Study on the stability of noradrenaline and on the determination of its acidity constants

Noradrenaline is a catecholamine which has been largely recognised to play a very important role in biological systems. In view of the neurotransmitter's alleged importance, this work aimed at showing the influence of time on its spectral behaviour using different analytical methods and determining its acidity constants through spectrophotometric titration and by the so-called point-by-point analysis, where the samples are freshly prepared for each pH value investigated at the instant required. Because the catecholamines are light-sensitive and likely to react with the oxygen in the surrounding air, both methods used preclude its incidence onto the samples being analysed under the presence of a nitrogen atmosphere maintained over the solutions. The constants obtained through point-by-point analysis were log beta1 = 30.71+/-0.16, log beta2 = 22.00+/-0.15 and log beta3 = 11.69+/-0.16.     

Study on the stability of the serotonin and on the determination of its acidity constants

The present work aimed at describing the spectral behaviour of the serotonin and to evaluate its acidity constants using three different methods, using two spectrophotometry titrations and a third method that involved point-by-point analysis, which permitted to monitor closely and determine the evolution of the serotonin species in solution as a function of time. The three methods allowed estimation of three acidity constants associated to the same number of functional groups that form part of the molecule. The results given by the point-by-point analysis were: log(beta1) = 24.95 +/- 0.12; log(beta2) = 20.20 +/- 0.10; log(beta3) = 10.89 +/- 0.018.     

Spectrophotometric study on the stability of dopamine and the determination of its acidity constants

The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.046+/-0.147, 10.579+/-0.148 and 12.071+/-0.069.     

八乙基金属卟啉的轴向配位反应研究及稳定常数测定

测定了八乙基金属卟啉 (OEP) M中当M=Zn, Cu, FeCl 或 MnCl时在CH2Cl2, N,N-二甲基甲酰胺(DMF), 二甲亚砜(DMSO), 吡啶(Py)等4种不同非水溶剂中的紫外-可见光谱, 探讨了溶剂的性质对八乙基金属卟啉光谱的影响. 结果表明八乙基锌、铁、锰卟啉在CH2Cl2中能够与配位溶剂如DMF, DMSO或pyridine 等发生轴向配位反应生成五配位或六配位的金属卟啉配合物. 用微量光谱滴定法测定了Zn、 Fe、 Mn等金属卟啉和配位溶剂发生轴向配位反应的稳定常数(logK). 讨论了中心金属离子的电负性以及配位溶剂的给电子能力对稳定常数的影响.     

Fluorimetric determination of acidity constants of naphthoic and anthroic acids

A fluorimetric method of determining the acidity constants (pK_a values) of some naphthoic and anthroic acids is presented, based on the changes in fluorescence of solutions of these acids in water on changing the pH. The method is useful in the absence as well as in the presence of excited-state proton transfer. In the latter case, when the excited-state protonation occurs in the same pH region as the ground-state dissociation, resolution of the two processes can be accomplished by addition of a suitable quencher such as iodide. The method permits the accurate determination of the pK_a value of fluorescent compounds, even when they are poorly soluble in water, because of the high sensitivity of the spectrofluorimetry.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

Multiwavelength spectrophotometric determination of acidity constants of some azo dyes

A multiwavelength spectrophotometric titration method was applied to study the acidity constants of some azo dyes in water. The UV-vis absorption spectra of azo dye solutions were recorded in the course of their pH-metric titration with a standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated. The quantitative effects of the substituents on the acidity of the studied azo dyes were investigated by the linear free energy relationship (LFER) using Hammet sigma constant (sigma) and field and resonance effects of Kamlet and Taft (f and Re, respectively).     

Spectrophotometric determination of aqueous acidity constants of three azo dyes

The two successive steps acidity constants of three azo dye derivatives D1–D3, were determined by a spectrophotometric method using 0.10 M KNO₃ as supporting electrolyte in water solvent. The electronic spectra of the compounds were recorded in the course of their pH-metric titration with a standard base solution. The protolytic equilibrium constants can be solved very satisfactorily from absorbance data, by combining the data obtained in the spectrometric determinations with graphic methods as the absorbance diagrams. An A-diagram shows the relative absorbance changes at two wavelengths as a function of the pH. For a one-step system, the absorbance at any wavelength must be proportional to the absorbance at any other wavelength, so an A-diagram for all wavelength combinations for such a system must be linear (rank, s=1). However, if a system is governed by two or more equilibria, the A-diagrams corresponding to multi-step titrations will change direction every time a new equilibrium becomes dominant in the system. Analysis of the each uniform sub-region can then be used in evaluating of the corresponding equilibrium. A-diagrams for all wavelength combinations suggest that these systems have the rank two(s=2). The results revealed that the K_a(1) and K_a(2) values of different azo dyes follow the order D3>D2>D1.      

Spectrophotometric determination of acidity constants by two-rank annihilation factor analysis

Annihilation of the contribution of one chemical component from the original data matrix is a general method in rank annihilation factor analysis (RAFA). However, RAFA is not applicable for studying the protonation equilibria of multiprotic acids. In this work, a two-rank annihilation factor analysis (TRAFA) method was proposed for determination of the acidity constants of diprotic acids. After recording the electronic absorbance spectra of the acids at different pH, the contributions of both H₂A and A²⁻ were annihilated from the absorbance data, which made feasible the determination of two successive acidity constants. The method was validated by analysis of simulated data and its application to the determination of the acidity constants of calmagite, as a reference compound. A close agreement was obtained between the resulted values by TRAFA and the declared values. Indeed, the method was used for determination of the acidity constants of two new chromenone derivatives in binary solvents mixtures of methanol and water. The effects of changing solvent composition on acidity constant data were explained by linear solvation free energy relationships (LSFER) utilizing solvatochromic parameters.     

Potentiometric determination of aminal stability constants

Potentiometric titration was used to determine the logarithms of the stepwise equilibrium constants for the species formed between morpholine and formaldehyde in aqueous solution, ionic strength 0.5 and 2.5M (KCl) at 25 degrees C. The instrumental and computational techniques developed for metal-ligand stability constant determination were applied. Formaldehyde is equivalent to the metal-ion and is represented by M while neutral morpholine is equivalent to the ligand and is represented by L. The stability constants of the following equilibria were determined by non-linear regression (figures in parentheses are at ionic strength 2.5 M KCl): M + L left arrow over right arrow ML (hemi-aminal) logK(1) = 2.90 +/- 0.02 (2.980 +/- 0.004); ML + L left arrow over right arrow ML(2) (bis-aminal); log K(2) = 1.3 +/- 0.2 (1.41 +/- 0.07); MLH left arrow over right arrow ML + H(+) (protonated hemi-aminal) pK(a) = 5.87 +/- 0.01 (6.411 +/- 0.005); ML(2)H left arrow over right arrow ML(2) + H(+) (protonated bis-aminal) pK(a) = (7.6 +/- 0.2). the pK(a) of the protonated bis-aminal could only be determined at the higher ionic strength. The results are in good agreement with reported values determined using the classic formol titration. The automated titration system acquired the full time course of the pH change upon each titrant addition allowing a kinetic analysis to be performed as well as an equilibrium analysis. The forward and reverse rate constants for M + L left arrow over right arrow ML were 0.77M(-1) sec(-1) and 8.1 x 10(-4) sec(-1). respectively.     

Analytical data Spectrophotometric determination of the acidity constants of 2-amino cyclopentene-1-dithiocarboxylic acid and some of its derivatives

The acidity constants of 2-amino cyclopentene-1-dithiocarboxylic acid (ACDA) and some of its derivatives have been determined spectrophotometrically, at 25 degrees and at different mole fractions of ethanol in water. In all solvent mixtures used, the acidity constants vary in the order ACDA > N-methyl-ACDA > N-ethyl-ACDA > N-butyl-ACDA.     

Spectrophotometric determination of acidity constants of some anthraquinones and anthrones in methanol-water mixtures

The acidity constants of some synthetic derivatives of 9,10-anthraquinone and 9-anthrone in methanol-water mixtures at 25 degrees have been determined spectrophotometrically. A linear reverse relationship is observed between pK(a1) of all acids and the mole fraction of methanol. The influence of substituents in the molecular structure on the ionization constants is discussed.     

Potentiometric determination of acidity constants of some Schiff bases in tetrahydrofuran-water mixtures

Using potentiometric measurements, dissociation constants of two Schiff bases, N,N′-bis(1′-hydroxy-2′-acetonaphthone)propylenediimine (L₁) and bis(1′-hydroxy-2′-acetonaphthone)-3,3′-diiminodpropaneamine (L₂) were determined in binary mixtures of tetrahydrofuran-water at 25.0 ± 0.1°C and an ionic strength of 0.100 M tetrabutylammonium perchlorate (TBAP). The results show that the pK _a values of these acids increases as the percentages of the tetrahydrofuran increase in solvent mixtures. There is a linear relationship between acidity constants and the mole fraction of tetrahydrofuran in the solvent mixtures. Effect of solvent composition on acidity constants is also discussed.     

Microcomputer-controlled polarographic instrumentation and its use in the determination of stability constants of crown ether complexes

A computer-controlled polarographic system is described, based on a commercially available polarograph interfaced to a microcomputer. Experiments are controlled and monitored entirely from software, including automatic evaluation of the Tast polarograms and addition of solutions to the polarographic cell from a motor burette. The program was written in FORTH, a computer language especially apt for laboratory automation. The system is used in the determination of stability constants of crown ether complexes.     

Derivative spectrophotometric method for determination of acidity constants of single step acid-base equilibria

A general derivative spectrophotometric method for determination of acidity constants is developed. The method appears suitable in cases when classical spectrophotometry cannot be employed due to little differences in the absorption spectra of conjugated acid-base pairs. Based on theoretical considerations, seven variants of the method have been established and their validity was checked, determining acidity constants of lorazepam and flurazepam as model compounds.     

Potentiometric determination of the dissociation constants of an asymmetric sorbent containing l-proline, and the stability constants of its Cu(II) complexes

The dissociation constants of the carboxyl groups (pK(a1) = 2.2, n = 1.8) and amino groups (pK(a2) = 9.5, n(2) = 1.6) of a sorbent prepared by reacting l-proline with a cross-linked chloromethylated styrene polymer have been determined by potentiometric titration. The potentiometrically measured stability constants of the Cu(II) complexes of the resin (logbeta(1) = 6.9 and log beta(2) = 12.4) were found to be close to the values for the Cu(II) complexes of N-benzyl-l-proline. For complexed resins of alpha-amino-acid type the pH-values of decomplexation do not appear to be directly correlated with the stability constants.     

Temperature variation effects on the determination of acidity constants through the internal standard–capillary electrophoresis method

The internal standard (IS) CE (IS‐CE) method is an interesting alternative to other methods for the determination of acidity constants of compounds. Although some of the advantages of this method have been already reported, the method has not been tested yet as regards to temperature effects. This has been the aim of this work, where it is demonstrated that the method can be applied successfully for the determination of pK_as at different temperatures, if the acidity constant of the IS at the desired temperature is known. The fact of obtaining the acidity constants at different temperatures allows the calculation of some thermodynamic quantities, such as the molar standard enthalpy and the molar standard entropy in a fast way. It is also demonstrated that if the IS and the test compound have similar standard enthalpy increment, the IS compensates uncontrolled possible temperature fluctuations (e.g., due to Joule heat) inside the capillary obtaining reliable acidity constant values at the desired temperature.     

Catalytic-thermometric determination of stability constants of complexes using the iodine-azide reaction

A catalytic-thermometric method for the determination of the stoichiometry and stability constants of complexes with sulphur-containing ligands is proposed. The method is based on the inhibition of the catalytic activity of the ligands on the iodine-azide reaction when a metallic ion is present. A study of the errors associated with the method has been performed and used to improve experimental design. The conclusions have been applied to the determination of the stoichiometry and the stability constants of the Ni(II)-DDTC and Ni(II)-PDTC complexes.     

Ultrafiltration and determination of Zn- and Cu-humic substances complexes stability constants

This study exhibits that size fractionation of humic substances (HS) and their metal complexes by ultrafiltration is an efficient procedure for simultaneous determination of stability constants. Using sequential-stage ultrafiltration and a radiotracer technique the HS–Cu and HS–Zn complexes studied can gently be size-fractionated and their free metal fractions simply be discriminated. The conditional stability constants Ki obtained for size fractions of these HS metal complexes exhibit a clear molecular size dependence. Accordingly, the highest Ki values (6.6 for Zn and 6.4 for Cu) are found in the HS fractions of >105 kDa. Moreover, the overall stability constants K found for Cu (log K=5.5) and Zn complexes (log K=4.5) of the aquatic HS complexes studied are quite comparable to those reported in the literature.     

Simultaneous determination of partition coefficients and acidity constants of chlorinated phenols and gualacols by gas chromatography

The partition coefficients, P, in the n-octanol/water system and the acidity constants, K_a, of 15 chlorinated phenols and guaiacols at 20° were simultaneously determined by investigating the pH dependence of the apparent partition coefficients, P_a. For determining P_a, the relative responses of individual phenols and guaiacols in the aqueous phase after equilibration were measured by glass-capillary gas chromatography with electron-capture detection and compared with those in the octanol phase before partition. Curvefitting, linear regression, and non-linear regression were used for treating the partition data. The values of pK_a and log P were compared with the values which could be found in the literature, and are discussed in relation to their affinity to methanol/water and acetonitrile/water mixtures.