1. After adding powder and titrating the sample to colourless with my TF testkit the sample somehow turns back to pink after 1 - 2 minutes is this normal? and why?
2. I borrowed a photometer from my neighbour reading came out to be Cl 2.5 ppm and for the test done with my TF testkit my reading came to be Cl 6.0 ppm, which of the readings should i believe?