Determination of acid dissociation constants and reaction kinetics of dimethylamine-based PPCPs with O₃, NaClO, ClO₂ and KMnO₄

Dimethylamine-based pharmaceutical personal care products (DMA-based PPCPs) are a group of N-nitrosodimethylamine (NDMA) precursors. The acid dissociation constant (pKa) values of four DMA-based PPCPs were determined by potentiometric titration over the pH range of 3–11. The pKa values of ranitidine, nizatidine, doxylamine and carbinoxamine corresponding to the DMA moiety were 8.4, 6.8, 9.4 and 9.1, respectively. Competition reaction kinetics and pseudo-first-order reaction kinetics were used to determine the reaction rate constant (k) of the DMA-based PPCPs with O₃, NaClO, ClO₂ and KMnO₄. Comparing the degradation rate constants of the four DMA-based PPCPs, the results of ClO₂ oxidation were close, and for the other three oxidants, the order was k_ranitidine ≈ k_nizatidine > k_doxylamine ≈ k_{carbinoxamine}. Comparing the reaction rate of the four oxidants, for ranitidine and nizatidine, the order was k_NaClO > k_O3 > k_KMnO4 > k_ClO2, and for doxylamine and carbinoxamine, the order was k_O3 > k_NaClO > k_ClO2 > k_KMnO4.