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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  

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