DPD Powder Question

[DavidD]

The kit from the store said to use 2. I was just thinking this morning that I was going to run out at the rate I'm testing. (who's obsessed? not me.)

I know what you mean about the obsessed part. I test way more than I need but it just doesn't take that long to do and I'm taking Carl's "stay ahead..." advice to heart. Any time I test for CL using OTO, I also test using FAS-DPD since there isn't any "shade" matching involved. It's either pink or clear, cut and dry.

Dave

[PoolDoc]

General notes about DPD powder:

1. Those who said that if the sample turns pink, no more DPD is needed are probably right. But, I'm not entirely sure of that. More below.
2. The quantity of powder in TWO Taylor scoops is approximately the same as in ONE of my scoops. When I switched to my own container serveral years ago (after Taylor adopted a container that guaranteed spoilage of the powder) that was the smallest scoop I could find.
3. Moisture is what kills the powder. After years of being told it wouldn't help, I finally put the desiccant packs into my containers. The very next season, Taylor added desiccant lining to their container, when they replaced the horrible container that had prompted my switch to my own containers.

By the way, thanks for the info about dumping DPD powder. It had never even dawned on me that that could be an issue. I'm sometimes all thumbs, but it happens I've never done that, and it just didn't occur to me. I'll have to think about that, and see if there's a way to avoid that problem.

Ben
"PoolDoc"


More about the powder:

All of the Taylor tests I use -- OTO, phenol red, ALK, CAL, CYA, SALT -- EXCEPT the FAS-DPD are essentially industry standard tests. They've made a few modifications, like stabilizing the phenol red a bit against high chlorine levels, but mostly, they are standard reagents used in standard ways. However, the FAS-DPD test does not appear to be standard. Using ferrous ammonium sulfate with DPD is standard method, but as a rather complex lab method which requires use of upwards of 7 different chemicals. Taylor's done some proprietary blending and buffering to reduce this to just two reagents -- the DPD powder and the FAS solution. But, unlike the other chemicals I use from Taylor, I do NOT know exactly what's in either one.

I've looked, but as far as I can tell, this test is unique to Taylor. Hach sells a DPD powder for use in it's colorimetric (color matching) tests. LaMotte and Palintest sell the DPD tablets originated by AT Palin, the originator of the DPD method. But only Taylor seems to have the simplified titrimetric test used in my PS23x series kits.

The DPD powder is mostly NOT DPD! It actually contains much less than 1% of N,N-Diethyl-p-phenylenediamine which is the actual DPD indicator. The rest is phosphate buffers, metal sequestrants, and other unspecified 'stuff'. They do NOT share the details of what this 'stuff' is, nor how it affects the test.

For this reason, I'm not entirely confident that it's always safe to say that, "If you get a pink color, all is well". I'm pretty sure that that's true if chlorine levels are 5ppm or less, but I'm much less sure when chlorine levels exceed 15 ppm!