µmols to ppm help...

[mcarroll]

I'm reading too many things giving me nitrate and phosphate concentrations in µmols but I speak ppms.

I'm sure since I learned to read ppm's in the hobby that it's wrong or at least antiquated.

Can someone tell me how to convert ppm's of NO3 and PO4 to µmols?

I have only gathered enough to know that it's not a straightforward conversion factor kind of thing.

Thanks for any answers or pointers!!


P.S. If you can also explain, if both units are usable for expressing concentration, why we measure in ppm instead of µmols, it would hopefully be interesting.

 

[Randy Holmes-Farley]

We should not use ppm since it is inherently flawed. Scientists use molar units.

It is straightforward, you just have to know how to do it.

mw nitrate (molecular weight) = 62 g/mole = 62 mg/mmol
mw phosphate = 95 g/mole = 95 mg/mmol

The short and rough estimate (which ignores the density of the seawater and equates ppm and mg/L) is to divide the weight based concentration (ppm) by the grams per mole.

1 ppm --> 1 mg/L

for nitrate:

1 ppm --> 1 mg/L / 62 mg/mmol = 0.016 mmole/L = 16 umole/L

 

[mcarroll]

Thanks!!!

molecular weight corresponds to atomic weight?

"Standard Atomic Weights:"

• Nitrogen – 14
• Phosphorus – 31
• Oxygen – 16

Nitrate = NO3 = (14 + (16 • 3)) = 62 mg/milli-mol
PO4 = (31 + (16 • 4)) = 95 mg/milli-mol

Got it so far, I think.

I tend to see nutrient nitrate concentration in the literature raised to 6.0 µmols/L fairly often.

To go backwards to ppm (for my test kits), I would go back to milli-mol, so:

6.0 µmols/L = 0.006 milli-mol/L​

And then convert to mg/L:

.006 milli-mol/L • 62 mg/milli-mol = .372 ppm​

Did I get that right?

 

[mcarroll]

6.0 µmol/L of PO4 would then be 0.57 ppm?

 

[Randy Holmes-Farley]

Molecular weight comes by adding up the atomic weights of the atoms in the molecule.

Yes, 6.0 µmol/L of PO4 ~ 0.57 ppm?

 

[mcarroll]

Thanks! This is going to be a huge help!!

 

[Randy Holmes-Farley]

You're welcome.

Happy Reefing.

 

[mcarroll]

We should not use ppm since it is inherently flawed. Scientists use molar units.

If I'm reading a modern scientific article about coral that uses mg/l or ppm rather than molar units, can I infer anything about the quality of the work, or in your estimation is that putting too much on it?

 

[Randy Holmes-Farley]

If I'm reading a modern scientific article about coral that uses mg/l or ppm rather than molar units, can I infer anything about the quality of the work, or in your estimation is that putting too much on it?

I'm not sure, but probably not. It might be more of an aquaculture "cultural thing".

 

[mcarroll]

Here's exactly how concentration of nitrogen is expessed in one article:
"above 5 μmol l−1"

Is that ≆ to "above 0.31 ppm" nitrate?

 

[Randy Holmes-Farley]

yes.

 

[mcarroll]

Sweet!

 

[mcarroll]

Oh...forgot my meta questions:

What exactly do the units mean as expressed?

I would write what I think they mean as: µmol/L

They use all lower case, no slash and what's the -1 for? It seems like I see almost all chemistry units expressed with a -1 at the end.

 

[nervousmonkey]

The -1 is used to designate that the unit with the -1 after it is the denominator. So 5 μmol l−1 is also equal to 5 μmol/l.

 

[Randy Holmes-Farley]

Yes, it means to the minus one power, or 1/unit.

L^(-1) = 1/L