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2XY ↔ X2Y+ + Y-

2XY ↔ X+ + XY2-

a compound causing an incréase in X+ (or X2Y+) and a decréase in Y- (or XY2-) is an acid and one causing the reverse is a base. For example in liquid sulfur dioxide (SO2), thionyl compounds (formally supplying SO2+) behave as acids, and sulfites (supplying SO32-) behave as bases.

In this more general sense, aprotic compounds (those which do not donate protons), can still réact with bases, and the terms "acid" and "base" can still be used for réactions in aprotic or non-aquéous environments.