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I am playing around with some test kits and the API's Nitrate test kit is bothering me greatly: I cannot discern between the different colors. So I want to make reference solutions from which I can later use to compare to the actual test sample: liquid-to-liquid instead of liquid-to-paper.

My concern is the accuracy of the reference samples. Will they hold their colors over time or will the reactants/indicator continue to chemically react/degrade over time, thus causing the colors to gradually get darker/lighter as time passes.

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In other words, the API's Nitrate Test Kit calls for a 1-minute waiting period after adding solution #2 before you read the resulting color change (or lack thereof). Will waiting 2 minutes, 5 minutes, 7 hours, 1 month, etc. cause the solution's color to change more and more and more...ad nauseam?
 

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When I made my reference solutions, the color faded to an off yellow over several weeks. I would not keep the colored solutions for reference. I would make standard reference solutions and then make fresh tests each time until I got a good idea on the levels, without having to use controls.

Ben
 

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It should also be noted that this is a common problem for many people. I have never reached a point where the API was of value to me. With calibrated test solutions, the readings are only useful when dealing with less than 25 PPM of nitrate. Any level beyond that is just not any help. The best read I get is by cutting the sample 50-50 with distilled water and then it is somewhat better to read but also throws in a certain amount of question as to the accurate measurements when doing the 50-50.
I now just go with using Jungle brand test strips as I have more confidence in what I see. Liquid tests can be more accurate but only if you can read the result.
When dealing with the trouble of diluting the sample and then doing the color check, I found it much simpler to just do a water change and assume the nitrate would be lower.
 
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