# API Nitrate Test Calibration Results



## trag (Jan 9, 2008)

I calibrated my API brand Nitrate test this weekend. The result was that the test seems to report about half of the actual nitrates present and above 80ppm concentration, all concentrations yield the same color (40ppm) in the indicator solution.

I used distilled water. I also tested my distilled water for nitrates with a result of 0.

I carefully rinsed my equipment between each test.

Yes, I shook the Solution 2 bottle before each test.

I used a timer to measure the 1 minute shaking time and the 5 minute sitting time.

I conducted two tests per test, i.e. ran the same solution in two test tubes for each solution.

One possibility is that I somehow halved my solution through error or poor math, but I can't figure out how. I wrote down each step before performing it, so I could double check my math and so I wouldn't forget what I did, and I labeled each bottle of solution.

Another possibility is that my KNO3 powder is only 50% KNO3. This seems unlikely but I suppose it is possible. I bought it from an Ebay seller who had plenty of feedback praising its purity and the pyrotechnics guys (bought it out of the model rocketry category) would notice if KNO3 was less than 90% pure.

I started with Rex Grigg's instructions for calibrating and modified them.

I measured out 1.63 g of KNO3 as well as I could. My scale does tenths of a g. So I carefully added KNO3 until it just reached 1.6g. Then I added KNO3 is a separate pile, until the reading reached 1.7g. Then I removed a little more than half of the second pile.

I added this to 500ml of water (not one liter) for a 2000ppm solution.

I mixed 50 ml of the 2000ppm solution with 450 ml of distilled water for a 200ppm solution.

I mixed 1 ml of 200ppm solution with 19 ml of distilled water for a 10ppm solution.

I mixed 2 ml of 200ppm solution with 18 ml of distilled water for a 20ppm solution.

I mixed 4 ml of 200ppm solution with 16 ml of distilled water for a 40ppm solution.

I mixed 8 ml of 200ppm solution with 16 ml of distilled water for a 80ppm solution.

This should hit each of the major colors on the API indicator strip.

My 10 ppm solution indicated between 5 and 10 ppm.

My 20 ppm solution indicated 10 ppm.

My 40 ppm solution indicated 20 ppm.

My 80 ppm solution indicated 40 ppm.

My 200 ppm solution indicated 40 ppm.

My 2000 ppm solution indicated 40 ppm.

Anyone else calibrated their API nitrate test? Any suggestions on how to verify my solutions? I still have about 400 ml of the 200 and 2000 ppm solutions.


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## wet (Nov 24, 2008)

It looks good. How confident are you in the 1mL, 19mL, and so on measurements? Pharmacies and hobby supplies have needle-less syringes (cheap!), 'case you're not aware.

It's been a while since I calibrated or used kits, but the AP test is tough to read. I think you should stop caring about the unreadable values, and I recall preferring to always read at the lower values. So, say the test reads 40ppm: don't translate it yet. Take one part sample with three parts distilled water, and measure again. Likewise, verify using the same logic with your other reference samples above. Then do it all over again and see if the numbers and readings still match up. If so, a third time doesn't hurt, and you can have some confidence in the numbers.

However, I'd suggest never using a kit more than as a ball park. Your calibrating the kit as you have makes it a smaller ball park, but it's still the trends and direction of the numbers we care about more than exact numbers. (And calibrated kits are great, inexpensive tools that can do that job wonderfully.)


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## Diana K (Dec 20, 2007)

If the test shows any of the unreadable high range results do not bother re-testing, do a water change. After a 50% water change, then re-test. 
I also cannot tell the difference between hot pink, deep pink, rich pink, magenta and whatever other names those are. "If it is too dark to tell, do a water change."

Interesting that you got just half the correct reading. 
That it follows all through the results does show that you have done all the mixing and diluting correctly. After the first one. 
I am not quite following how you tried to measure the KNO3. 
1.631 grams KNO3 mixed with 1 liter water = 1000.29 ppm. (Barr report)
If you mixed 1.631 grams with .5 liter, then you would have 2000 ppm. 
I think I would have measured out 1.6 grams, then measured out 0.1 gram, then aimed for dividing that 0.1 into three piles, and just using one of them. That would not be far off .031 grams.


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## trag (Jan 9, 2008)

wet said:


> It looks good. How confident are you in the 1mL, 19mL, and so on measurements? Pharmacies and hobby supplies have needle-less syringes (cheap!), 'case you're not aware.


I used what amounts to a plastic graduated cyclinder. It's meant for dosing medications. We have a bunch of them because we have an eight-year-old. I could see the 1ml being substantially less accurate than the other measurements, because a proportionately larger amount of fluid is likely to stick and remain in the cyclinder because of surface tension.



> So, say the test reads 40ppm: don't translate it yet. Take one part sample with three parts distilled water, and measure again. Likewise, verify using the same logic with your other reference samples above. Then do it all over again and see if the numbers and readings still match up. If so, a third time doesn't hurt, and you can have some confidence in the numbers.


I will try that with my remaining solutions when I have time for more calibrations. Afterwards, I may also try remixing my main solution to be sure I didn't goof somehow.


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## trag (Jan 9, 2008)

Diana K said:


> If the test shows any of the unreadable high range results do not bother re-testing, do a water change. After a 50% water change, then re-test.
> I also cannot tell the difference between hot pink, deep pink, rich pink, magenta and whatever other names those are. "If it is too dark to tell, do a water change."


Sound advice. 



> Interesting that you got just half the correct reading.


Yes, that's what makes me suspect some mix up or the purity of my powder. I'm tempted to order a pound of KNO3 from Aquarium Fertilizers just to compare the results.



> That it follows all through the results does show that you have done all the mixing and diluting correctly. After the first one.


Good point.



> I am not quite following how you tried to measure the KNO3.
> 1.631 grams KNO3 mixed with 1 liter water = 1000.29 ppm. (Barr report)
> If you mixed 1.631 grams with .5 liter, then you would have 2000 ppm.
> I think I would have measured out 1.6 grams, then measured out 0.1 gram, then aimed for dividing that 0.1 into three piles, and just using one of them. That would not be far off .031 grams.


Essentially the same way you describe. I just left the 1.6g on the scale at the same time. I had two piles on the scale. One was 1.6g. The other was the additional material to make 1.7 grams, or .1g in the second pile. Then I divided the second pile as best I could. I may not have hit 1.63g, but I'm pretty sure it's somewhere between 1.6 and 1.7g, unless the scale reads 1.6 from 1.55 to 1.65. If that's the case then I probably hit something closer to 1.58g.

I guess that's where an analytical balance would be nicer.


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## ray-the-pilot (May 14, 2008)

trag said:


> Sound advice.
> 
> I guess that's where an analytical balance would be nicer.


I have an analytical balance and used the API Test to measure NO3 levels to 1 ppm. Unfortunately very few people have the required expertise to achieve that level of confidence.

I would estimate, from my experience, that using the box directions and getting to +/- 2x is about right and is not too bad.

There are a number of things that you can do to improve your estimate but dividing a pile of KNO3 into three parts is kind of amusing!


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