Make sure you use 2 heaping scoops of R-0870 DPD powder, swirl to mix, some granules are fine at the bottom, they don't all dissolve at 1st.
Check your DPD Titrant reagent.
R-0871 is FAS-DPD titrating reagent. It should be a clear colorless liquid. If it turns a dark yellow color, it has gone bad.
Make sure you hold the bottle completely vertical and squeeze with just enough pressure to allow a drop to come out and hang on the tip for just a moment before falling. IE. Minimal required pressure to reach a fully formed drop, which changes as the bottle empties, or when it's full and new.
At high FC levels you need to add drops quickly, about once per second, swirling the entire time. You can slow down as you reach the end.
Use a white backdrop in good light to verify your endpoint and that the water is indeed clear.
If you suspect FC is very very high, in order to save on reagent and DPD powder, use a 5ml water sample. Each drop of R-0871 reagent will equal 1ppm of FC instead of 0.5ppm as it would with a 10ml sample. This test is also less accurate, but OK for getting close as it produces about a 20% yield in accuracy.
So with a FC level of say 15ppm, you could be 12-18ppm, vs a 10ml sample will yield an accuracy range of 13.5-16.5ppm FC. So still reasonable enough at 20% and way more accurate than OTO.
But do use your OTO Taylor Basic kit, or whatever OTO block you may have if not that one and verify how high/low your CL is.
Another thought would be that you might be fighting with ammonia in there, hence the odd behavior. But I suspect the above should cure it, as you clearly have CL present, since it turns pink when you add the DPD powder. I'm just betting there isn't enough DPD being added and the titrating isn't happening fast enough.
