Acid dissociation constant

The acid dissociation constant is a number that measures the strength of an acid. The constant is written as , and its negative logarithm as . Chemists usually talk about pKa instead of Ka because of the size of the numbers involved.

A higher Ka, or lower pKa, indicates a stronger acid: an acid with a pKa of 2 is ten times as acidic as an acid with a pKa of 3.

Some acids, called polyprotic acids, have more than one 
(acidic) hydrogen, so they dissociate in more than one step. Each step has its own pKa value:
 Sulfuric acid () is a biprotic acid, meaning it has two hydrogen atoms it can give up. It has a pKa of -2.8 to give up its first hydrogen and become bisulfate.
 Bisulfate () is the conjugate base of sulfuric acid. It still has one of the acid's two hydrogens, and has a pKa of 1.99 to give it up and become sulfate.
 Sulfate () has no hydrogen atoms and no pKa.

 Relationship to pH 
In chemistry, the "p" indicates the negative logarithm of a number. Both pH and pKa are negative logarithms used to measure strength of acids and bases, but pH is a property of a solution, rather than a specific acid or base.

A different solution of the same acid can have differing pH depending on the amount of acid, or what else it is mixed with, but the acid will always have the same pKa as long as it's in the same solvent (and at the same temperature).

 References 

Category:Chemistry