Environmental Engineering Reference
In-Depth Information
(iii) Atmospheric “mixing ratio”
Note that ppmv is also equal to (P i /P T ) 10 6 since volumes are related to
partial pressures through the ideal gas law. P i is the partial pressure of the
pollutant in the gas. Analogously ppmv is also equal to (n i /n T ) 10 6 , where
n represents the number of moles. Note that (P i /P T ) 10 6 is also called the
“mixing ratio
χ i .”
COMPARTMENT: WATER
The three most common units of a pollutant concentration in water are
moles of i per liter (or per dm 3 ) of solution.
(i) Molarity, C iw =
moles of i per kilogram of water.
(iii) Mole fraction of i , x i =
(ii) Molality, m i =
n T ) . Note that for most “dilute solutions,”
mole fraction and molarity are related through x i =
n i /(n i +
C i w V w , where V w is the
molar volume of water (0.018 L/mol).
A common, but less precise unit of pollutant concentration in water is:
(iv) Parts per million by mass, ppmm
=
(mg of i per kg of solution). Note that
for water since the density is 1 kg/L, ppmv is equivalent to mg of i per liter
of solution, ppmv = mg/L.
For acids and bases, the preferred unit of concentration is “normality,” which is
the number of equivalents per liter of solution. The number of equivalents per mole
of acid equals the number of moles of H + the acid can potentially produce.
COMPARTMENT: SOIL/SEDIMENT
Concentration of a pollutant on soil/sediment is expressed as
(W i /W soil ) 10 6 , where W i is the grams of pollutant i and W soil is
the total amount of soil in grams. Note that this is equivalent to milligrams
of i per kilogram of soil or sediment. Hence ppmm
(i) ppmm
=
mg/kg.
(ii) Moles of i per square meter of surface area of soil/sediment
=
(mg of i
per kilogram of soil) ( 10 3 S a / M i ) , where S a is the specific area of the soil
(m 2 per kilogram of soil) and M i is the molecular weight of i .
(iii) Percent by weight of i
=
=
( gof i /g of soil)100.
 
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