK ₁ = 6.03 ± 0.03 and pK ₂ = 9.86 ± 0.05 (L₃), and pK ₁ = 6.19 ± 0.05 and pK ₂ = 9.98 ± 0.06 (L₄). The chemical analytical characteristics of copper(II) complexes of L₁–L₄ were determined by spectrophotometry. It was found that, with an increase in the elecrophilic properties of the substituents introduced into the aromatic ring of the molecule, the sensitivity of the reaction increased; the substituents had no effect on the ratio of Cu : L = 1 : 2. A photometric procedure was developed for determining copper in nickel-based alloys.     

Analytical applications of the reaction of thorium with benzenephosphonic acid

Benzenephosphonic acid quantitatively precipitates thorium as Th(C₆H₅PO₃)₂·3H₂O at pH values as low as 0.5. The compound may be dried at 140° to 180° C and weighed, as a gravimetric means of determining thorium. On ignition, Th (C₂H₅PO₃)₂ 3 H₂O undergoes decomposition at 240° to 300° C to form Th(C₆H₅PO₃)₂·2H₂O, at 450° to 650° C to form Th(HPO₄)₂·2H₂O and finally at 800° to 1000° C to form Th(HPO₄)₂. The latter compound is stable to 1200° C.Potentiometrically (pK₁' = 0.91, pK₂' = 6.41) and spectrophotometrically (pK₁' = 0.96, pK₂' = 6.51) determined pK' values are reported. Absorption spectra of C₆H₅PO₃H₂, C₆H₅PO₃H^- and C₆H₅PO₃^-2 are reported. The solubility of Th (C₆H₅PO₃)₂·3H₂O was studied as a function of pH and the average value of the solubility product (K_sp = 4s³) was found to be 3.24·10^-31.