Nitrate test - wait time?

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

"Stu" wrote in message om...
I'm assuming I should go with the first (highest) reading,
but has anyone else experienced this?

No, you always go with the manufacturer instructions.
If not sure - try to bring a water sample to LFS and
ask them to run one test for you.

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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"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