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BORIC ACID (H3BO3) and its structural features:

Orthoboric acid, H3BO3, commonly known as boric acid, and Metaboric acid, HBO3 , are two common oxy acids of  boron . Orthoboric acid naturally found in volcanic steam vents called Suffioni. 
(1) In dilute Solution Boric acid exist as monomeric form;
(2) When concentration of the solution of acids  is very high then boric acid exist as polymeric metaboric and many more ions.
(3) In Solid State It exist sp3 hybridised BO3-  2D Sheet .
(1) H3BO3 is soluble in water and behaves as weak monobasic acid. It does not donate protons but rather it accepts OH- .Therefore it acts as a Lewis acid [B(OH)3] .It is not a proton donor because it accept lone pair or hydroxyl ion from water.

(2) Since B(OH)3 only partially reacts with water to form H3O+ and [B(OH)4]- it behaves as a weak acid. Thus it cannot be titrated satisfactorily with NaOH as a sharp end point is not obtained.  

(3)  B(OH)3 (Boric acid ) does not  titrated even strong alkali like NaOH but If certain polyhydroxy compounds such as glycerol, mannitol or sugar are added to the titration mixture then B(OH)3 behaves as a strong monobasic acid and hence can be titrated with NaOH and end point is diluted using phenolphthalein as indicator.
(4) The added compound must be a cis-diol to enhance the acidic proprieties. In this way the cis-diol forms very stable complexes with [B(OH)4formed in forward direction above, thus effectively removing it from solution. Hence reaction proceeds in forward direction (Le-Chatelier principle.)

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