Geoscience Reference
In-Depth Information
Note:
The material content in solid and gas phases is different from those in liquid phases. For
example, the material content in solid phases is often quantified by a ratio of masses and is
expressed as ppm or ppb. The material content in gas phases is often quantified by a ratio of
moles or volumes and is expressed as ppm or ppb. It is preferable to report gas-phase concen-
trations at standard temperature and pressure (STP; 0°C and 769 mmHg or 273 K and 1 atm).
■
EXAMPLE 3.1
Problem:
A certain chemical has a molecular weight of 80. Derive the conversion factors to quan-
tify the following:
1. 1 ppm (volume/volume) of the chemical in air in molar and mass concentration form.
2. 1 ppm (mass ratio) of the chemical in water in mass and molar concentration form.
3. 1 ppm (mass ratio) of the chemical in soil in mass ratio form.
Solution:
1.
Gas phase
—The volume ratio of 1 ppm can be converted to the mole or mass concentra-
tion form using the assumption of ideal gas, with a molar volume of 22.4 L/g mol at STP
conditions (273 K and 1.0 atm.).
3
1m chemical
1,000,000 mofair
1 ppm
v
=
3
3
1m chemical
1,000,000 mofair
mol
22.4
1000 L
m
≡×
−
44610
5
3
1pp
m
v
≡
.
mol/m
3
3
L
80 g
gmol
≡
−
5
3
3
3
≡
≡
4
.46
×
10
mol/m
0 0035
.
g/m
≡
3
.
5
mg/m
3 5
.
µg/L
The general relationship is 1 ppm = (MW/22.4) mg/m
3
.
2.
Water phase
—The mass ratio of 1 ppm can be converted to mole or mass concentration
form using the density of water, which is 1 g/cm
3
at 4°C and 1 atm:
1gchemical
1,000,000gofwater
1 ppm
=
1gchemical
1,000,000gofwater
(
)
(
)
≡
1g/cm
3
33
3
1 ppm
≡
1,000,000 cm /m
1
g/m
≡
1
g/L
≡ 0 0125
mol
3
mol/m
3
≡
1
g/m
.
80 g
3.
Soil phase
—The conversion is direct:
1gchemical
1,000,000gofsoil
1 ppm
=
=
11gchemical
1,000,000gofsoil
1000 g
kg
1000 mg
g
1 ppm
=
1mg/kg
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