Elimination of Oxygen Interference in the Electrochemical Detection of Monochloramine, Using In-Situ pH Control at Interdigitated Electrodes

Disinfection by chloramination of water systems is an alternative to chlorination that is frequently used in North America. In such a case, monochloramine is used as the primary source of chlorine for disinfection. Regular monitoring of the residual concentrations of this species is crucial to ensure adequate disinfection. An amperometric sensor for monochloramine would provide fast, reagent free analysis, however the presence of dissolved oxygen in water complicates sensor development. In this work, we have explored the use of in-situ pH control as a method of eliminating oxygen as an interferent by conversion of monochloramine to dichloramine. The electrochemical reduction of dichloramine occurs outside the oxygen reduction window and is therefore not affected by oxygen concentration. Potential sweep methods were used to investigate the conversion of monochloramine to dichloramine at pH 3. The pH control method was used to calibrate monochloramine concentrations between 1 and 10 ppm, with a detection limit of 0.03 ppm. Tests were carried out in high alkalinity samples, wherein it was found that the sensitivity of this method effectively remained unchanged. Monochloramine was also quantified in the presence of common interferents (copper, phosphate and iron) which had no significant impact on the analysis