Mitch and Sedlak (2002) studied the formation of N-nitrosodimethlyamine (NDMA) from dimethylamine (NH(CH3)2, or DMA) in response to the addition of chloramines to water as disinfectants. They proposed that the rate-limiting step in the reaction sequence is the formation of unsymmetric dimethylhydrazine (UDMH) according to the following elementary reaction of monochloramine (NH2Cl, or MCA) with DMA:
NH2Cl + DMA → UDMH

The rate constant for the formation of UDMH via this reaction was reported to be 0.081 M-1s-1.

(a) What is the characteristic reaction time (according to the second definition) for the loss of DMA by this reaction under the conditions of the authors’ experiments, which included an initial concentration of 1mM of each of the reactants?

(b) What is the characteristic reaction time under more realistic conditions that could occur in water distribution or wastewater treatment applications, namely 3 x 10-5 M of each species?