01 Jan 09
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.
01 Jan 09
2 edits
Originally posted by Andrew Hamiltonit depends on context ... ppm is often a little ambiguous ... it can even be in meters per meter!
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.
in the earth's crust I would expect mass would be meaningful in most discussions, (but volume could be more relevant in other discussions)
02 Jan 09
Originally posted by KazetNagorraDespite it being dimensionless, you still need to know if it's mass, or number of particles, etc.
Usually ppm is dimensionless, it often refers to a number density fraction. Compare to percent.
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.
02 Jan 09
Originally posted by AThousandYoungYes, it will have to be derived from the context.
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.
03 Jan 09
Originally posted by AThousandYoungLike stated earlier, ppm is a dilution.
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.
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.
03 Jan 09
Originally posted by mlpriorThat'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.
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.
Mass/mass.
03 Jan 09
Originally posted by mlpriorBut 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.
exactly....the units still cancel out so it is dimensionless.
03 Jan 09
1 edit
Originally posted by mlpriorthis approximation is accurate when the dilution is very weak, and the solution is in water.
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.
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.
03 Jan 09
Originally posted by KazetNagorraPeople and industry use mg/L all the time and it is perfectly acceptable for a ppm number.
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]
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.
03 Jan 09
Originally posted by AThousandYoungA ratio is dimensionless.
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.