Ammonia & Nitrite: When to check test results?

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pseudorand

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I just measured ammonia and nitrite using my API test kit. I waited exactly 5 minutes as instructed and recorded one reading. A came back 5 (or so) minutes later and decided to double-check my reading. This time I read a higher value. (Nitrate looked the same both times.) Note that I did not re-run the test, I simply looked at the same vial against the color chart again.

Some test kits, such as my Red Sea iodine kit, specifically say results are stable for only5 minutes and not to rely on them after that time. But the API ammonia and nitrite test kits say no such thing.

So which is the correct result? Do these tests continue to get darker with time, or do they converge on the correct reading and simply need more than 5 minutes?
 
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pseudorand

pseudorand

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I usually wait about 15-minutes for the reading, using Red Sea test kit.
But how long do the instructions for the Red Sea test kit say to wait? Do you get different results if you wait exactly that long and then read again at 15 minutes? If so, how do you know which is more accurate?

I typically set up ammonia, nitrite and then nitrate. All take 5 minutes per the instructions, but I start my timer after the nitrate test is set up and read all three when that goes off. This means I'm reading ammonia at about 9 minutes. I had assumed it converges on a value within 5 minutes and this should make no difference.

But today I was going to test ammonia only, so I set it up and started my timer. Then I figured I should test nitrite and nitrate while I was at it. Much to my surprise, I got different answers.
 
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pseudorand

pseudorand

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The API should be read after the time indicated - not a later time. In my experience, Ammonia continues to change as do some of the others.
Good to get confirmation of my sanity, but I'm still interested in the chemistry of it, or at least confirmation that this collective observation agrees with theory. It will make my test procedure more of a PITA (multiple timers), but it will also explain quite a few disagreements between the chemical ammonia test (not zero) and the is-everything-in-the-tank-dead test (no positives so far, at least in this tank).
 
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Randy Holmes-Farley

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If nitrite may be present, both may be way overestimating nitrate. You did not post a value for it.

API tests are not noted for accuracy, especially at the low end, often reading some ammonia when other kits do not.

Kit timing can certainly be different. Don't try to outsmart the manufacturer in that aspect.
 
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