Poly(acrylamidoxime) adsorbents are often invoked in discussions of mining uranium from seawater. It has been demonstrated repeatedly in the literature that the success of these materials is due to the amidoxime functional group. It is known that amidoximate is the amidoxime species that complexes to uranium. However, a wide range of values have been reported for the acid dissociation constant, pKa, that is necessary to measure the binding constant to uranium, log K1. Herein, we report the acid dissociation constants of acetamidoxime, as a representative of the amidoxime functional group present in poly(acrylamidoxime) adsorbents. Two dissociation constants were obtained in water at ionic strength 0.5 M (NaNO3) at 25 �C. The first one corresponed to the dissociation of protonated acetamidoxime, pKaH = 6.03, and the second one corresponed to the dissociation of neutral acetamidoxime, pKa = 12.10. With this information we were able to estimate its log K1at 9.6 by using a linear free energy relationship. Additionally, two protonation sites have been proposed in the literature for a protonated amidoxime functional group: the amine nitrogen and the imino nitrogen. We performed calculations with explicit hydration in water and determined that the amidoxime functional group is protonated at the imino nitrogen atom in solution.Acknowledgment:  This work was sponsored by the US Department of Energy, Office of Nuclear Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC.

Acetamidoxime dissociation constants: Experimental and computational study

248th National Meeting (2014)

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