An Introduction to Environmental Chemistry

82 Chapter Four

Dissociation and pH

As dissociation is an equilibrium process it

is logical that where parameters influence

equilibrium the degree of dissociation will

also alter. Thus, pH has a huge bearing on

the extent to which ionizable species

dissociate since pH is a measure of free

proton concentrations (or more correctly the

free proton activity) (see Section 2.6).

Re-arranging equation 1:

c c eqn. 6

c

HKHA

a A

+=. -

Taking the log of this equation yields:

eqn. 7

multiplying this equation by –1 yields:

eqn. 8

and thus:

pH p A eqn. 9

a HA

=+

• K c
  c
  log 10

-=--log 10 cK+ log 10 log 10 c -

c

H HA

a A

log 10 cKlog 10 log 10 c

c

H HA

a A

+=+-

CH 3

OH

o-cresol

OH

phenol

NO 2

OH

o-nitrophenol

pKa = 10.2 pKa = 9.96 pKa = 7.22

Increasing acidity

Fig. 1Variation in pKavalues for phenols containing different additional functional groups.

Dissociation Delocalization around the aromatic ring

Dissociation Delocalization around the aromatic ring

Delocalization onto

nitro-group

N

O

• 
  

O

O

(a)

(b)

CH 3

OH

CH 3

O– H+

CH 3

O

CH 3

O

• 
  

H

• 
  

H

CH 3

O

• 
  

NO 2

OH

NO 2

O– H+

NO 2

O

NO 2

O

• 
  

H

• 
  

H

NO 2

O

• 
  

N

O O–

O

Fig. 2Delocalization of negative charge in the phenate ion where (a) methyl and (b) nitro functional

groups are present.

(continued)