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Science Forum

  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. Standard member flexmore
    Quack Quack Quack !
    01 Jan '09 10:55 / 2 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. Subscriber AThousandYoung
    Do ya think?
    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. Subscriber AThousandYoung
    Do ya think?
    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. Subscriber AThousandYoung
    Do ya think?
    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. Standard member flexmore
    Quack Quack Quack !
    03 Jan '09 09:36 / 1 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. Standard member 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.