the acidity nature of Boric acid is not clearly mentioned in any of the websites.
https://chemistry.stackexchange.com/questions/10614/mechanism-of-acidity-boric-acid
https://en.wikipedia.org/wiki/Boric_acid
what are the ions of boric acid? in Bronsted sense, they would be
H2BO3- ,HBO32- , BO33-
, the ions should be instead
H3BO3(OH)- ,H3BO3(OH)22-- , H3BO3(OH)33-
Which of the following is true?
even if bronsted ions are formed ,if their water solvation bond appear more coordination-covalent character than hydrogen -bonding character,(which can easily happen since the ionic species is charged) then the ion would appear lewis-like as well.
is this the case here? is then, there exists no sharp boundary between Bronsted and Lewis acidity?
All Brønsted-Lowry acids are Lewis acids, but not all Lewis acids are Brønsted-Lowry acids. In the reaction of boric acid with water, we have
B(OH)3 + H2O → B(OH)-4 + H+.
B(OH)3 is an electron receptor and a Lewis acid, but in this case, the water molecule is the proton donor and Brønsted-Lowry acid. The Lewis theory does not depend on the presence of an H atom in the acid that can act as a proton donor.
Therefore Boric acid is not a Brønsted-Lowry acid but a Lewis Acid.
If I refer to the Wikipedia text, there is spectroscopic evidences that B(OH)4- exist in strongly basic solution (supposedly water.) The process shown by Jürgen Wintner corresponds to pKa = 9.24. Nevertheless there are two other pKa values (12.4, 13.3) for boric acid in water, showing that these weaker acidities come from B-O-H protons in B(OH)4- , therefore corresponding to Bronsted acidities. In solvents other than water, the picture is probably much different, but a bibliography search would be necessary.
As Jurgen noted, boric acid per se acts as a Lewis acid toward water. The resulting complex (B(OH)4)-H+ is a Brønsted acid.
So the answer depends on what, precisely, your question is: are you asking about boric acid, or an aqueous solution of boric acid?
The Bronsted acidity of boric acid in water as described by James Demers is perfectly correct, and I agree with his answer. The qiestion is "in what medium?". For example I am sure that BF3, a typical Lewis acid, can be considered as a Bronsted acid in water : the adduct [BF3H2O] can be written as (BF3OH)-H+. All Lewis acids can be treated the same way.
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