When orthoboric acid is dissolved in water, it partially dissociates to give metaboric acid:
The solution is mildly acidic due to ionization of the acids:
However, Raman spectroscopy of strongly alkaline solutions has shown the presence of  ions, leading some to conclude that the acidity is exclusively due to the abstraction of  from water:
Equivalently,
: +    +  (K = 7.3×10; pK = 9.14)
Or, more properly,
This reaction occurs in two steps, with the neutral complex aquatrihydroxyboron  as an intermediate:
This reaction may be characterized as Lewis acidity of boron toward , rather than as Brønsted acidity. However, some of its behaviour towards some chemical reactions suggest it to be tribasic acid in the Brønsted sense as well.
Boric acid, mixed with borax  (more properly ) in the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately.