Acid dissociation constant

The acid dissociation constant is a number that measures the strength of an acid. The constant is written as $K_a$, and its negative logarithm as $\mathrm{p}{K}_a$. Chemists usually talk about pK_a instead of K_a because of the size of the numbers involved.

A higher K_a, or lower pK_a, indicates a stronger acid: an acid with a pK_a of 2 is ten times as acidic as an acid with a pK_a 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 pK_a value:

• Sulfuric acid (Template:Chem2) is a biprotic acid, meaning it has two hydrogen atoms it can give up. It has a pK_a of -2.8 to give up its first hydrogen and become bisulfate.
• Bisulfate (Template:Chem2) is the conjugate base of sulfuric acid. It still has one of the acid's two hydrogens, and has a pK_a of 1.99 to give it up and become sulfate.
• Sulfate (Template:Chem2) has no hydrogen atoms and no pK_a.

Relationship to pH

In chemistry, the "p" indicates the negative logarithm of a number. Both pH and pK_a 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 pK_a as long as it's in the same solvent (and at the same temperature).