Interferences by Monochloramine and Organic Chloramines in Free Available Chlorine Methods. 2. N,N-Diethyl-p-phenylenediamine

Both inorganic monochloramine and organic chloramines interfere in the free available chlorine (FAC) measurement with N,N-diethyl-p-phenylenediamine (DPD). The reaction between chlorocyanuric acid and DPD, even with 100-fold excess of the free cyanuric acid and at 9 °C, was as fast as the free chlorine-DPD reaction. Thus, 100% of the chlorocyanuric acid appears as FAC. The rate of the reaction of the other chloramines decreased with increasing basicity of the nitrogen substrate. First-order interference rate constants were measured for 10 chloramines and ranged from 4.6 × 10−5 s−1 for N-chloroalanine to 0.29 s−1 for N-chloro-5,5-dimethylhydantoin. Interference from monochloramine occurred by both direct and hydrolysis-based pathways. The interference appears to be catalyzed by trace iodide present in the phosphate buffers. The presence of Hg(II) in the buffer decreased the iodide effect, which appears most important with the strong base nitrogen chloramines such as monochloramine.