It should be fine and fairly pure, at least to 99.5% or so.

If you get 5-10-20ppm and the same reading for each with the test kit, somethings up.

Either the test kit is junk, or you make a mistake in making to calibration solution.
Many test kits are poor at lower ranges and better at 0-20-40-60-80-100ppm ranges. the Lamotte NO3 test kit is good however and I'd suggest using that.


Regards,
Tom Barr