Acidity Constant Estimation of Weakly Acidic Polyelectrolyte by Linear Approximation: A Case Study for Polyphosphate via Gran’s Approach

The acidity constant values (K_a or pK_a) and polyionic species distribution are valuable tools to identify the extent of ionization and complexing properties of polyionic species. They are of great importance to predict the charged state of analyte and choose the appropriate condition in the design of functional materials for their intended purposes. Polyphosphates, being one of the anionic types of polyelectrolytes, have attracted interest for utilization in a quite wide range of applications in the boundaries of nanomaterial, bio-analytical chemistry, and physical sciences. In this study, it is aimed to estimate the intrinsic pK_a^o of polyphosphoric acid (PPH) with high molar mass and the charge of its polyanionic conjugate base (PP⁻), (ZPP-), according to activity coefficient estimation by Davies using potentiometric titration along with the theoretical consideration of Gran’s approach. Besides, a series of PPH titration was simulated via CurTiPot and the data were further treated in MATLAB to visualize the 3-D pH surfaces of PPH/PP⁻ pairs. As a result, the outcomes of experimental work and simulations were consistent very well, and pK_a^o of PPH and ZPP- were found as 2.22 ± 0.01 and − 1.105 ± 0.003, respectively, at a 95% confidence interval.