After addition of the powder test reagent, when should the free chlorine residual be determined?

The determination of free chlorine residual after adding the powder test reagent should occur within one minute to ensure an accurate reading. This prompt timing is crucial because chlorine reacts quickly with the test reagents, and the resulting color intensity may change rapidly. If the sample is measured too late, the reaction may be altered due to factors such as the depletion of available chlorine or interference from other materials present in the water sample. By measuring the free chlorine residual within one minute, you ensure that the reading reflects the chlorine concentration accurately at the time of the reagent's addition, providing reliable data for water quality assessment and treatment processes.