After adding powdered test reagent, when should the total chlorine residual be measured?

The total chlorine residual should be measured after waiting 3 minutes for several reasons related to chemical reactions and accurate testing. When powdered test reagents are added to water in order to determine total chlorine levels, a specific amount of time is necessary for the reagents to react completely with the chlorine present in the sample.

Waiting for 3 minutes allows the chemical reaction to stabilize, ensuring that the measurement reflects the true concentration of chlorine in the water. If the measurement is taken too soon, the reaction may not be complete, which could lead to an inaccurate reading of the chlorine levels. Thus, allowing this time ensures that the reaction between the chlorine and the reagent has reached equilibrium, enabling a more reliable and precise measurement of total chlorine residual.

The other time frames either do not align with standard testing practices or could potentially lead to inaccuracies due to incomplete reactions.