API Test Kit Question

[johnt2k14]

Hi gang,

Has anyone experienced "odd" test results from the famous API Test Kit? It looks like I am getting false positives for nitrAtes and I don't know why.

(Note: I did/do vigorously shake the test bottle and tube for the test)

History:
Started a freshwater tank about 6 months ago. To establish a base line at the start I tested my tap water with the API Test Kit and it showed little or no nitrAtes. After 2 months the tank cycled and all was well.

Another month or two and things were looking good. I continued testing with the API Test Kit. NitrAtes would rise normally and a pwc would reduce them as I expected.

Recently, about a month ago, I saw a huge spike in the nitrAtes; somewhere between 40 and 80 ppm. I did a 50% pwc and it got to 40 ppm. The next day it was still around 40ppm as expected and I did another 50% pwc but this time saw no change in the nitrAtes!?!?!?

I tested my tap water with the same API Test Kit and it was reading 40ppm right out of the tap! Bought a new API Test Kit and retested. The tap water was still 40ppm!

Tried testing against some bottled spring water from the store and zero nitrAtes were seen. So I didn't think it was the test kits. Had to be the tap water, right?

Sent samples of the water to Karr Labs and their report came back with nitraAtes at only 6ppm. The same day I took the samples for Karr I tested with the API Test kit and it was still 40ppm.

So I'm left scratching my head. Is the API Kit testing a different nitrAte compound than Karr Labs? Or did I just get a bad batch of API testing chemicals?

So, has anyone else seen "odd" results from their API Test Kit?

-John

 

[Bbarb27]

I have had similar experiences

and I attribute it to not shaking the second bottle of reagent well enough when initially using the test. If I haven't shaken it well enough during the initial uses, my readings are always too high when I am nearing the last 1/3 of the available tests in the bottle. Opening a new test kit and re-testing (after vigorously shaking the newly opened bottles) always results in a lower nitrate reading for me. Vigorously shaking the "old" (semi depleted) bottles does no good, so I'm assuming there is a higher than normal concentration of chemicals in the bottle due to poor mixing during earlier uses.

Years ago I used a nitrate test that required adding a powdered reagent (I can't recall what) after mixing the two liquids, and those tests seemed far more accurate than the newer API tests. I like the API tests, but the nitrate test seems prone to false readings.

 

[johnt2k14]

Thanks for the response.

I have shaken the heck out of that second bottle every time. I just tried my tap water again, shaking the second bottle for 60 seconds prior to counting out the 10 drops and then shaking the test tube for 60 seconds. The test is still turning quite red.

 

[Delapool]

Did it say what test was used at the lab? Also what the accuracy and detection limit was on it?

Every so often I'll get an odd result on a test but the retest is usually fine. It's strange.

 

[johnt2k14]

I am not sure how the lab tested for nitrates. Why would you ask that Delapool? Are all nitrates not created the same?

-John

 

[Delapool]

At work we do a lot of assay tests on rocks and depending on the test method you can get a different result as they may not measure the same thing.

As well the quality of the test is often lower at low values.

I don't know if nitrates vary but I'd be certain that test methods and accuracy can, so was just curious if they listed it.

 

[PerfectDepth]

Sent samples of the water to Karr Labs and their report came back with nitraAtes at only 6ppm. The same day I took the samples for Karr I tested with the API Test kit and it was still 40ppm.

I would trust the lab's results. This is way more common with these NO3 kits than many people realize. I don't think I've ever had an API nitrate kit that didn't read higher-than-actual levels. The one I'm currently using reads 15-20 ppm at only 5 ppm actual concentration. With distilled water and some KNO3, you can easily make reference solutions to calibrate your kit if you really want more accuracy.

I belong to a local aquarium group on facebook, and people are always insisting that our tap water has sky-high nitrate levels, around 40 ppm or higher based on their test kits, but it's really only 8 to 9 ppm at its highest. The EPA regulations only allow a maximum of 10 ppm, so 40 ppm would be a major issue for the local water company.

 

[johnt2k14]

Delapool - sounds like an interesting line of work. What kind of tests would you run for the rocks in my head

And yeah, I checked the lab's website and they list a number of techniques used for their tests but they don't map the technique to the test.

PerfectDepth - The water company assured me they test the water monthly and nitrates are no where near 40ppm. Like you said, EPA regulations would have stirred up something so I believe them and I trust the 3rd party lab work. The tap water is very likely fine.

Creating a reference solution sounds like a good way to go, if I want to stick with API. I would rather try a different manufacturer and see what happens. Anyone got a suggestion for another kit to try. I was going to try Seachem. Any pros/cons?

-John

 

[Delapool]

Delapool - sounds like an interesting line of work. What kind of tests would you run for the rocks in my head

And yeah, I checked the lab's website and they list a number of techniques used for their tests but they don't map the technique to the test.

PerfectDepth - The water company assured me they test the water monthly and nitrates are no where near 40ppm. Like you said, EPA regulations would have stirred up something so I believe them and I trust the 3rd party lab work. The tap water is very likely fine.

Creating a reference solution sounds like a good way to go, if I want to stick with API. I would rather try a different manufacturer and see what happens. Anyone got a suggestion for another kit to try. I was going to try Seachem. Any pros/cons?

-John

Lol - no test there!

No worries, I was curious on what they used. Every so often I'm tempted as I'd like to know how accurate my potassium test kit is (suspect it over-reads).

If you did use them again I'd see if they have a lab chemist who may be able to explain the differences between your test and theirs. It's interesting you got 0 in bottled spring water.