Which substance can interfere with a DPD chlorine test?

The DPD (N,N-diethyl-p-phenylenediamine) chlorine test is a common method used for measuring free or total chlorine in water. Certain substances can interfere with the accuracy of this test, leading to incorrect readings. Manganese is one of those substances that can cause interference.

When manganese is present in the water sample, it can react with the DPD reagent used in the test. Specifically, manganese forms complexes that can result in the formation of a color that is similar to the one produced by free chlorine, which can cause an overestimation of chlorine levels. This interference can lead to misleading conclusions about the amount of chlorine present, impacting water quality assessments and treatment decisions.

In contrast, chloride, iron, and calcium do not have the same propensity to produce misleading coloration in a DPD test, thus they do not interfere in the same way as manganese. Understanding the compatibility of different substances with the DPD test is crucial for accurate water analysis and management of wastewater treatment processes.