A variety of concentration units are used in analytical chemistry, and the most common are provided in this table (Ref. 1). The reference below provides additional details.
Parts per Million | Percent | |
1 ppm | = | 0.0001 % |
10 ppm | = | 0.001 % |
100 ppm | = | 0.01 % |
1000 ppm | = | 0.1 % |
10 000 ppm | = | 1.0 % |
100 000 ppm | = | 10.0 % |
1 000 000 ppm | = | 100.0 % |
Parts per Billion | Percent | |
10 | = | 0.000 001 % |
100 | = | 0.000 01 % |
1000 | = | 0.0001 % |
10 000 | = | 0.001 % |
100 000 | = | 0.01 % |
1 000 000 | = | 0.1 % |
Parts per Trillion | Percent | |
100 | = | 1 × 10–8 % |
10 000 | = | 0.000 001 % |
1 000 000 | = | 0.0001 % |
100 000 000 | = | 0.01 % |
Because the mass of one liter of water is approximately one kg, mg/L units of dilute aqueous solution are nearly equal to ppm units. The precise equivalence is obtained by dividing by the density, ρ:
ppm = (mg/L)/ρ
where the solution density, ρ, is in g cm–3. Some sources will substitute specific gravity for density in the above equation. The specific gravity is the ratio of the solution density to that of the density of pure water at 4 °C. Because the density of pure water at 4 °C is very nearly 1 g cm–3, the specific gravity is numerically equal (within an uncertainty of 25 ppm) to the solution density when the latter is expressed in units of g cm–3.
The following table provides guidance in the use of base-10 concentration units (presented in the three preceding tables), since there are differences in usage worldwide.
Number | Number of Zeros | Name (Scientific Community) | Name (United Kingdom, France, Germany) |
1000. | 3 | thousand | thousand |
1 000 000. | 6 | million | million |
1 000 000 000. | 9 | billion | milliard, or thousand million |
1 000 000 000 000. | 12 | trillion | billion |
1 000 000 000 000 000. | 15 | quadrillion | thousand billion |
Molarity *, M: | (moles of solute)/(liters of solution) |
Molality, m: | (moles of solute)/(kilograms of solvent) |
Normality, N: | (equivalents* of solute)/(liters of solution) |
Formality, F: | (moles of solute)/(kilograms of solution) |
|
F = ppm/(1000 RMM)
where RMM is the relative molecular mass of the solute i.
m = [ppm/(1000 RMM)] [1/(1 – tds/1 000 000)]
where tds is the total dissolved solids in ppm in the solution.
M = [ppm/(1000 RMM)] ρ
where ρ is the solution density.