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 pK a can be obtained simply by taking logarithms (to the base 10) of the previous equation: log 10 K a = log 10 [H + ] + log 10 [A - ] - log 10 [HA] Therefore -log 10 [H + ] = -log 10 K a + log 10 [A - ] - log 10 [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 pK a values, we are able to express the strength of an acid (i.e. its tendency to dissociate) with reference to the pH scale. If K a is large, then pK a will have a low numerical value. E.g., Hydrochloric acid, HCl has a pK a = -3 Acetic acid, CH 3 COOH has a pK a = 4.77 A strong acid is one which is largely, or completely, dissociated, and which therefore has a high K a value (and low pKa). A weak acid is one that is only slightly dissociated in solution, and has a low K a 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(OH 2 ) + = ~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

Colloid Negative charge Positive charge % constant% variable Humus Vermiculite Smectite Illite Kaolinite Fe & Al Oxides