FT-IR Titration Unveils pH Mysteries: New Insights into Protonated Mono and Polyamine Solutions

In a recent study published in Applied Spectroscopy on November 20, 2023, researchers from Technische Universität Dresden in Germany, led by corresponding author and faculty member Martin Müller, have delved into the determination of pKa values for protonated mono and polyamines in concentrated solutions, utilizing Fourier transform infrared (FT-IR) titration and classical potentiometric (POT) titration techniques. The article, titled "Determination of the pKa Value of Protonated Mono and Polyamine in Solution Using Fourier Transform Infrared Titration," yields information on the intriguing behavior of propanolamine hydrochloride (PAMH) and poly(allylamine hydrochloride) (PAAMH) under varying pH conditions (1).

Outdoor pH test | Image Credit: © Shawn Hempel - stock.adobe.com

The research employed in situ attenuated total reflection (ATR)-FT-IR spectra on PAMH and PAAMH solutions, starting with their fully protonated forms and incrementally adjusting the pH with sodium hydroxide addition. By monitoring the variation of diagnostic infrared (IR) bands, particularly the δ(NH³⁺) band, the dissociation process of the NH³⁺ groups was tracked. The dissociation degree αIR of the ammonium groups was determined from the decrease of the most intense band area A.

The study innovatively plotted pH against αIR, fitting the curve with a modified Henderson–Hasselbalch function pH = pK_a + B log (αIR/1 – αIR), enabling the extraction of crucial parameters—pK_a and cooperativity factor B. The obtained pKa values from FT-IR titration were qualitatively aligned with those from POT titration, providing comprehensive insights into the dissociation processes of these chemical species (1).

The article systematically explores and discusses quantitative deviations in pK_a values between polyelectrolyte (PEL) and respective monoelectrolyte. The researchers delve into the potential effects of PEL molecular weight, ambient ionic strength, and titration concept (FT-IR and POT), drawing on classical models of weak PEL.

This research not only refines our understanding of protonated mono and polyamines but also contributes valuable data to the broader field of chemical analysis using spectroscopy. The article, available in the latest issue of Applied Spectroscopy, showcases the detailed work of the research team in unraveling the mysteries of pH behavior in these intriguing chemical systems.

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Reference

(1) Müller, M.; Wirth, L.; Urban, B. Determination of the pKa Value of Protonated Mono and Polyamine in Solution Using Fourier Transform Infrared Titration. Appl. Spectrosc. 2023, November 20, 2023. DOI: 10.1177/00037028231213673