In any reaction between two charged species, the concentration of electrolytes present in solution can substantially impact reaction rates and shift equilibria. This occurs as a result of perturbations to the electrostatic force between the two charged species resulting from the additional ions in solution. For electrolyte concentrations below an ionic strength of 100 mM, Debye-Hückel theory accounts for these effects reasonably well. However, this theory breaks down for electrolytes with highly charged ions such as polyanionic polyoxometalates (POMs). We demonstrate a precise analytical method to study this phenomenon by examining changes in the pKa values of weak acids, such as 4-nitrophenol (NP), as a function of solution ionic strength. Visible absorption spectroscopy allows quantification of both the protonated and deprotonated forms of NP which has a high extinction coefficient when deprotonated. At higher ionic strengths, the acid dissociation equilibrium is shifted in favor of the deprotonated phenolate as the association with protons becomes slower. Common salts including NaCl, NaClO4, and Na2SO4 are all consistent with Debye-Hückel theory, while POMs such as Na5AlW12O40 13H2O and K13Na2P5W30O110 15H2O deviate substantially. Explanations for these observed deviations are explored.

Evaluating the effective ionic strength of highly charged polyions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.