Page 58 - PC 101 Interactive practical book

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

Exp.: Determination of Boric acid (H3BO3)

P Pr ri in nc ci ip pl le e : :

Boric acid (H 3BO 3) is a very weak monoprotic acid (pK a = 9.2) so it can not

be titrated against a standard alkali.

H 3BO 3 BO 2 + H + H 2O
+
-
metaborate ion

But if a sufficient amount of Glycerol, Mannitol or similar organic

polyhydroxy compound is added to boric acid, it will be converted into

a relatively stronger monoprotic acid that can be directly titrated against

standard NaOH using ph.ph. indicator.

(very weak acid) (relatively stronger acid)
{ pH near 7 } { pH ≈ 4.2 }

Then:

[Glyceroboric complex] H + NaOH  [Glyceroboric complex] Na + H 2O

Notes:

1) The reaction between glycerol and boric acid may be 2:1 (as shown in the

above equation) or 1:1 (as shown in the following equation):

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