Boric acid is a weak monoprotic
acid (Ka=6.4×10−10). Therefore, it cannot be
directly titrated with standard strong alkali (NaOH). But by the addition of an
organic poly hydroxy compound such as 1,2- or 1,3-diol, (Like
Sugar, Mannitol, Catechol) it is
converted to a much stronger acid, which can be titrated using phenolphthalein.
This is due to the complex formation between hydrated borate ion and either
1,2- or 1,3-diol for example .
Ploy hydroxy
(Cis-diol) increases acidity of boric acid due to formation of complex with
metaborate and reaction goes to farward to produce more H+.
IIT UPDATE: (2006):
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