PPD a database of protein ionization constants

     
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General Definition of pKa
 

The pKa is a measure of the tendency of a molecule or ion to keep a proton, H+, at its dissociation center(s). It is related to dissociation capabilities of chemical species.

The equilibrium acid dissociation constant, Ka, expresses the ratio of concentrations for the reaction:



where, by convention, it is assumed that the concentration of water is constant, and it is absorbed into the Ka definition.

The acid dissociation constant varies by orders of magnitude. For example at 25°C:

acetic acid    
phenol    

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):

The Ka values of the compounds above are then easily converted to pKa values:

acetic acid    
phenol    

There is an essential difference between interpreting the pKa values for molecules vs. ions. A molecule which loses a proton ionizes:

An ion which loses a proton, however, de-ionizes:

Note that there is no intrinsic reason to rule out pKa values less than 0 or greater than 14. For example, sulfuric acid, , has a negative pKa for the loss of its first proton:

    pKa < 0

although normally experiments can only measure pKa between 1 and 13.

 

 
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