... compared to Hach DPD and Taylor OTO. R-0871 is ~ 3-months old; it started all at once.

Not shaking the reagent enough?

Neither the OTO or the DPD test is as accurate as the FAS/DPD. Why do you think it is the one in error?

I spent years comparing results between Hach DPD kit and two different models of Hach spectrophotometers at a waste-water treatment-plant; they were always quite close. Started using FAS/DPD because I no longer have access to a spectrophotometer, and did not feel the necessary dilutions were worth the effort for a swimming-pool (lazy!) And no longer need to comply with state environmental-procedures.

pH has been a consistent 7.6, and no other oxidizer was present. Distilled-water is used for the dilutions. There are stains from the DPD powder inside the plastic-cylinder as I have yet to find a way to remove them, but they should not affect the test.

Pool is concrete, and Chlorination is by Tri-Chlor generator. OTO test is used by the tenant to alert me if the Chlorine level falls to 3. OTO is difficult for me to evaluate, even with dilution.

A new bottle of R-0871-2 is 'in-the-mail.'

Well, I'm still not sure how you decided the FAS/DPD test is the inaccurate one but, hopefully, your new R-0871 will help.

As an aside, why are you using dilutions? There should be no reason to do that and dilutions will ruin the accuracy of ANY of those tests.

You can use a 3:1 mixture of muriatic acid and water to remove the powder stains from the plastic cylinder but, you are correct, it has no influence on the validity of the test.

Curious, exactly how low is it as compared to the others?

Tailor FAS/DPD measured less than half Hach DPD. Had good correlation between the two all summer, then one day the Taylor measurement started measuiring low for no apparent reason.

Good point on dilutions. Have been using grocery-store distilled-water, and having faith that that is what it is. Have some Hach deionized-water around here somewhere; will check for a difference. Ran out of distilled-water one day checking my aquarium, and used bottled drinking-water. The water measured 8-mg/l Nitrate-Nitrogen, almost at the legal threshold of 10. I use the same methodology and precision-pipettes acceptable to EPA regulators.

The Hach DPD scale has a maximum reading of 2.5-mg/l. I try to operate between 4 & 8.

Thank you for the tip on cleaning the cylinder with acid. Maybe I will not burn myself!

Mystery solved!

The purple-powder in the jar labeled R-870 is supposed to be white.

The Chlorine-measurements between a newly-arrived Taylor FAS/DPD kit and the Hach kit agreed.

And the new cylinder did not get any black stains.

Thank you for your interest!

That is very interesting. OK, so we now know that when the R-0871 DPD powder turns purple...

it's BAD!

LOL... now we can add "what color is your DPD powder?" to the line of questioning for the next person who says the testing is off. Thank You!

Do you think it got moist from chlorine water at some point?

It was purple two-months ago when I first opened it, and is almost black now. It never did clump.

Do not remember spilling pool-water into the jar. The air here contains plenty water, and I try to replace the lid expeditiously.

Will add a pouch of desiccant to the new container.

That's a fabulous idea. I need to re-evaluate where I keep my kit this winter. It is moist here nearly 24/7 most of the year. I have extra powder, I might do some testing with moistures of varying sources...

As noted in Taylor's Recognizing a Compromised Reagent article, for the R-0870 powder "as it degrades, the color will turn from an off-white color to gray or purple-gray and will gradually become a purple- brown color."

Thank you for the link, chem geek.

Would you happen to know if the LaMotte Iron Insta-Test measures chelated, free, or total iron? Test results measured zero, whereas the Seachem MultiTest measured somewhat more. Before its powdered-reagent turned brown.

You cannot make a blanket statement that says R-0870 powder that is off-color is bad.

My experience is that the R-0870 starts to darken in color almost immediately upon opening and using it. The more it is exposed to moisture and/or relative humidity, the more it continues to darken. That does not negate the validity of it's use.

I have R-0870 powder in my possession (edit: it's four years old!) that is charcoal-purple in color and it performs the FC test flawlessly. I also have R-0870 powder that is less than thirty days old to which I can compare it.

There are others on this forum with similar experiences although I doubt anyone has kept any for four years.

I don't know the details of those tests. You can ask the manufacturers of the tests for more info. In general, chelated metal ions still get measured in the metal tests since the chelation of the metal is an equilibrium reaction and the metal gets unbound as it reacts with the chemicals in the test. However, whether it shows up depends on the rate or this release relative to the rate of reaction in the test.

I agree with Dave that off-color alone is not a problem for the powder. Mine is often grayish and works fine. The only reports of powder not working anymore that I recall were when they got really clumpy and purple though that color still works for Dave. What I referred to in my post was just what Taylor describes as what happens as the powder ages.