pKa concepts
Ionization = the process in which ions are formed
from neutral compounds;
Dissociation = the separation of the ions of an
electrovalent compound as a result of the action
of a solvent (usually water)
For a weak acid, which dissociates as follows:
HA ↔ H+ + A-
An interesting and extremely useful relationship between
pH and pKa can be obtained simply by taking logarithms
(to the base 10) of the previous equation:
log10Ka = log10[H+] + log10[A- ] - log10[HA]
Therefore
-log10[H+] = -log10Ka + log10[A-] - log10[HA]
Note: log a – log b = log (a/b)
giving the Henderson-Hasselbalch
equation:
The most convenient form of this Henderson-Hasselbalch
equation, is
Using pKa and pH relationship
• By using pKa values, we are able to express the
strength of an acid (i.e. its tendency to
dissociate) with reference to the pH scale.
• If Ka is large, then pKa will have a low numerical
value. E.g.,
Hydrochloric acid, HCl has a pKa = -3
Acetic acid, CH3COOH has a pKa = 4.77
• A strong acid is one which is largely, or
completely, dissociated, and which therefore has
a high Ka value (and low pKa).
• A weak acid is one that is only slightly
dissociated in solution, and has a low Ka value.
if we consider the situation where the acid
is one-half (50%) dissociated,
or where
[A-] = [HA]
(that is 50% negatively charged and 50% uncharged)
then, substituting in the
Henderson-Hasselbalch Equation
pH = pKa + log(A-/HA)
pH = pKa + log(1)
Therefore
pH = pKa + 0
and
pH = pKa
How to use H-H equation
• When pH = pKa, the charged and uncharged
species have ~equal concentrations.
• When pH > pKa, the ionized (charged) form is
dominant, so there will be more negative sites,
therefore, as pH increases, CEC increases
• When pH < pKa, the un-ionized, uncharged form
is dominant, so there will be fewer negative
sites,
thus, as pH decreases, CEC decreases (and AEC
increases)
Why care about pKa in soils?
• CEC increases as pH increases
Early studies showed soil CEC was constant
from pH 2.5 – 5
At pH > 5 the CEC of soil increased, especially in
soils containing organic matter or non 2:1 clays
• Organic and inorganic components of soil
have functional groups that dissociate at
various pH’s, leaving them with a negative
charge that can attract cations
Acidity of various soil functional groups
• Some inorganic surface functional groups are
more likely to deprotonate or dissociate than
others
pKa of Al(OH2)+ = ~5, (Al-OH-Si)+0.5 = ~7, SiOH = ~9.5
2:1 silicate minerals have more Si-OH groups and
contribute less to pH-dependent charge than 1:1
minerals and metal oxides
• SOM contributes the most negative charge
85-90% of charge due to deprotonation of COOH and
phenolic OH groups which have pKa’s of 4 – 6 and 9 - 11
http://www.biologie.uni-hamburg.de/b-online/ge16/10.gif
Colloid
Negative
charge
Positive
charge
% constant
% variable
Humus
200
0
10
90
Vermiculite
120
0
95
5
Smectite
100
0
95
5
Illite
40
0
80
20
Kaolinite
12
4
5
95
Fe & Al
Oxides
5
5
0
100