Just a thought: Why can't we use R-0001 and R-0002 instead of the DPD powder R-0870 in the water sample? It seems to me that the powder contains the both the phosphate pH buffers of R-0001 and the DPD of R-0002 (and some EDTA)? Is it strictly a concentration issue?
I did perform both tests yesterday, first with the powder and then replacing the powder with the combination R-0001/R-0002 and the results were within a few drops of each other. I was expecting the FAS to reduce the Wurster dye at the same rate with both DPD formulations.