Parts per million, ppm is just another unit of concentration (like mol dm–3, g dm–3 etc) i.e. 1 part solute in 1 million parts solvent.
For solutions, it simply means the grams of solute per million grams of solution.
i.e. 1 ppm of NaCl solution means 1 g of NaCl is present in 1 000 000 g of solution
For gaseous mixtures, it simply means the volume, cm3 (or dm3) of solute per million cm3 (or dm3) of mixture.
i.e. 1 ppm of O2 in air means 1 cm3 of O2 is present in 1 000 000 cm3 of air
It is generally used for extremely dilute solutions/ mixtures.
Eqn to obtain concentration in terms of ppm:
|
conc (ppm) |
= |
mass of solute (g/ kg) |
x 106 |
|
mass of solution (g/ kg) |
|
conc (ppm) |
= |
volume of solute (cm3 / dm3) |
x 106 |
|
volume of mixture (cm3 / dm3) |
|
Example 1: Determine the concentration of sucrose, in ppm, in a solution where 0.05 g of sucrose is dissolved in 1 dm3 of solution.
Since solution is very dilute, it is almost entirely water → density of solution ~ density of water (1 g cm–3) Mass of 1 dm3 sucrose solution = 1000 g Conc. of sucrose = 0.05/1000 x 1 000 000 = 50 ppm
Essentially: 1 ppm = 1 mg dm–3 and 1 ppb = 1 μg dm–3
Alternate Method Conc. of sucrose = 0.05 g dm–3 = 50 mg dm–3 = 50 ppm |
Practice:
It was determined that 1 dm3 of water contains 0.005 g of dissolved oxygen. Express the concentration of dissolved O2 in ppm. [5 ppm]
