13
The larger the pK
a
value, the weaker the acid.
For ionizable solutes, the compound may exist as several different species in each phase at any given pH.
The most appropriate descriptor for ionizable compounds is the distribution coefficient, log D, since it is a measure
of the pH-dependant differential solubility of all species in the octanol/water system.
Log D is the ratio of the sum of the concentrations of all forms of the analyte in each of the two phases:
Log D provides a good indication of the ionization state of a compound in solution at a specific pH and is more
representative of the hydrophobic character in buffered conditions than log P.
Knowledge of the combination of log P, log D and pK
a
data for the analytes will help to guide the stationary phase
selection process.
*Note: k
b
, the equilibrium constant for a base gives us an indication of the basicity of a molecule. Historically, k
b
has been
defined as the association constant for protonation of the base B, to form the conjugate acid, HB
+
.
log P
oct/water
HA A
-
+ H
+
K
a
=
= log
log D
oct/water
= log
B + H
2
O HB
+
+ OH
-
K
b
=
pk
b
14-pk
a
[Analyte]
[Analyte]
[
HA
]
[ionized Analyte]
+
[un
–
ionized Analyte]
[ionized Analyte]
+
[un
–
ionized Analyte]
octanol
water
≈
water
water
octanol
octanol
[HB
+
] + [OH ]
-
[B]
log P
oct/water
HA A
-
+ H
+
K
a
=
= log
log D
oct/water
= log
B + H
2
O HB
+
+ OH
-
K
b
=
pk
b
14-pk
a
[Analyte]
[Analyte]
[
HA
]
[ionized Analyte]
+
[un
–
ionized Analyte]
[ionized Analyte]
+
[un
–
ionized Analyte]
octanol
water
≈
water
water
octanol
octanol
[HB
+
] + [OH ]
-
[B]
