FAS/DPD titration reagents

[CaOCl2]

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.

 

[JasonLion]

I don't know the chemistry enought to be sure but I believe that the problem is that the R-0001 R-0002 combination will stop working at FC levels around 25 and higher, while R-0870 will work up to 50. For me the powder is easier to handle than counting ten drops from two different vials.

 

[chem geek]

It's even worse than that. According to Taylor's interference instructions here, the DPD reagents can bleach out with chlorine levels above 10 ppm, not 25. The FAS-DPD interference instructions here indicate that above 25 ppm the bleaching out will create a brown color (so at least you still know there is chlorine present) and that using more powder let's you test even higher chlorine levels -- somewhere else it said up to 50 ppm if you use more powder.