Quote Originally Posted by JimK View Post
This part caught my attention. Are you adding 2 scoops of DPD powder to the water sample per Taylor's instructions? This part of you post sounds like you are adding 1 (incorrect) or 2 (correct) scoops.

I'm not sure why your sample would return to pink after turning clear. I've been using this test kit for years and it has never behaved that way. One possibility I just thought of is that it will return to pink if the DPD powder hadn't completely dissolved before adding the reagent (it will return to pink as the remaining powder dissolves).
The amount of DPD powder is not critical as long as the pink color forms. If you are using a 10 ml sample one scoop is more than sufficient and often is with a 24 ml sample also.

If there is very high CC it is common for the color to return to pink on FC part of the test. The sample will also return to pink if allowed to stand. This is normal behavior.

Also, the pink will not return if all the DPD has not dissolved since there would be no more chlorene for it to react with once the titration is complete.