polyprotic acid
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Imagine someone carrying several coins and handing them out one at a time rather than all at once, parting with the easiest coin first and the most reluctant one last. A polyprotic acid behaves like this with its hydrogen ions: it has more than one to give, and it lets them go in steps.
Formally, a polyprotic acid is an acid that can donate more than one hydrogen ion per molecule, releasing them in successive dissociation steps, each governed by its own acid dissociation constant. The first proton comes off most easily, so the first Ka is largest; each later proton is harder to remove from an increasingly negative ion, so the constants shrink with each step. Acids that give two are diprotic, three are triprotic.
These acids matter because their stepwise release shapes titration curves with more than one jump and lets a single substance buffer at several different pH values. The honest caveat is that the steps overlap when their pKa values are close together: only when successive pKa values are well separated do you see cleanly distinct equivalence points, otherwise the jumps blur into one another.
Phosphoric acid is triprotic, with pKa values near 2.1, 7.2, and 12.3; titrating it with base reveals two clear jumps before the third proton, which is too weakly acidic to give a sharp endpoint, comes off gradually.
Well-separated pKa values give multiple distinct equivalence points.
Each step has its own pKa, and they always increase down the series because pulling a proton off an already-negative ion is harder. A species midway between two protons, like bicarbonate, is amphiprotic — it can act as either acid or base.