For reactions involving two or more charged species, the solution ionic strength can greatly impact reaction rates. In solutions with simple salts such as NaCl, Debye-Huckel theory accounts for these effects reasonably well. However, this theory breaks down for more complex highly charged species such as polyanionic polyoxometalates (POMs). We have developed an analytical method to probe the ionic strength of POMs and other polyions by observing changes to the equilibrium distribution of a protonated vs. deprotonated dye, 4-nitrophenol (NP), in the presence of different POM/electrolyte concentrations. Higher ionic strengths shift the distribution in favor of the strongly colored deprotonated phenolate as the association with protons becomes slower. By measuring the absorption spectrum of the solution, perturbations to the dissociation constant of NP at different ionic strengths can be quantified. Common salts such as NaCl, NaClO4, and Na2SO4 obey Debye-Huckel theory while POMs deviate substantially. Multiple POMs are compared and explanations for the deviations are explored.

Ionic strength of large polyions

2019 Fall National Meeting

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