| Title: | "Effect of temperature and solvent composition on acid dissociation equilibria, I: Sequenced (s)(s)pKa determination of compounds commonly used as buffers in high performance liquid chromatography coupled to mass spectroscopy detection" |
| Author(s): | Padro JM; Acquaviva A; Tascon M; Gagliardi LG; Castells CB; |
| Address: | "Laboratorio de Separaciones Analiticas, Division Quimica Analitica, Universidad Nacional de La Plata y CIDEPINT, La Plata, Argentina" |
| DOI: | 10.1016/j.aca.2012.03.015 |
| ISSN/ISBN: | 1873-4324 (Electronic) 0003-2670 (Linking) |
| Abstract: | "A new automated and rapid potentiometric method for determining the effect of organic-solvent composition on pK(a) 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 pK(a) values of the acidic compound within the complete solvent-composition range in about 2h. The experimental design, procedures, and calculations needed to convert the measured pH into the thermodynamic pK(a) values are thoroughly discussed. This rapid and automated method allows the systematic study of the effect of solvent compositions and temperatures on the pK(a). 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 degrees C and 60 degrees 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 pK(a) values are in excellent agreement with those previously reported. The results were fitted to empirical functions between pK(a) and temperature and composition. These equations, which can be used to estimate the pK(a) of these substances at any composition and temperature, would be highly useful in practical work during chromatographic method development" |
| Keywords: | "Algorithms Buffers Chromatography, High Pressure Liquid/*methods Ethylamines/*chemistry Formates/*chemistry Hydrogen-Ion Concentration Mass Spectrometry/methods Models, Chemical Solvents/*chemistry *Temperature Thermodynamics;" |
| Notes: | "MedlinePadro, Juan M Acquaviva, Agustin Tascon, Marcos Gagliardi, Leonardo G Castells, Cecilia B eng Research Support, Non-U.S. Gov't Netherlands 2012/04/17 Anal Chim Acta. 2012 May 6; 725:87-94. doi: 10.1016/j.aca.2012.03.015. Epub 2012 Mar 17" |
