Boric acid
All borates can be considered derivatives of boric acid, B(OH)3. Boric acid is a weak proton donor (p"K"a ~ 9) in the sense of Brønsted acid, but is a Lewis acid, i.e., it can accept an electron pair. In water, it behaves as a Lewis acid, accepting the electron pair of a hydroxyl ion produced by the water autoprotolysis.