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lalitha's picture

How is 1mg / ml = 1000ppm ??? How is the calculation of ppm arrived at in weight/ volume?

Dr. Analytical
Dr. Analytical's picture


A good question.  Many people are confused about this topic.

First, we must discuss the easier definition of  one part-per-million ("ppm").

If we have one ug (microgram) of a substance in a total of one gram (that is one million micrograms), then we have a 1 ug/g mixture (one part substance in one million parts sample).  Other equilvalent units are:

These are on a "weight-to-weight" basis.  This is often labeled w/w.

The same is true for dissolving a liquid in a liquid. A 1 uL/L solution is 1 ppm.  (nL/mL is equivalent).  This is on a volume-to-volume (v/v) basis.

Now, when we dissolve a solid in a liquid, we have a 1 ug/mL solution.  If the liquid is water, which has a density of 1 g/mL, then the solution is 1 ug/g.  However, if the liquid is not water, and has a density different than one, then the solution is not a true 1 ppm solution.

This shows the problem with the "ppm" type of label.  For weight-to-volume solutions, the ppm label is only true if the solvent is water.  I do not recommend that people use "ppm" (or ppb).  Rather, they should always use the actual units, so that there is no confusion: ug/mL, mg/kg, etc.

For your original question, 1 mg/mL = 1000 ug/mL = 1000 ppm ONLY if the solvent is water.

MicroLady's picture
Geesh...  Finally the answer

Geesh...  Finally the answer I've been searching for!  Thank you Dr. Analytical!  I have been needing to make a 1ppm solution and felt like it should be easy enough (was too embarrassed to ask any of my colleagues) given the unit to pretty much detail what it required: 'one part per million parts.'  Thanks again for hitting the nail on the head!

Stepan's picture
 In Air chemistry 1 ppm is

 In Air chemistry 1 ppm is used as mL of gas or vapour (let say CO2) in 1 m3 of air (basically V/V*1e6)