Determination of the pK_a Value of Protonated Mono and Polyamine in Solution Using Fourier Transform Infrared Titration

The pK(a) values of propanolamine hydrochloride (PAMH) and poly(allylamine hydrochloride) (PAAMH) in concentrated solutions were determined by both Fourier transform infrared spectroscopy (FT-IR) titration and classical potentiometric (POT) titration and compared. Starting with the respective fully protonated forms PAMH and PAAMH and increasing the pH value by sodium hydroxide addition in situ attenuated total reflection FT-IR (ATR FT-IR) spectra on PAMH and PAAMH solutions show the variation of diagnostic infrared (IR) bands. From the decrease of the most intense delta(NH3+) band the dissociation process of the NH3+ groups could be followed. Thereby, from the respective normalized band area A the dissociation degree alpha(IR) of the ammonium groups could be determined. Plotting pH versus alpha(IR) and fitting this curve by a modified Henderson-Hasselbalch function pH = pK(a) + B log (alpha(IR)/1 - alpha(IR)) the parameters pK(a) and cooperativity factor B were obtained. pK(a) values from FT-IR titration were qualitatively in line with respective pK(a) values from POT titration. Quantitative systematic pK(a) deviations between polyelectrolyte (PEL) and respective monoelectrolyte and the tentative effects of PEL molecular weight, ambient ionic strength, and titration concept (FT-IR and POT) are discussed based on classical models of weak PEL.