Spectrophotometric, potentiometric and chromatographic pKa values of polyphenolic acids in water and acetonitrile-water media

The pK_a values of nine polyphenolic acid substances in water and acetonitrile-water mixtures using potentiometric, spectrophotometric, and liquid chromatographic (LC) measurements, have been determined. Also, a new method based on the absorbance spectra at the maximum of chromatographic peaks previously obtained has been applied. This method can be applied to data obtained from LC-UV (diode array detection (DAD)) instruments and retains all the advantages of LC and spectrophotometric methods, such as the possibility of working with impure samples. Finally, the advantages of the different methods to the determination of pK_a values of nine polyphenolic acids have been described in order to verify the applicability of the proposed methods in aqueous and acetonitrile-water media.     

Determination of pKa values of some antihypertensive drugs by liquid chromatography and simultaneous assay of lercanidipine and enalapril in their binary mixtures

In this study, pK_a values were determined using the dependence of the retention factor on the pH of the mobile phase for three ionizable substances, namely, enalapril, lercanidipine and ramipril (IS). The effect of the mobile phase composition on the ionization constant was studied by measuring the pK_a at different methanol-water mixtures, ranging between 50 and 65% (v/v), using LC-DAD method. Two simple, accurate, precise and fully validated analytical methods for the simultaneous determination of enalapril and lercanidipine in combined dosage forms have been developed. Separation was performed on an X-Terra RP-18 column (250 mm × 4.60 mm ID × 5 μm) at 40 °C with the mobile phase of methanol-water 55:45 (v/v) adjusted to pH 2.7 with 15 mM orthophosphoric acid. Isocratic elution was performed in less than 12 min with a flow rate of 1.2 mL min⁻¹. Good sensitivity for the analytes was observed with DAD detection. The LC method allowed quantitation over the 0.50-20.00 μg mL⁻¹ range for enalapril and lercanidipine. The second method depends on first derivative of the ratio-spectra by measurements of the amplitudes at 219.7 nm for enalapril and 233.0 nm for lercanidipine. Calibration graphs were established for 1-20 μg mL⁻¹ for enalapril and 1-16 μg mL⁻¹ lercanidipine, using first derivative of the ratio spectrophotometric method. Both methods have been extensively validated. These methods allow a number of cost and time saving benefits. The described methods can be readily utilized for analysis of pharmaceutical formulations. The methods have been applied, without any interference from excipients, for the simultaneous determination of these compounds in tablets. There was no significant difference between the performance of the proposed methods regarding the mean values and standard deviations.     

Determination of pKa values of quinolones from mobility and spectroscopic data obtained by capillary electrophoresis and a diode array detector

This work compares the values of dissociation constants obtained from electrophoretic mobilities of a series of quinolones at different pH values and those obtained using absorbance spectra at the maximum of the eletrophoretic peaks. The results obtained show that the two methods are complementary and constitute a valuable means of obtaining better precision. The two methods proposed can be used simultaneously without an increase in the experimental time and allow confirmation of the results obtained.     

Calculations of pKa values of carboxylic acids in aqueous solution using density functional theory

With the specific aim of calculating the acidity equilibrium constant (K_a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK_a (−lg K_a) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK_as of 66 carboxylic acids in aqueous with the average error of 0.5 in pK_a units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK_a value higher than 3.     

Determination of pKa values of fluoroalkanesulfonamides and investigation of their nucleophilicity

The pK_a values of a series of fluoroalkanesulfonylamides were measured by potentiometric titration. Different kinds of alkyl halides and tosylates were employed to investigate the nucleophilicity of fluoroalkanesulfonylamides. Fluoroalkanesulfonylamides with longer fluoroalkyl chain have weaker nucleophilicity.     

Spectrophotometric determination of acidity constants of 4-(2-pyridylazo)resorcinol in binary methanol-water mixtures

The acidity constants of 4-(2-pyridylazo)resorcinol (PAR, Scheme 1) in binary mixtures of methanol-water at 25 °C and an ionic strength of 0.1 M have been determined spectrophotometrically. DATa ANalysis (DATAN) program applied for determination of acidity constants. As percent of methanol increases in solvent mixtures the pK_a constants also increased. There is linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures. Effect of solvent composition on acidity constants and pure spectrum of each component are also discussed.     

Uncertainty estimation in measurement of pKa values in nonaqueous media: A case study on basicity scale in acetonitrile medium

The difficulties in estimating uncertainty of pK_a values determined in nonaqueous media are reviewed and two different uncertainty estimation approaches are presented and applied to the pK_a values of the compounds on a previously established self-consistent spectrophotometric basicity scale in acetonitrile. One approach is based on the ISO GUM methodology (the “ISO GUM” approach) and involves careful analysis of the uncertainty sources and quantifying the respective uncertainty components. The second approach is based on the standard-deviation-like statistical parameter that has been used for characterization of the consistency of the scale (the “statistical” approach). It is demonstrated that the ISO GUM approach somewhat overestimates the uncertainty. The statistical approach is based on long-term within-laboratory statistical data and it is demonstrated that it underestimates the uncertainty. In particular it neglects the laboratory bias effects that are taken into account at least to some extent by the ISO GUM approach. Thus, together these two approaches allow to “bracket” the uncertainties of the pK_a values on the scale. The uncertainties of the pK_a values are defined in two different ways. Definition (a) includes the uncertainty of the pK_a of the reference base (anchor base of the scale) pyridine. Definition (b) excludes it. It is demonstrated that both definitions have their virtues. Definition (a) leads to the uncertainty ranges of 0.12-0.22 and 0.12-0.14 pK_a units at standard uncertainty level for different bases using the ISO GUM and statistical approach, respectively. Definition (b) leads to the uncertainty ranges of 0.04-0.19 and 0.02-0.08 pK_a units, respectively. The uncertainty of the pK_a of a given base is dependent on the quality of the measurements involved and on the distance from the reference base on the scale. The importance of the correlation between the pK_a values of bases belonging to the same scale is stressed.     

Determination of pKa values of active pharmaceutical ingredients

The acid dissociation constant is an important physicochemical parameter of a substance, and knowledge of it is of fundamental importance in a wide range of applications and research areas. We present a critical review of published methods and approaches for the determination of acid dissociation constants, with a special emphasis on the pK_a values of active pharmaceutical ingredients.     

Determination of pKa value, kinetic studies of decomposition and oxygen transfer for chromium(VI) peroxide

The determination of pK_a value for the unstable chromium(VI) peroxide, CrO(O₂)₂(H₂O) in aqueous solution is presented. The pK_a value is found to be (1.55 ± 0.03). The kinetic decomposition of chromium(VI) peroxide is dependent on the concentration of hydrogen peroxide in the pH range between 2.5 and 4.0. We have proposed the possible explanation for the formation of triperoxo chromium complex of hydrogen peroxide which is dependent on decomposition. Activation of coordinate peroxide in chromium(VI) peroxide observed in the kinetic studies is by reduction of thiolato-cobalt(III) complex. The rate constant (M⁻¹ s⁻¹, 15 °C) for the oxygen atom transfer reaction from CrO(O₂)₂(OH)⁻ to (en)₂Co(SCH₂CH₂NH₂)²⁺ is found to be (25.0 ± 1.3).     

Equilibration of mixtures of polyamic acids studied using size-exclusion chromatography and light scattering

The transamidation reaction in a polyamic acid solution has been investigated using size-exclusion chromatography and low-angle light scattering. Mixtures of a high-molecular weight (DP = 150) and a low-molecular weight (DP = 10) polymer and of the high-molecular weight polymer with monomer were studied. Mixtures were made at high and low concentrations. The polyamic acid studied is the product of the polycondensation of 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (BPDA) with oxydianiline (ODA). In all cases the molecular weight distribution equilibrated with time to a most-probable distribution with a DP consistent with the stoichiometry of the mixture. Equilibration required about 2 weeks for mixtures of 10% by weight at ambient temperatures. The effect of addition of a small amount (5%) of low-molecular weight material to sample of high-molecular weight is dramatic; for DP = 150 the molecular weight is decreased by more than one-half. In an entangled solution or melt, this would reduce the viscosity by an order of magnitude.     

Ab initio calculations of pKa values of some organic acids in aqueous solution

The acidity of different classes of organic compounds in aqueous solution has been calculated. The calculations are carried out at the SCF level with inclusion of entropic and thermochemical correction to yield free energies of dissociations.

The polarized continuum model is used to describe the solvent. The model furnishes pK_a values in relatively good agreement with experimental data. Scaling different parts of solvation energies provides a significant improvement in results and signifies the importance of balance of individual contributions from electrostatic, cavity, dispersion and repulsion interactions.     

Determination of pKa values of tenoxicam from H NMR chemical shifts and of oxicams from electrophoretic mobilities (CZE) with the aid of programs SQUAD and HYPNMR

In this work it is explained, by the first time, the application of programs SQUAD and HYPNMR to refine equilibrium constant values through the fit of electrophoretic mobilities determined by capillary zone electrophoresis experiments, due to the mathematical isomorphism of UV-vis absorptivity coefficients, NMR chemical shifts and electrophoretic mobilities as a function of pH. Then, the pK_a values of tenoxicam in H₂O/DMSO 1:4 (v/v) have been obtained from ¹H NMR chemical shifts, as well as of oxicams in aqueous solution from electrophoretic mobilities determined by CZE, at 25 °C. These values are in very good agreement with those reported by spectrophotometric and potentiometric measurements.     

Dissociation constants of Br∅nsted acids in D2O and H2O: studies on polyaza and polyoxa-polyaza macrocycles and a general correlation

Using data on a series of polyaza and polyoxa-polyaza macrocyclic compounds, a satisfactory linear correlation was established between the dissociation constants of their acid forms obtained in deuterium oxide (pK_D) and in water (pK_H). The isotopic effect, ΔpK=pK_D - pK_H, increases with increasing pK, being larger for the weaker acids. The pK_D vs. pK_H correlation may be used to determine pK_D or pK_H values when one of the constants is known; the statistical errors of the estimates are not significatively higher than those of the experimental determinations. By including data available in the literature for a series of different acids (mineral and carboxylic acids, phenols, protonated amines, amino acids, etc.; a total of 138 pairs of data for 70 different compounds), a good general pK_D vs. pK_H correlation was also obtained (correlation coefficient 0.998), showing that the dependence of the isotopic effect on the nature of the acid is not as important as previous studies, based on limited and not always satisfactory Δpk vs. pK_H correlations, suggested.     

Ag+-TLC/GC分析食品中的反式脂肪酸

研究了用银离子薄层色谱和气相色谱测定食品中反式脂肪酸组成和含量的方法.采用气相色谱直接分离脂肪酸甲酯时,18:0～18:2n-6区域的一些顺、反18:1异构体和c/t-18:2异构体的脂肪酸甲酯出峰时间过于接近,部分色谱峰重叠,采用CP-Sil88 100 m极性毛细管柱仍无法完全分离.采用银离子薄层色谱将反式脂肪酸甲酯预分离后,再用气相色谱检测能够避免顺反异构体的色谱峰重叠,可得到更合理的测定结果.实验表明,色谱峰面积和反式脂肪酸浓度的线性关系良好,相对标准偏差为0.29%～0.48%,回收率为92.52%～104.27%.实验测得牛奶和奶油中反式脂肪酸量分别为0.17 g/100 g和5.45g/100 g.11t-18:1在两样品中含量最高,分别占总反式脂肪酸的38.03%和51.20%.     

Determination of bile acids and lysolecithin in artificial and natural gastric juice by HPLC for characterization of antacida

Work-up procedures and HPLC separation systems for determination of taurocholic, glycocholic, chenodeoxycholic, and cholic acids and lysolecithin in artificial and natural gastric juice are described. These compounds are used for testing the binding capacity of antacida to the individual analytes. Work-up is simple, no extraction or filtration being required. The optimized combinations of stationary and mobile phases allow selective and sensitive determination of the respective bile acids in gastric juice. For optimization, the capacity factors of two related bile acids were evaluated for numerous commercial stationary phases. The mechanism of retention is different for free and conjugated bile acids. Special aspects of routine analysis are discussed.     

Selective determination of total normal microcystin by colorimetry, LC/UV detection and/or LC/MS

Microcystins, hepatotoxic cyclic heptapeptides, are produced by freshwater cyanobacteria, and are classified four groups according to the amino acid structure at unit 7. Normal microcystins contain N-methyldehydroalanine (Mdha) or dehydroalanine (Dha) at unit 7, and command the great part of all microcystins. As unusual microcystin classes, [Dhb⁷]microcystins, [ - and -Ala⁷, or N-MeAla⁷]microcystins and [ -Ser⁷]microcystins have been found.

On tumor initiation and/or promotion activities of microcystins, the tumor promotion activity of normal microcystins has been found, but cancer-related activities of microcystins belonging in the other classes have not been clear.

To determine normal microcystins as hepatotoxic tumor promoters, a selective determination method was developed. Only Mdha or Dha in normal microcystins was reacted with glutathione (GSH). The GSH-normal microcystins conjugates were reacted with trinitrobenzene sulfonate (TNBS). The TNB–GSH-normal microcystin conjugate can be determined as the total normal microcystin by colorimetry. After methanolysis of the conjugate, dimethyl TNB–glutamate from the conjugate was determined by liquid chromatography/ultraviolet detection (LC/UV) and/or liquid chromatography/mass spectrometry (LC/MS). The detection limits of the total normal microcystin by colorimetry, LC/UV and/or LC/MS were 1 μg, 10 and 0.1 ng, respectively.     

Mixing effects on the protonation of some polycarboxylates in NaClaq + KClaq at different ionic strengths

Protonation constants of succinic, 1,2,3-propanetricarboxylic and 1,2,3,4-butanetetracarboxylic anions were determined in NaCl_aq + KCl_aq mixtures, at three ionic strengths, I = 1.2, 3 and 4.5 mol L⁻¹. Experimental evidences showed that the function log K^H = f(y) (y = [Na⁺]/([Na⁺] + [K⁺])) is not linear, indicating mixing effects on the protonation constants. The Guggenheim zeroth approximation holds that the above function can be written as: