I Lab Services 
The acid ionization constant varies by orders of magnitude.  For example at 25
o
C: 
acetic acid 
K
a
 = 1.8 x 10
 5
phenol   
K
a
 = 1.0 x 10
 10
It is easier to refer to such extreme numbers on a logarithmic scale and, again by convention,  p  
is used to denote the negative logarithm (base 10): 
pK
a
 =  log (K
a
) 
The K
a
 values of the compounds above are then easily converted to pK
a
 values: 
 acetic acid 
pK
a
 =  log(1.8 x 10
 5
) = 4.745 
phenol   
pK
a
 =  log(10
 10
) = 10.0 
There is an essential difference between interpreting the pK
a
 values for molecules vs. ions.  A 
molecule which loses a proton ionizes: 
HA + H
2
O 
  
 H
3
O
+
 + A
and so a low pK
a
 value denotes good aqueous solubility. 
An ion which loses a proton, however, de ionizes: 
and so a high pK
a
 value denotes good aqueous solubility. 
Note that there is no intrinsic reason to rule out pK
a
 values less than 0 or greater than 14.  For 
example, sulfuric acid, H
2
SO
4
, has a negative pK
a
 for the loss of its first proton: 
H
2
SO
4 
  
 HSO
4
 + H
+
 pK
a
 < 0 
although normally, experiment can only measure pK
a
 between 1 and 13. 
4.19.3 Understanding pK
a
 Calculation Results 
For example, if you calculate pK
a
 for uric acid: 
O
10
H
6
N
2
N
H
9
5
8
O
7
1
12
O
N
N
11
H
H
4
3
the obtained results can be interpreted in the following way: 
Ionic form: H4L 
It means that the ionic form of the specified structure has 4 protons that can dissociate in the 
molecule according to the ACD/Labs algorithm.  
pKa1(HL/L+H; 4) = 17.05   0.20 
ACD/I Lab via ChemSketch  
User's Guide 
50 






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