Page 38 - PC 101 practical notes 24-25..

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

C Ch ho oi ic ce e o of f A Ac ci id d- -B Ba as se e I In nd di ic ca at to or rs s

For accurate titrimetric method, the indicator should be selected so as to

make the titration error small as possible. To achieve that the pH-range of the

indicator should fall in the vertical part of the titration curve (i.e. the part

marked by sharp change of pH) because the equivalence point is located in this

part so the difference between (the equivalence point) and (the end point

indicated by the selected indicator) will be small as possible.

Example:

Titration of NaOH with standard HCl using M.O. or ph.ph. indicator:

NaOH + HCl → NaCl + H 2O pH
(pH = 7)

The calculated equivalence point pH range of ph.ph.

corresponds to pH=7.

7
But since the vertical part of this

titration curve ranges from about 3.5 to pH range of M.O.

10.5 so either M.O. (3 – 4.5) or

ph.ph. (8.5 – 10) can be used as indicator M.O. End mls added of
Equivalence
Point
Point

(Theoretical) (Experimental) standard HCl

for this titration. Titration Error
(very small)

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