Test color changes with more time

nerdyguy83

nerdyguy83

AC Members
May 11, 2006
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Erie, PA, US
I am using an AP test kit to check a tank I am fishless cycling. I check a couple times a day because I am impatient. I seeded the tank with sponges and gravel from another tank, so I am expecting the cycle to finish up any day now. My nitrate tests are a little odd. I record the results, but then I also sometimes leave a tube out so that I can compare side by side with later readings. The thing is, the nitrate tests get darker the longer I leave them out, but only to a certain point. The original reading after the 5 minutes allotted by the instructions is 5ppm. Another 15 minutes later it reads 10ppm, and now a few hours later it reads 20ppm. Is it getting darker because there is actually that much nitrate in there, or just because that is what it does over time once exposed to nitrate of any quantity (i.e. the 5 minute reading is the only accurate one)?
 
After the reading from 5 minutes. the test is no longer giving an accurate reading. its like checking the milk and leaving it out. its no good. the best thing to do if you want to compare the results later is to write down the result and then compare those when you test again. Also, make sure that you are rinsing the test supplies out with clean water... as not to "infect" the new test.
 
nerdyguy,

I find it strange that it will change after you've already added the chemicals. I've left my test tubes out overnight (gasp!) and haven't noticed a color change in the nitrates like that. Other tests, yes, but not the nitrates.

What's the expiry date on your bottle?

If you leave the water out for 24 hours and then test it, it will be off, yes. I once let a sample sit in a closed bottle for a day or so and when I got around to testing it the nitrates were off the chart. I was advised by the more learned members here that water samples over (I think it was) 30 minutes would not be accurate.

Roan
 
I'm not seeing any kind of an expiration date on the bottles. The only think printed on them is a lot number, but it is different on every one of the solutions. I find it odd that I let three nitrate tests sit out over night and they all went to 20ppm and then stopped instead of getting continually darker until they maxed out at 160ppm.
 
nerdyguy83 said:
I'm not seeing any kind of an expiration date on the bottles. The only think printed on them is a lot number, but it is different on every one of the solutions. I find it odd that I let three nitrate tests sit out over night and they all went to 20ppm and then stopped instead of getting continually darker until they maxed out at 160ppm.
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What's the lot number? The last two sets of digits are the month and year, ie:

27A0405 means April of 2005 and AP says the shelf life is 1 year from that date.

Hrm, guess I need to replace that bottle of NO2 test liquid :)

Roan
 
My tests are good until January and February of next year. So there must just be something wrong with the bottles.
 
nerdyguy83 said:
My tests are good until January and February of next year. So there must just be something wrong with the bottles.
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Nod nod, could be you got a bad batch. Fire off an email to AP?

Roan
 
I don't think it's a bad batch ... My test does the same thing. The manual will tell you when to check the vial for an accurate test. Anything after that will be inaccurate. My test does the same thing if I leave the vials out overnight in the morning they darken up quite a bit.

I've had the water tested with three different tests and they're always negative it's just how the thing works sometimes....
 
My test kit only does that with the ammonia test. It turns brown if left for extended periods of time. Nitrites and Nitrates will not change color.

Come to think of it, even the ammonia needs more than 15 minutes to have any significant color change.

Edit: Also, the nitrate kit requires a bunch of shaking per the instructions. I have never tried not doing this and I am not sure if that would cause the kind of results you are seeing.
 
I use Hagen's Nutrafin test kits, and my NO3 test will get darker if I let it sit for more than the instructed 5 minutes.

This is a 3 step test, so it requires a lot of shaking too, specially regeant #3. Once, I did my test with out shaking it and got an oddly low nitrate reading. After that, every time I tested my water it read too high. Even my tap water would be at almost 50ppm!!!!
 
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