Determination of the Carboxyl Dissociation Degree and <i>pK</i><sub>a</sub> Value of Mono and Polyacid Solutions by FTIR Titration
Martin Müller, Luise Wirth, Birgit Urban
Abstract
Abstract Dissociation degree α and pK a of two model carboxylic acid systems are determined by the Fourier transform infrared (FTIR) titration concept and compared to the classical potentiometric (POT) titration concept. pH dependence of poly(acrylic acid) (PAA) is studied in comparison to that of propionic acid (PA) being structurally related to PAA repeating units. Starting with the respective acid forms and increasing the pH value by adding NaOH portions FTIR spectra of PA and PAA show decrease of ν(CO) band due to carboxyl (COOH) groups and increase of ν a (COO − ) band due to carboxylate (COO − ) groups. From the integrated band areas A the dissociation degree can be calculated with F taking different absorption coefficients of ν(CO) and ν a (COO − ) into account. pH is plotted versus α IR and this curve fitted by functions related to Henderson–Hasselbalch equation. As fit parameters pK a value for PA and PAA and optionally empirical cooperativity factor B are considered. pK a values found for PA and PAA by FTIR titration are in agreement to respective pK a values found by own and referenced POT titration data. The found pK a deviation between PA and PAA and PAA molecular weight trends are discussed based on the known two‐phase‐model including Gibbs–Donnan‐potential concept.