Maybe somebody already answered this, but I think all common
reagent-based test kits are reacting to nitrate, NO3. Now, some kits
are calibrated to report the amount of actual nitrate in ppm, while some
report nitrate nitrogen - that is, an adjustment is made for the weight
of the nitrate ion vs. just the nitrogen component. Without dragging
out a periodic table, I think the adjustment factor is on the order of
4.4 or something like that.
As to phosphate vs. orthophosphate, I haven't a clue. I slept through
most of my organic chemistry classes |-/
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