acid dissociation constant
/ AS-id dis-oh-see-AY-shun KON-stunt /
Imagine handing out party invitations and watching how many guests actually let go and come to the door. Some acids are eager and release their hydrogen ions readily; others cling tightly and barely let any go. The acid dissociation constant is the number that scores exactly how willing an acid is to release its hydrogen ion into water.
Formally, for an acid HA dissolving in water, the acid dissociation constant Ka is the equilibrium constant for the reaction in which HA gives up a hydrogen ion to form H+ and its conjugate base A-. It equals the product of the concentrations of the products divided by the concentration of the undissociated acid. A large Ka means a strong, generous acid; a tiny Ka means a weak, reluctant one. Chemists often quote it as pKa, the negative logarithm, so the numbers stay small and comparable.
This constant matters because it predicts, before you ever mix anything, how acidic a solution will become and at what pH a buffer or indicator will work best. The honest caveat: Ka is a true constant only at a fixed temperature and, strictly, only when written in terms of activities — in ionic solutions a conditional value measured at your actual conditions is often more trustworthy than a textbook number.
Acetic acid, the acid in vinegar, has a Ka of about 1.8 times ten to the minus five, or a pKa near 4.76 — so in a glass of dilute vinegar only a small fraction of the molecules have actually let go of their hydrogen ion at any moment.
A pKa near 5 marks a typical weak acid: mostly intact in solution.
Smaller pKa means stronger acid, because pKa is a negative logarithm. It is a frequent slip to read a higher pKa as a stronger acid — the relationship runs the other way.