Page 59 - PC 101 practical notes 24-25..

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

2) M.O. indicator is unsuitable for the titration of glyceroboric acid against

NaOH because the pH of glyceroboric acid is about 4.2 (i.e. the titration

curve will start at pH 4.2 and go upward) so the vertical part of the

titration curve will be above the pH-range of M.O.

3) If the titration is done directly on boric acid without adding glycerol, the

end point will appear very rapidly due to the hydrolysis of the produced

sodium metaborate leading to the formation of NaOH again giving pink

color with ph.ph.

H 3BO 3 + NaOH NaBO 2 + 2H 2O
Sod. metaborate
Question:

By mistake, boric acid determination is done directly without adding glycerol.

Will the result be lower or higher than the true value? (lower).

P Pr ro oc ce ed du ur re e : :

1) Transfer 10 ml of the sample into a clean conical flask.

2) Add 5 ml neutral glycerol

3) Add 10 drops ph.ph. indicator.

4) Titrate ≠ 0.1 N NaOH. measuring cylinder

{ Color change at E.P.: from colorless to pink }

C Ca al lc cu ul la at ti io on ns s : :

Boric acid + 2 glycerol → [Glyceroboric complex] H + 3H2O

[Glyceroboric complex] H + NaOH → [Glyceroboric complex] Na + H2O
titrant

 1 Boric acid ≡ 1 glyceroboric acid ≡ 1 NaOH

 1 NaOH ≡ 1 Boric acid
standard sample
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