DPD Powder is white but performs inconsistently.

[DistantHorizon]

Anyone ever have DPD powder go bad while it still looks white?

I'm getting inconsistent free chlorine test results. On several occasions I've gone to test chlorine and found it way too low, and added enough to correct. Then, testing the next day, I find that the reading is higher than it should be, given the amount I added.

After a few scares, I have determined that sometimes when I add the test powder, the water doesn't get very pink, and the resulting test isn't going to report much chlorine. Dumping water, rinsing and starting over usually does the trick, though I occasionally need to try a third time.

Both DPD powder (Taylor R-0870) and R-0871 are last year's, but the problem sure seems to be occuring when I add DPD.

I have another, unopened DPD powder I'm going to try before I order another, but both it and the one I'm having trouble with were purchased at the middle of last year's pool season, and have been stored in my 70-80 degree house.

I'm also curious, why do vials of DPD powder have varying granule sizes?

 

[duraleigh]

It doesn't matter if the pink color is intense or not...although it is indicative of how much chlorine is in the pool.

The amount of DPD powder is commonly misunderstood.

If you have ANY chlorine in your pool, put enough DPD powder in your test vial to turn the solution pink....that's it. If one scoop turns the solution pink, that's enough and the test is valid.

If it takes two scoops, the test is still valid.

If it turns dark pink, the test is valid

If it turns light pink, the test is valid

In the presence of very high chlorine, the test may "flash" pink for a few seconds on the first scoop. add another scoop. Add another scoop if it flashes pink again.

Residual powder in the solution is irrelevant....the test remains valid.

If the solution absolutely doesn't turn pink, the test is valid.....you have no chlorine!

My point is to emphasize that this test is really pretty predictable and is subject to very little interference. Taylor chemistry will only warrant the product to one year from purchase but many on the forum have had good experience using the reagents longer if the kit is kept in a controlled environment.

I cannot say why your test results have been unpredictable but the test itself is almost always very dependable and deadly accurate.