amphiprotic species
/ am-fih-PROH-tik SPEE-sheez /
Think of someone who can both lend money and borrow it, depending on who they are dealing with. An amphiprotic species is the chemical equivalent: it can either give away a hydrogen ion or take one up, acting as an acid toward bases and as a base toward acids.
Specifically, an amphiprotic species is one that can both donate and accept a proton because it carries a hand-off-able hydrogen and also has a lone pair or negative site able to receive one. Water itself is the classic case, and the intermediate ions of polyprotic acids — like bicarbonate, sitting between carbonic acid and carbonate — are everyday examples. Which role it plays in any given mixture depends on the partner it meets.
These species matter because they are the hinge points of polyprotic titrations and the building blocks of natural buffers. The honest caveat is a small vocabulary trap: amphiprotic specifically means it trades protons, while the broader word amphoteric can also cover substances that react with acid and base by other routes, so the two terms overlap but are not perfect synonyms.
The bicarbonate ion can give up its proton to become carbonate when met by a base, or accept one to become carbonic acid when met by an acid — the same ion playing both parts depending on its company.
One species, two roles — acid to a base, base to an acid.
The pH of a pure amphiprotic salt solution lands near the average of the two flanking pKa values, almost independent of concentration. This neat result is why such solutions are handy starting points for setting a known pH.