Environmental Engineering Reference
In-Depth Information
Practical tips
Report concentrations with the right unit and right significant figures. Use
common sense in reporting when to choose mg/L,
g/kg,
ng/kg; or even % if the concentration is high. Avoid very large or very small
numbers.
It is a good practice to use mass/volume (mg/L), mass/mass (mg/kg), and
mass/volume (mg/m 3 ) for contaminants in water, soil, and air, respectively.
Avoid using ppm, ppb, and ppt because they can be ambiguous. If chemicals
in air are concerned, use ppm v , ppb v , and ppt v to denote that the chemical
concentration in air is based on the volume ratio.
There are many other contaminant specific units, such as mg/m 3 for atmo-
spheric particulate matter (PM 2.5 or PM 10 ) and lead (here ppm is an invalid
unit because PM and lead cannot be expressed in volume); mg/L as CaCO 3
for water hardness, acidity, and alkalinity; nephelometric turbidity units
(NTU) for turbidity;
m
g/L, ng/L; or mg/kg,
m
S/m for conductivity; and pCi/L for radionuclides.
The salinity unit is parts per thousands (ppt or %), which should not be
confused with parts per trillion (ppt).
m
EXAMPLE 2.1. Maximum contaminant level (MCL) according to the U.S. EPA for 2,3,
7,8-TCDD (dioxin) in drinking water is 0.00000003 mg/L. Convert this concentration to ppt
and molarity (M). What is the equivalent number of dioxin molecules per liter of water? The
molecular weight of dioxin is 322 g/mol.
SOLUTION:
10 6 ppt
1 ppm ¼ 0:03 ppt
1 ppm
1mg=L
0:00000003mg=L ¼ 0:00000003mg=L
0:00000003mg=L ¼ 0:00000003 mg
1g
10 3 mg
1mol
322 g ¼ 9:3210 14 mol
L
L
¼ 9:3210 14 M
02210 23 mol 1 , we can obtain the number of
Using the Avogadro's constant of 6
:
dioxin molecules per liter of water:
3210 14 mol
02210 23 1
6110 10 dioxinmolecule
L
9
:
L 6
:
mol ¼ 5
:
Dioxin is probably the most toxic contaminant on earth. The above calculations
indicate that the unit of molarity (M) is a very large unit for such a low
concentration. Even ppm (mg/L) is still not an appropriate unit. The above
calculation also reveals that even though the MCL is so low at the ppt level, the
number of molecules is significant, that is, 56.1 billion toxic dioxin molecules per
liter of drinking water!
 
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