Summary: convert your concentration to a mole, mass, or volume fraction and multiply by 1 million.
Parts per million must be measured in consistent units:
mass per 1 million mass units
volume per 1 million volume units
moles per 1 million mole units
For example, if you add 1 milligram of substance to 1 kilogram of solution, the concentration is:
1 mg/1 kg = 0.001 g / 1000g = 1e-6
Multiply the resulting mass ratio by 1 million and you convert this to ppm
1e-6*1million = 1 ppm
I don't think it is correct to directly use molarity or molality times 1 million, since both of these are ammount/volume: the units are not consistent. You have to convert the "volume" in the concentration to a "mass" or "molar" amount consistent with the concentration measurement.
If you have a concentration in terms of a mole fraction, it is just the mole fraction times 1 million. A mole fraction of unity (one) is the same as 1 part per part (pure!).
If you have a concentration in terms of a volume fraction or mass fraction, you just multiply by a million. It is important to note that whether you do parts by mole, volume, or mass you will get different numbers for a given system. It is important to be clear which one you are talking about. You can read more on wikipedia, look up ppm.
· 1 decade ago