INVESTIGADORES
CASTELLS Cecilia Beatriz
artículos
Título:
Effect of temperature and solvent composition on acid dissociation equilibria. I. Sequenced pKa determination of compounds commonly used as buffers in HPLC coupled to MS detection
Autor/es:
PADRÓ, J. M.; ACQUAVIVA, A.; TASCÓN, M.; GAGLIARDI, L. G; CASTELLS, C. B.
Revista:
ANALYTICA CHIMICA ACTA
Editorial:
ELSEVIER SCIENCE BV
Referencias:
Lugar: Amsterdam; Año: 2012 vol. 725 p. 87 - 94
ISSN:
0003-2670
Resumen:
A new automated and rapid potentiometric method for determining the
effect of organic-solvent composition on pKa has been developed. It
is based on the measurements of pH values of buffer solutions of variable
solvent compositions using a combined glass electrode. Additions of small volumes
of one precisely thermostated solution into another, both containing exactly
the same analytical concentrations of the buffer components, can produce
continuous changes in the solvent composition. Two sequences of potential
measurements, one of increasing and the other of decreasing solvent content,
are sufficient to obtain the pKa values of the acidic compound
within the complete solvent-composition range in about two hours. The
experimental design, procedures, and calculations needed to convert the measured
pH into the thermodynamic pKa values are thoroughly discussed. This
rapid and automated method allows the systematic study of the effect of solvent
compositions and temperatures on the pKa. It has been applied to
study the dissociation constants of two monoprotic acids: formic acid and triethylamine:HCl
in acetonitrile/water mixtures within the range from 0 to 90% (v/v) at
temperatures between 20 ºC and 60
°C. These volatile compounds are frequently used to
control the pH of the mobile phase in HPLC, especially in methods coupled to
mass-spectrometry detection. The obtained pKa values are in excellent
agreement with those previously reported. The results were fitted to empirical functions
between pKa and temperature and composition. These equations, which
can be used to estimate the pKa of these substances at any
composition and temperature, would be highly useful in practical work during
chromatographic method development.