Chemistry, Third edition

may be used instead of the mass of the solution in grams. For dilute aqueous solu-

tions, therefore, we can use:

1 ppm 1 mg of substance dissolved in 1 dm^3 of water

and

1 ppb  1    g of substance dissolved in 1 dm^3 of water

These units are more convenient for low concentrations because they avoid using

the very low numbers that would be involved if the concentrations were to be

expressed in mol dm^3 ; water pollutants are often given in ppm units. Also, we may

use these units to describe trace concentrations of metals when we are not sure of the

exact nature of the species involved. For example, drinking water may be found to

have a total aluminium content at a concentration of 3 ppm. The nature and concen-

tration of the different aluminium compounds present in a sample of drinking water

is dependent on pH and can be a very complex problem to solve; the ions Al^3 ,

AlOH^2 and Al(OH) 2 are but a few species that may be found. Without exact

knowledge of the species present, calculations of molar concentration cannot be

made, but it is always possible to express concentration in terms of the total mass of

metal present.

OTHER UNITS OF CONCENTRATION 147

BOX 9.3

How little is ‘little’?

One lump of sugar (6g) dissolved in:

Tea pot 1% 10 g/ kg 10 mg/ g

1 per cent

0.6 dm^3

Bucket 0.1% 1 mg/ g

per thousand 1g/kg 1 milligram

0.001 g

6 dm^3

Tank lorry 1ppm 1    g/ g

1 per million 1 mg/ kg 1 microgram

0.000 001 g

6000 dm^3

Super-tanker 1ppb 1 ng/ g

1 per billion 1     g/ kg 1 nanogram

0.000 000 001 g

6 million dm^3

Reservoir 1ppt 1 pg/ g

1 per trillion 1 ng/ kg 1 picogram

0.000 000 000 001 g

6 billion dm^3

Bay 1ppq 1 fg/ g

1 per quadrillion 1 pg/ kg 1 femtogram

0.000 000 000 000 001g

6 trillion dm^3

Reproduced courtesy of LGC, http://www.lgc.co.uk