DPD powder experiment

Been reading here for a long time about deterioration of DPD powder. When it gets clumpy and dark gray or black, it's no good or maybe questionable. When in doubt, run an experiment. Taylor R-0870, same as in the PS-233 kit. Took a small amount from same batch and let it sit in a container, sometimes open, sometimes closed, in a high temperature and humidity environment (Northeast PA with no A/C) for probably 6 weeks. Looked awful. Did a 25 ml sample of water just now with good powder and yucky powder after declumping it. Both tests identical, 17 drops, 3.4 ppm. You can draw your own conclusion.
Al

I did a similar thing last year. I got new dpd powder at the beginning of the year, but still had some old studd left from the year before. I tested tho old grey stuff against the new and came up with the same results - but by mid year (about now) I did the same test again and came up with about a 1/3 decrease in the reading with the old powder. It was really dark grey and clumpy by then. Unfortuantely, there seemed no way to tell by eyeballing it when the stuff really started to deteriorate in quality.

Thanks Al! I kind of suspected but wasn't sure so I kept dumping the "lumpy". Not any more. :)

Dave

Interesting about the 1/3 decrease in reading. I still have the powder samples and will try a couple more comparisons, probably at end of this season and next year. Chemistry isn't my bag so I have no idea. Chem_Geek...any thoughts on this?
Al

I have started to keep the DPD powder vial in a silica gel bottle, and am waiting to see how it turns out.

Pat

My understanding is that as long as the color has not turned to a dark grey it is still usable. Light grey and clumpy is ok.

I think the 'bad' powder is obvious. Form your own opinion. Best I could do for lighting and a gray scale without fussing too much but it is white to black. I know...I've got to get a life. But...this is fun stuff.
Al

Attachment 450

Earlier I indicated that I kept a container of the dpd powder for over one year before degradation. The stuff I have now I bought two years ago and lasted two full seasons, with what appear to be accurate results (add 5 ppm of bleach, get a 5 ppm rise in measurement). I keep it in a dry location on the bottom floor of the house. I do not put my test vials in the kit container so there is no moisture locked in. I am looking forward to Al's report on the stuff he keeps in the fridge.

A final followup on the original experiment. The caked, dark DPD powder that was used in the original test was now a hard chunky lump that had to be ground up. Yesterday did a 25 ml sample test of that vs. good powder and the results were the same at FC=1.0 Today, after bringing up the FC, both measurements were 3.6 So.....looks like the DPD powder is pretty stable even though it looks pretty bad. I certainly don't recommend letting the powder get this bad but some dark gray discoleration should be nothing to worry about.
Al

Thanks for the experiment and the results !!!

I have a PS233 kit, but did not use it this year. I am building a pool now and am curious what will still be good in it. I know the DPD powder is bad, but what is the shelf life on the rest of it?

Don't be too sure about the powder. May be perfectly good considering what I found in the experiment. I'm not sure of the shelf life of the individual components but I do keep bulk chems in the fridge and only open them to refill smaller bottles. Under these conditions they should keep almost indefinitely. Certainly you'll get a couple seasons out of them. Just keep them cool and out of the sun.
Al