DPD Powder


LifeTime Supporter
Jul 30, 2007
When I got my first TF-100 test kit last year the powder was a light gray. I always keep my kit stored inside but even so, I'm in FL and humidity is high. I also have a few packets of silica gel I keep in my test kit thinking it should help with humidity.

When I broke out the kit after the winter I noticed the powder had darkened, but I assumed it was still good. It seemed to be working fine and I never had a reading of more than .5 CCs. But last week (my bad) I let the FC drop to 2 and I got a touch of algae. Still, my CCs tested <1. I brushed and shocked the pool and the FC held overnight on the first night. Since then I have been waiting for the chlorine to drop back to maintenance level before turning the SWG back on. No one has been swimming because we're both sick so it has taken a while.

Anyway, today I broke out the new powder I just received and was surprised to see it is a white color. When I tested I had a reading of 1 CC, so I suspect the old powder I was using was not giving me a good reading of CCs.

When the powder turns dark does that mean it is bad?
What is the shelf life of the powder supposed to be?


LifeTime Supporter
Platinum Supporter
TFP Expert
Normally it turns from light grey to a purplish color, but that does not change the effectiveness. If it's been kept indoors and closed it should last for a couple of year, maybe more. I've not heard anyone mention theirs turning white though.

If you can post up your full set of numbers, we might be able to get a better idea of what's going on.


Mod Squad
LifeTime Supporter
Platinum Supporter
TFP Expert
In The Industry
Apr 1, 2007
Sebring, Florida
Sue, I think you were originally talking about the difference in your test results (.5cc vs. 1.0cc).

That's one drop difference and it's my experience that's about as close as you can get it. The test will vary that much from day to day. I have used some dark powder that's over four years old and the results have always been valid. It doesn't take long for it to turn light grey and then that dark purplish grey but it still seems to work.

Taylor says replace it every year and I can't say not to. My experience tells me you can go longer than that, however.
The powder reacts with moisture and atmospheric oxygen and darkens in a very similar way that an open bottle of permanent haircolor turns dark once it has been opened. (Not surprising since DPD is one of the dye bases used in some haircolors). It does lose a bit of 'potency' as it darkens (oxidizes, actually) but as long as you are getting a good pink color when you add it to your sample of pool water it will work fine in much the same way that an opened bottle of hair color will still color the hair properly even though it has darkened in the bottle. Some of the active ingredients has oxidized but there is still a lot of unoxidized material present.