# Water Chemistry Problem

The question:

Nitrate concentrations exceeding 44.3 mg NO3-/L are a concern in drinking water due to the infant disease, methemoglobinemia.

Nitrate concentrations near three rural wells were reported as 0.01 mg NO3- N/L, 1.3 mg NO3- N/L, and 20.0mg NO3- N/L. Do any of these three wells exceed the 44.3 ppm level?

The problem:

I have no problem with conversions, it's just the units in this case. I've asked peers about the NO3- N/L unit and they said the N stands for nitrogen. If this is so, I don't know what the implications are. Whats the difference between the NO3-/L and NO3- N/L unit?

## Answers and Replies

Borek
Mentor
0.01 mg NO3- N/L

Doesn't make sense to me.

I can easily imagine NO3- reported as 0.01 mg N/L, but the way you have it written it is - to say the least - strange.

alxm
I have no problem with conversions, it's just the units in this case. I've asked peers about the NO3- N/L unit and they said the N stands for nitrogen. If this is so, I don't know what the implications are. Whats the difference between the NO3-/L and NO3- N/L unit?

Hmm, I've https://www.physicsforums.com/showpost.php?p=2162757&postcount=2"..

But to elaborate a bit more; you have the whole nitrogen cycle, so nitrogen in water (with living stuff in it) is being converted between nitrate, nitrite, ammonia, etc.
Therefore they distinguish between "N/L" nitrogen per liter (the mass of N from all compounds) "NO3- N/L", nitrate nitrogen per liter (just the nitrate nitrogen) and nitrate/L (the mass of nitrate, including oxygen).

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But for every mole of NO3-, there is 1 mole of N... So is there no conversions to be made here? I can just simply look at the values and if they are less than 44.3, they don't exceed the limit?

Borek
Mentor
So is there no conversions to be made here?

ppm is usually weight/weight, so you need some conversion.

But a mg/L is a ppm, at least concerning aqueous solutions.

there is 10^6 mg of water in a litre.

I think I'm still missing something though. It can't be as easy as just looking at the given values...

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alxm
But for every mole of NO3-, there is 1 mole of N... So is there no conversions to be made here?

Your units are in mg/L, not mol/L. One mole of N doesn't weigh the same as 1 mole of NO3-

Ah that is a good point, it's starting to become a little clearer.

So:

(0.01 mg NO3- N/L) x (1 mol N / 14000 mg N) x (1 mol NO3- / 1 mol N) x (62000 mg NO3- / 1 mol NO3-)

= 0.04 mg NO3-/L

Am I understanding this concept correctly?