It has to do with some of the interferences and methods used that create and break down
NITRITE (NO2), not Nitrate (NO3)

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If You See Me Running You Better Catch-Up
"Stu" wrote in message
om...
: My nitrate test kit says to wait five minutes before comparing the
: colour against the chart. I assume usually this would be for the
: colour to develop and if anything, show a higher reading (and
: stabilise). However I've noticed that after the five minutes I get a
: lower reading.
: Eg last night I did a test and the reading immediately after adding
: reagent 3 was around 10ppm. Five minutes later it read closer to 5ppm.
: Last week when I did a test the immediate reading was 20ppm, after
: five minutes 10ppm, and after a couple of hours (I forgot to rinse the
: vial) read 0-5ppm.
:
: I'm assuming I should go with the first (highest) reading, but has
: anyone else experienced this?
:
: Cheers,
: Stu