Page 63 - Materials Chemistry, Second Edition

46 Practical Design Calculations for Groundwater and Soil Remediation

TABLE 2.4
Conversion Table for Henry’s Constant
Desired Unit for Henry’s Constant Conversion Equation
*
atm/M, or atm∙L/mole H = H RT
*
atm∙m /mole H = H RT/1,000
3
M/atm H = 1/(H RT)
*
atm/(mole fraction in liquid), or atm H = (H RT)[1,000γ/W]
*
*
(mole fraction in vapor)/(mole fraction in liquid) H = (H RT)[1,000γ/W]/P
Source: [10].
*
Note: H = Henry’s constant in the dimensionless form
γ = specific gravity of the solution (1 for dilute solution)
W = equivalent molecular weight of solution (18 for dilute aqueous solution)
R = 0.082 atm/(K)(M)
T = system temperature in Kelvin
P = system pressure in atm (usually = 1 atm)
M = solution molarity in (g⋅mol/L)
Henry’s law has been widely used in various disciplines to describe the distri-
bution of solute in the vapor phase and the liquid phase. The units of the Henry’s
constant (or Henry’s law constant) reported in the literature vary considerably.
The units commonly encountered include atm/mole fraction, atm/M, M/atm,
atm/(mg/L), and dimensionless. When inserting the value of Henry’s constant
into Equations (2.19) and (2.20), it is important to check if its units match those
of the other two parameters. Engineers with whom I have conferred normally
use the units they are familiar with and often have difficulties in performing
the necessary unit conversions. For your convenience, Table 2.4 is the conversion
table for Henry’s constant. Use of Henry’s constant in dimensionless form has
increased significantly. It should be noted that it is not a “(mole fraction)/(mole
fraction)” dimensionless unit. The actual meaning of the Henry’s constant in a
dimensionless format is (concentration in the vapor phase)/(concentration in the
liquid phase), which can be [(mg/L)/(mg/L)]. To be more precise, it has a unit of
“(unit volume of liquid)/(unit volume of air).”
Henry’s constant of any given compound varies with temperature. The
Henry’s constant is practically the ratio of the vapor pressure and the solubil-
ity, provided that both are measured at the same temperature, that is

Vaporpressure
H = (2.21)
Solubility

This equation implies that the higher the vapor pressure, the larger the
Henry’s constant is. In addition, the lower the solubility (or less soluble com-
pound), the larger Henry’s constant will be. For most organic compounds,
the vapor pressure increases and the solubility decreases with temperature.
Consequently, Henry’s constant, as defined by Equations (2.20) or (2.21),
should increase with temperature.