Determination of ionization constants (PKA) of β-hydroxy-β-arylalkanoic acids using high-pressure liquid chromatography

Abstract

pKa values of five β-hydroxy-β-arylalkanoic acids and ibuprofen were determined using the RP-HPLC method. Stationary phase was octadecyl modified (C-18) silica gel, and mobile phase was a mixture of methanol and one of nine different buffers (60:40, v/v). values were measured after mixing methanol with an appropriate buffer. The mean retention time of each compound was plotted against of each mobile phase. The inflection point of each sigmoidal curve represented of the compound. Using in already known equations for the specific methanol/buffer mixture, values were calculated. Obtained pKa values for synthesized compounds were in a narrow range from 3.34-3.81 and pKa for ibuprofen was 4.45. Predicted pKa values for these compounds in SPARC software showed good correlation with experimental pKa values (R 2 =0.8048).

Determination of acid dissociation constants of Alizarin Red S, Methyl Orange, Bromothymol Blue and Bromophenol Blue using a digital camera

“…Absorbance (or RGB colour absorbance)-pH graphs were used to calculate the respective pK a values where the pK a value is represented by the pH at half the inection of the sigmoid graph. 17,25,26,42 For a more accurate determination of pK a the 2 nd derivative graphs were also plotted.…”