Page 49 - PC 101 practical notes 24-25..

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

Exp.: Determination of HCl & CH3COOH mixture

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

 According to the two-indicators method:

a) First step: (E.P M.O.)  for HCl only

Titration of 10 ml of the mixture ≠ standard NaOH using M.O. indicator.

E.P M.O. ≡ HCl only

b) Second step: (E.P ph.ph.)  for Total (HCl & CH3COOH)

Titration of another 10 ml of the mixture ≠ standard NaOH using ph.ph. ind.

E.P ph.ph. ≡ HCl + CH 3COOH

 Reaction equations:

HCl + NaOH → NaCl + H 2O

CH 3COOH + NaOH → CH 3COONa + H 2O

Notes:

st
1) During the determination of HCl only (1 E.P. using M.O.):
NaOH neutralizes HCl only without interference of acetic acid because the

+
hydrogen ion (H ) of HCl suppresses the ionization of acetic acid by
common-ion effect as shown in the following equations:

⎯
HCl ⎯→ H + Cl
-
+
⎯→
-
+
CH 3COOH  H + CH 3COO

and when the neutralization of HCl is completed, the pH of the solution

reaches the pH-range of M.O. and so M.O. changes its color.

nd
While during the detn. of total HCl & acetic acid (2 E.P. using ph.ph.):
NaOH neutralizes HCl first without any change in the color of ph.ph. and

then the titration is completed to make neutralization of acetic acid also.

49 | P a g e