When measuring Free Chlorine Residual using the DPD colorimetric method (Standard Method 4500-Cl G), how quickly should the reading be taken after adding the DPD reagent to the sample?

• Wait 15 minutes in total darkness
• The timing does not matter as long as the color turns pink
• Read immediately (within 1 minute) before combined chlorine begins to react
• Wait exactly 5 minutes for full color development