What would cause positive results using DPD when no iodine is present in the sample?
Positive interferences to iodine method
Oxidants such as chlorine, bromine, ozone, chlorine dioxide, or hydrogen peroxide can react with DPD and cause false positive results. The most common interferent is oxidized manganese, which can be removed by pretreating 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. 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.