Why should samples for corrosive tests be taken only after post-chlorination?

Samples for corrosive tests should be taken only after post-chlorination primarily because disinfectants, such as chlorine, may react with corrosion-control chemicals that are added to the water treatment process. During the water treatment process, corrosion inhibitors are commonly used to prevent the leaching of metals like lead and copper from pipes into drinking water. If these corrosion-control chemicals have not had sufficient contact time with the disinfectant, their effectiveness may be compromised, and the samples may not accurately reflect the water's properties after all treatments, including disinfection, have been conducted.

Taking samples after post-chlorination allows for a better assessment of how the final treated water behaves when exposed to chlorine, as well as how it interacts with any corrosion inhibitors that have been added. This practice ensures that the water quality, particularly in terms of corrosivity and the potential leaching of harmful metals, is accurately evaluated in its end state, right before distribution.

While concerns about organic materials and other chemical interferences are important in broader water testing contexts, they do not specifically address the unique interactions between disinfectants and corrosion-control agents that can occur before disinfection. Similarly, while accurate measurements of lead and copper are critical, the timing of samples in relation to post-chlorination specifically addresses