Originally posted by AThousandYoungIt's whatever it needs to be in context. It can be a ratio of masses, ratios of volumes - whatever.
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.
Originally posted by Andrew HamiltonYep its a pain in the hole to work with too..... We used to assay for parts per billion gold and ppm silver/antimony/arsenic, those numbers were heaps of fun to try and work with when you have mass, volume, density etc in the equation too. You'd have to translate everything at least twice. And usually I'd end up spending nearly as long trying to work out what units my reserve estimate was in as I would doing the actual maths...
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.
Generally you have to take it in the context of the other data, eventually you get used to it and stop converting, instead just having an Idea of the how the difference between X ppm and Y ppm of whatever element affects the particular data your dealing with, it took me a ages though.
With regards to trace element concentrations as far as I remember from college taking it as mass usually makes the brain hurt a little less.
Originally posted by AThousandYoungHere's an example for gasses:
Can you give me an example of it being used as something other than mass/mass and the context that makes this clear?
"The concentration of NOx (oxides of nitrogen) in air samples which were collected from one automobile exhaust was determined by filtration of the particulates and colourimetric analysis of the air samples. The air samples had a mean of 27.8 NOx ppm with a standard deviation of 13.9 NOx ppm at start up and a mean of 5.4 NOx ppm with a standard deviation of 3.7 NOx ppm after idling the automobile engine for 15 minutes."
Originally posted by spruce112358I love it when you talk colorimetry....tell me more!
Here's an example for gasses:
"The concentration of NOx (oxides of nitrogen) in air samples which were collected from one automobile exhaust was determined by filtration of the particulates and colourimetric analysis of the air samples. The air samples had a mean of 27.8 NOx ppm with a standard deviation of 13.9 NOx ppm at start up and a mean of 5.4 ...[text shortened]... with a standard deviation of 3.7 NOx ppm after idling the automobile engine for 15 minutes."
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Originally posted by spruce112358Is that pressure/pressure?
Here's an example for gasses:
"The concentration of NOx (oxides of nitrogen) in air samples which were collected from one automobile exhaust was determined by filtration of the particulates and colourimetric analysis of the air samples. The air samples had a mean of 27.8 NOx ppm with a standard deviation of 13.9 NOx ppm at start up and a mean of 5.4 ...[text shortened]... with a standard deviation of 3.7 NOx ppm after idling the automobile engine for 15 minutes."
Originally posted by KazetNagorrappm is quite often used in the mass/mass sense to describe concentrations.
Normally concentrations are not measured in masses but in moles per volume. So it's actually (mole/volume)/(mole/volume).
Parts-per notation is often used in the measure of dilutions (concentrations) in chemistry; for instance, for measuring the relative abundance of dissolved minerals or pollutants in water. The expression “1 ppm” means a given property exists at a relative proportion of one part per million parts examined, as would occur if a water-borne pollutant was present at a concentration of one-millionth of a gram per gram of sample solution.
http://en.wikipedia.org/wiki/Parts_per_million
However the same article goes on to say
Similarly, parts-per notation is used also in physics and engineering to express the value of various proportional phenomena. For instance, a special metal alloy might expand 1.2 micrometers per meter of length for every degree Celsius and this would be expressed as “α = 1.2 ppm/°C.”
Originally posted by KazetNagorraYou claimed concentrations were usually in molarity, and therefore if you use ppm to refer to concentrations it's implied that it's not mass/mass. I disagree with that.
Sure, I wouldn't claim it's never used as a mass/mass ratio.
Normally, when parts-per notation is used with respect to concentrations, it's meant in the mass/mass sense.