# ppm is by mass or volume?

Andrew Hamilton
Science 01 Jan '09 10:48
1. 01 Jan '09 10:48
I keep looking up the abundance of various trace chemical elements in the Earth’s crust and I read it expressed as ppm (parts per million) but cannot remember if “ppm” is by mass or by volume -so which is it? -there is a big difference!
I tried to look this up but couldn’t find a link that would tell me.
2. flexmore
Quack Quack Quack !
01 Jan '09 10:552 edits
Originally posted by Andrew Hamilton
I keep looking up the abundance of various trace chemical elements in the Earth’s crust and I read it expressed as ppm (parts per million) but cannot remember if “ppm” is by mass or by volume -so which is it? -there is a big difference!
I tried to look this up but couldn’t find a link that would tell me.
it depends on context ... ppm is often a little ambiguous ... it can even be in meters per meter!

in the earth's crust I would expect mass would be meaningful in most discussions, (but volume could be more relevant in other discussions)
3. 01 Jan '09 12:02
Usually ppm is dimensionless, it often refers to a number density fraction. Compare to percent.
4. AThousandYoung
TCB SnichStich DTLA
02 Jan '09 16:20
Originally posted by KazetNagorra
Usually ppm is dimensionless, it often refers to a number density fraction. Compare to percent.
Despite it being dimensionless, you still need to know if it's mass, or number of particles, etc.

For example, suppose you have a solution of glucose in water. If it's 1 ppm, that could mean one gram of glucose per million grams of water, or it could mean one molecule of glucose per million molecules of water. These are different concentrations, because glucose molecules are not the same mass as water molecules.

It's a ratio of masses I believe.
5. 02 Jan '09 17:07
Originally posted by AThousandYoung
Despite it being dimensionless, you still need to know if it's mass, or number of particles, etc.

For example, suppose you have a solution of glucose in water. If it's 1 ppm, that could mean one gram of glucose per million grams of water, or it could mean one molecule of glucose per million molecules of water. These are different concentrations, ...[text shortened]... cose molecules are not the same mass as water molecules.

It's a ratio of masses I believe.
Yes, it will have to be derived from the context.
6. AThousandYoung
TCB SnichStich DTLA
02 Jan '09 18:30
Originally posted by KazetNagorra
Yes, it will have to be derived from the context.
This is why I don't like the "parts per" method of describing concentrations.
7. 02 Jan '09 21:38
Would it not be expressed as moles? That would seem to make the most sense.
8. 02 Jan '09 22:55
Originally posted by jimslyp69
Would it not be expressed as moles? That would seem to make the most sense.
This is the same as a number density fraction.
9. 03 Jan '09 04:12
Originally posted by AThousandYoung
Despite it being dimensionless, you still need to know if it's mass, or number of particles, etc.

For example, suppose you have a solution of glucose in water. If it's 1 ppm, that could mean one gram of glucose per million grams of water, or it could mean one molecule of glucose per million molecules of water. These are different concentrations, ...[text shortened]... cose molecules are not the same mass as water molecules.

It's a ratio of masses I believe.
Like stated earlier, ppm is a dilution.

It is generally mg/L when disolving a solid into solution.

A solution that is 10 ppm for example would be 10 mg/L or 10:1000000 dilution.
10. AThousandYoung
TCB SnichStich DTLA
03 Jan '09 04:23
Originally posted by mlprior
Like stated earlier, ppm is a dilution.

It is generally mg/L when disolving a solid into solution.

A solution that is 10 ppm for example would be 10 mg/L or 10:1000000 dilution.
That's because a liter is 1000 mL, which (if water) is 1000 g. A mg is 1/1000 g, so mg/L is the same as grams solute/1 billion grams solution.

Mass/mass.
11. 03 Jan '09 04:31
Originally posted by AThousandYoung
That's because a liter is 1000 mL, which (if water) is 1000 g. A mg is 1/1000 g, so mg/L is the same as grams solute/1 billion grams solution.

Mass/mass.
exactly....the units still cancel out so it is dimensionless.
12. 03 Jan '09 09:27
Originally posted by mlprior
exactly....the units still cancel out so it is dimensionless.
But gram and liter don't cancel out, so you definately cannot use ppm in the way you mentioned. Also, 1 L is not equal to 1 kg and is only equivalent under certain special conditions (a certain fixed temperature and pressure) for certain fluids.
13. flexmore
Quack Quack Quack !
03 Jan '09 09:361 edit
Originally posted by mlprior
Like stated earlier, ppm is a dilution.

It is generally mg/L when disolving a solid into solution.

A solution that is 10 ppm for example would be 10 mg/L or 10:1000000 dilution.
this approximation is accurate when the dilution is very weak, and the solution is in water.

the more general method of "grams per million(or thousand) grams" is meaningful over this range, and also over a much wider range of diverse scenarios.
14. 03 Jan '09 21:43
Originally posted by KazetNagorra
But gram and liter don't cancel out, so you definately cannot use ppm in the way you mentioned. Also, 1 L is [b]not equal to 1 kg and is only equivalent under certain special conditions (a certain fixed temperature and pressure) for certain fluids.[/b]
People and industry use mg/L all the time and it is perfectly acceptable for a ppm number.

It is all how you are referencing your sample and set up your GC or HPLC or whatever you are using to measure a substance. After all, the machine is not going to directly measure ppm, it is going to measure a peak area unit and reference that to your internal standard and/or calibration curve.
15. sasquatch672
Don't Like It Leave
03 Jan '09 21:48
Originally posted by AThousandYoung
Despite it being dimensionless, you still need to know if it's mass, or number of particles, etc.

For example, suppose you have a solution of glucose in water. If it's 1 ppm, that could mean one gram of glucose per million grams of water, or it could mean one molecule of glucose per million molecules of water. These are different concentrations, ...[text shortened]... cose molecules are not the same mass as water molecules.

It's a ratio of masses I believe.
A ratio is dimensionless.