Comparative analysis of ionization constants determination using spectrophotometry and potentiometry: 3-aminobenzoic acid, 1,3,5-benzenetricarboxylic acid and tyrosine

Abstract

The ionization constant (usually expressed in logarithmic form, pKa) is important physicochemical parameter which is used to characterize the acid-base chemistry of a compound. Since most drugs contain one or more ionizable functional groups, knowledge of pKa values is necessary in drug research. The most common techniques used for pKa determination are potentiometry and spectrophotometry. Potentiometry is a method of choice when ionization processes are overlapping, as in such case it is not possible to obtain the absorption spectrum of each species present in solution. The aim of this work was the comparative analysis of pKa determination using potentiometry and spectrophotometry for model compounds with overlapping ionization processes: 3-aminobenzoic acid, 1,3,5-benzenetricarboxylic acid and tyrosine. The potentiometric titrations were performed with pSOL Model 3 instrument (pION) equipped with pS software package for titration data analysis.1 Avdeef–Bucher four–parameter equation was used for electrode standardization.2 To overcome the above-mentioned limitation of spectrophotometry, the alternative approach was applied in this study. The new aminocaproate phosphate buffer (containing phosphoric and ε-aminocaproic acids) was used for the solutions preparation of the model compounds in pH range 1 – 12. This buffer has numerous advantages like UV-transparency, resistance to pH changes upon standing for several days, useful buffer capacity and constant ionic strength in the wide range of pH values. Absorption spectra were recorded according to specific procedure which was carefully designed to avoid systematic errors. Collected absorption spectra will be used for the development of the algorithm for the spectral deconvolution (using MATLAB). Such software can be very useful tool in the drug research, especially for the analysis of the compounds which pKa values cannot be determined by potentiometry.