Can monochloramine be determined using free and total chlorine DPD chemistry?
Calculating monochloramine using DPD chlorine chemistry
Hach does not recommend using the difference between a total and free DPD chlorine test for measuring monochloramine. This is because high levels of monochloramine interfere with the free DPD chlorine test, causing the reading to continually increase. Therefore an accurate reading for free chlorine in chloramination systems cannot be measured. Free chlorine method 10241 using indophenol is recommended.