What would cause positive results using DPD when no bromine is present in the sample?
Positive interferences to bromine method
Other oxidants such as chlorine, iodine, ozone, chlorine dioxide, or hydrogen peroxide can react with DPD and cause false positives. The most common interferent is oxidized manganese, which can be corrected for by treating the sample with potassium iodide and sodium arsenite. Sunlight can react with the DPD indicator during the 3-minute reaction time; keep the sample covered during the reaction time if testing outdoors. It is also a good idea to use the same sample cell for zeroing the instrument and reading the sample concentration. This avoids any concentration that might be due only to optical differences between the zero and read cell.