Timing of API nitrate test

[esoxlucius]

This is an observation i've been thinking about for a while and the more i think about it the more niggling it becomes. On the instruction booklet that comes with the API kit it says "wait 5 minutes for the colour to develop". This is what I do. But we've all probably done it where we do the test, put it down, just forget about it momentarily, come back after 5-10 mins to a vial of test solution that looks to be totally off the scale. Sound familiar?

So it seems the reading after 5 minutes is the one we want to determine our nitrate level, no less than 5 minutes, no more than 5 minutes. Hmmm.

But to me it begs a question, mainly for our resident microbiologist duanes .

You have a liquid solution that tests for total aquarium nitrate. I'd have thought that no matter how long you leave the test for the end reading would be what it is and it would stay that way, not get progressively worse the "extra" time you leave the test vial sat there. To me, the REAL nitrate reading is the colour it ends up at because the test solution tests for total nitrate, right? Am I missing something? I'm not getting the logic.

I mean i'm not complaining, because we'd all be in a great big sack of s**t if we did indeed have to go off the proper end reading. Our lives would be constant water changes. Thoughts?

 

[duanes]

Certain chemical reagents react to time differently when combined, these times are determined by research, and that duration must be adhered to, or the tests are not considered valid.
In the lab we used simple kitchen timers (Ding!) to alert us as to when tests were ready.
If we forgot (maybe left the room, and didn't hear the bell), and reagents cooked beyond the allotted test maximum duration, we considered the test results skewed, and were required to run the test over.
In some cases of even a few moments overtime cooking, colors would continue to darken so the results could not be trusted.
Different companies use different combinations of reagents to get results, so one "size" does not fit all brands. (in this case one lapse of time).
Somewhere in the companies research, it was determined that accurate result occurred after only a certain reagent combinations was used, and only after a certain specified lapse of time.
And.... in most cases in the lab, we were required to run the sample 3 times, and use a DI blank, and a known Standard reagent to determine accuracy. In the lab we also rinsed test tubes at least 3 times with DI water after each set, to prevent carry over.
For example in the ammonia test below,
left my tank water, middle a 2ppm known standard, right straight DI water

Most aquarium tests have been dumbed down enough to allow for sloppy technique, and accuracy may not be as important, but if a certain time is stated in the instructions, it should considered important.
A company might make tests for a variety of uses (drinking water, waste water, food grade)(not just aquarium water) that mandate more government required accuracy, Those test may not be "uber" dumbed down.
These more accurate tests sold from companies like Fischer Scientific have costs most aquarists would baulk at,

In the ammonia test above, results were read by a photospectrometer that could tell the color difference even if other chemicals were entered the mix.
I did not use my eyes to get the results.

 

[esoxlucius]

Now I do understand the logic. Thanks duanes for explaining that perfectly