What is pka

Some not-so-acidic compounds. Water is very, very weakly acidic; methane is not really acidic at all. The pKa measures the "strength" of a Bronsted acid. A strong Bronsted acid is a compound that gives up its proton very easily. A weak Bronsted acid is one that gives up its proton with more difficulty. Going to a farther extreme, a compound from which it is very, very difficult to remove a proton is not considered to be an acid at all.

When a compound gives up a proton, it retains the electron pair that it formerly shared with the proton. It becomes a conjugate base. Looked at another way, a strong Bronsted acid gives up a proton easily, becoming a weak Bronsted base.

The Bronsted base does not easily form a bond to the proton. It is not good at donating its electron pair to a proton. It does so only weakly. In a similar way, if a compound gives up a proton and becomes a strong base, the base will readily take the proton back again. Effectively, the strong base competes so well for the proton that the compound remains protonated. Setting pH and pKa equal gives another equation:.

When half of the acid dissociates, pH and pKa are equal. Select the best buffer for a situation by choosing one with a pKa value close to the target pH of the solution. Search for:. Acid Dissociation Constant. Related Posts. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.

Facebook Facebook Twitter Twitter. Updated November 11, A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.

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