J.L. Beltrán

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.

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

Abstract 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.

Use of cloud point extraction methodology for the determination of PAHs priority pollutants in water samples by high-performance liquid chromatography with fluorescence detection and wavelength programming

The cloud point extraction methodology has been used to develop a new procedure for the preconcentration of polycyclic aromatic hydrocarbons in water samples (bottled and network supply). Triton X-114 was used as extractant agent. The procedure, consisting of three steps (preconcentration into the surfactant, clean-up and concentration of the eluate), allows good detection limits to be reached (from nanograms per liter to even sub-nanograms per liter in some cases). Determination was carried out by HPLC, with fluorimetric detection and wavelength programming.

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

The pKa 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 pKa values of nine polyphenolic acids have been described in order to verify the applicability of the proposed methods in aqueous and acetonitrile–water media.

Evaluation of stability constants from multi-wavelength absorbance data: program STAR

Abstract The program STAR and STAR/FA were developed for the study of ionic equilibria from spectrophotometric data. Both programs are written in Turbo-Pascal for use on personal computers. STAR is a non-linear regression program for the refinement of complex formation constants of systems that can be described by up to 5 components and 25 species A p B q C r D s H t , where A, B, C and D can be either metal or ligand. The experimental data treated can contain up to 50 absorption spectra, measured at 50 wavelengths. Interaction with the user is provided by menu-driven operation and graphics presentation of data on the computer screen (or plotter). STAR/FA is a complementary program, used for the determination of the number of absorbing species by factor analysis of the absorbance data matrix. It can also be used for the smoothing of the experimental data. Two application examples are given: the interaction of boric acid with 2,3-dihydroxynitrobenzene (from literature data) and the dissociation and complex formation equilibria with Zn(II) ions of 8-hydroxyquinoline-5-sulphonic acid.

Determination of the dissociation constants of sulfonated azo dyes by capillary zone electrophoresis and spectrophotometry methods

The dissociation constants of 10 sulfonated azo dyes, six of the most common food colours used as additives (Food Yellow 4, Food Yellow 3, Food Red 9, Food Red 7, Food Red 17 and Food Blue 5), and four commonly used as textile dyes (Acid Orange 7, Acid Orange 12, Acid Red 26 and Acid Red 88), have been determined by two different systems, one by using capillary electrophoresis (CE) with diode array detection and the other by using UV-visible absorption spectrophotometry, which has been used as reference method to obtain the pKa values. The pKa values obtained by CE were determined in two ways, first on the basis of the electrophoretic mobilities (calculated from the migration times), and after we propose a new methodology, in which the dissociation constants are determined from the spectra corresponding to the maxima of electrophoretic peaks. The pKa values obtained by using these CE methods have been compared with those obtained by using the spectrophotometric method. The results show that the pKa values obtained by the CE proposed method are in general closer to the reference values than those obtained from the electrophoretic mobilities. Moreover, the proposed method retains the advantages of CE, as the possibility of working with small amounts of sample, despite its purity.

Determination of dyes in foodstuffs by capillary zone electrophoresis

A rapid method based on capillary zone electrophoresis coupled with photodiode-array detection has been developed to determine the dyes Tartrazine E-102, Sunset Yellow FCF E110, Amaranth E-123, New Coccine E-124, Patent Blue V calcium salt E-131 and Allura Red AC E-129 in foodstuffs. Separation was done by using a Bare CElect-FS75 CE column, using a 10 mM phosphate buffer at pH 11.0. Hydrodynamic injections at 0.5 p.s.i. for 4 s (21 nl of sample) and 20 kV separation voltage were used. The quantitation limits for the six dyes ranged from 3 to 6 microg/ml. A linear relationship between 3 to 95 microg/ml, with correlation coefficient better than 0.995 was obtained. This method has been applied to the determination of the studied dyes in beverages, jellies and syrups.

Ionization constants of pH reference materials in acetonitrile—water mixtures up to 70% (w/w)

Abstract Ionization constants of pH reference materials in acetonitrile—water mixtures containing 30, 40, 50 and 70% (w/w) acetonitrile were obtained. The ionization constant values determined were: p K 1 and p K 2 for tartaric and phthalic acid; p K 1 , p K 2 and p K 3 for citric acid and p K 1 for fabric and acetic acid. These values are essential to determine the standard reference value, pHs, for various standard reference buffer solutions. In the total composition range studied, p K values were linearly correlated with the mol fraction of acetonitrile and with the reciprocal of the dielectric constant of solvent mixtures. Considering the unlimited number of solvent mixtures, a multilinear regression procedure was applied to correlate p K values with % (w/w) and % (v/v) of acetonitrile, and the methodology of linear solvation energy relationships (LSERs) was used to relate p K data with solvatochromic parameters of acetonitrile—water mixtures.

PKPOT, a program for the potentiometric study of ionic equilibria in aqueous and non-aqueous media

Abstract The least-squares computer program PKPOT has been developed to run on a PC-compatible computer, for the study of ionic equilibria from potentiometric data. It allows for the refinement of equilibrium constants in systems described by up 5 components and 20 complex species A p B q C r D s H t . Other parameters (standard potentials of electrode, reactant concentrations, etc.) can also be refined. The program can deal with up to ten titration curves. It provides several statistical tests, as also graphics presentations of data and residuals distribution. The program allows for the determination of stoichiometric formation constants (at fixed ionic strength), or thermodynamic constants; in this case, corrections for changes on the activity coefficients are taken into account. The data analysed can be given as volume/e.m.f. or volume/pH. Several application examples are given, including titrations in aqueous and non-aqueous media.

Determination of sulphonated dyes in water by ion-interaction high-performance liquid chromatography

Abstract An ion-interaction high-performance liquid chromatography method for quick separation and determination of the sulphonated dye Acid Yellow 1, and the sulphonated azo dyes Acid Orange 7, Acid Orange 12, Acid Orange 52, Acid Red 2, Acid Red 26, Acid Red 27 and Acid Red 88 has been developed. An RP-ODS stationary phase is used, and the mobile phase contains an acetonitrile–phosphate buffer (27:73, v/v) mixture at pH 6.7, containing 2.4 m M butylamine as ion-interaction reagent. Good separations were obtained using isocratic elution and spectrophotometric detection at 460 nm. The detection limits for the eight dyes ranged from 7 to 28 μg/l for an injection volume of 100 μl. Spiked tap water samples (100 ml), containing different concentration levels (0.3–1.2 μg/l) of the dyes were analyzed after acidification (pH 3) and preconcentration in disposable solid-phase extraction C 18 cartridges.