PKa Concepts

8/9/2019 PKa Concepts

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pKa concepts

Ionization = the process in which ions are formedfrom neutral compounds;

Dissociation = the separation of the ions of anelectrovalent compound as a result of the actionof a solvent (usually water)



For a weak acid, which dissociates as follows

!" # !$ $ "%



"n interestin& and e'tremely useful relationship etween

p! and pKa can e otained simply y takin& lo&arithms

(to the ase *) of the previous e+uation

lo&*

Ka = lo&

*!$- $ lo&

*"% - % lo&

*!"-

.herefore

%lo&*!$- = %lo&*Ka $ lo&*"%- % lo&*!"-

/ote lo& a 0 lo& = lo& (a1)

&ivin& the !enderson%!asselalch e+uation



.he most convenient form of this !enderson%!asselalche+uation, is



2sin& pKa and p! relationship

3 4y usin& pKa values, we are ale to e'press thestren&th of an acid (i5e5 its tendency to dissociate)with reference to the p! scale5

3 If Ka is lar&e, then pKa will have a low numerical

value5 65&5,!ydrochloric acid, !7l has a pKa = %8

"cetic acid, 7!8799! has a pKa = :5

3 " stron& acid is one which is lar&ely, orcompletely, dissociated, and which therefore hasa high Ka value (and low pKa)5

3 " weak acid is one that is only sli&htly dissociatedin solution, and has a low Ka value5



if we consider the situation where the acid

is one%half (<*) dissociated,

or where

"%- = !"-

(that is <* ne&atively char&ed and <* unchar&ed)

then, sustitutin& in the

!enderson%!asselalch 6+uation

p! = pKa $ lo&("%1!")

p! = pKa $ lo&()

.herefore p! = pKa $ *

and p! = pKa



!ow to use !%! e+uation

3 >hen p! = pKa, the char&ed and unchar&edspecies have ?e+ual concentrations5

3 >hen p! @ pKa, the ionized (char&ed) form is

dominant, so there will e more ne&ative sites,

therefore, as pH increases, CEC increases

3 >hen p! A pKa, the un%ionized, unchar&ed form

is dominant, so there will e fewer ne&ative

sites, thus, as pH decreases, CEC decreases (and "67

increases)



>hy care aout pKa in soilsB

3 767 increases as p! increases

6arly studies showed soil 767 was constant from

p! C5< 0 <

"t p! @ < the 767 of soil increased, especially in

soils containin& or&anic matter or non C clays

3 9r&anic and inor&anic components of soil

have functional &roups that dissociate atvarious p!s, leavin& them with a ne&ative

char&e that can attract cations





"cidity of various soil functional &roups

3 Eome inor&anic surface functional &roups aremore likely to deprotonate or dissociate thanotherspKa of "l(9!C)$ = ?<, ("l%9!%Ei)$*5< = ?, Ei9! = ?5<

C silicate minerals have more Ei%9! &roups andcontriute less to p!%dependent char&e than minerals and metal o'ides

3 E9G contriutes the most ne&ative char&eH<%* of char&e due to deprotonation of 799! and

phenolic 9! &roups which have pKas of : 0 and %



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PKa Concepts