I want to help a friend at work understand the benefits of TFP by testing his water. He lives around 90 minutes from work. How long after he takes a sample and brings it to work can I test it and get reliable results?
If he caps the jar and keeps it out of sunlight, then 90 minutes should be fine. The main parameters that can change over time are the chlorine if exposed to sunlight and the pH which can rise if the sample is shaken or stirred while exposed to air.