principles of extraction and the extraction of semivolatile organics ...

40 principles of extraction
Solubility
Solubility is also used to estimate the Henry’s law constant [equation (2.4)].
Solubility is the maximum amount of a chemical that can be dissolved into
another at a given temperature. Solubility can be determined experimentally
or estimated from molecular structure [6,10–12].
The Henry’s law constant, H, calculated from the ratio of vapor pressure
and solubility [equation (2.4)] can be converted to the dimensionless Henry’s
law constant, H 0 , [equation (2.3)] by the expression
H 0 ¼ PvpðMWÞ
0:062ST
ð2:5Þ
where Pvp is the vapor pressure in mmHg, MW the molecular weight, S the
water solubility in mg/L, T the temperature in Kelvin, and 0.062 is the
appropriate universal gas constant [9].
For the analyst’s purposes, it is usually su‰cient to categorize the escaping
tendency of the organic compound from a liquid to a gas as high,
medium, or low. According to Henry’s law expressed as equation (2.4), estimating
the volatilization tendency requires consideration of both the vapor
pressure and the solubility of the organic solute. Ney [13] ranks vapor pressures
as
Low: 1 10 6 mmHg
Medium: between 1 10 6 and 1 10 2 mmHg
High: greater than 1 10 2 mmHg
while ranking water solubilities as
Low: less than 10 ppm
Medium: between 10 and 1000 ppm
High: greater than 1000 ppm
However, note that in Ney’s approach, concentration expressed in parts per
million (ppm) does not incorporate molecular weight. Therefore, it does not
consider the identity or molecular character of the chemical.
Rearranging equation (2.4) produces
Pvp ¼ HS ð2:6Þ
In this linear form, a plot (Figure 2.1) of vapor pressure (y-axis) versus solubility
(x-axis) yields a slope representing the Henry’s law constant at values