Page 93 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Notes:

1) During the determination of HCl only (E.P ), NaOH neutralizes HCl only without
M.O.
+
interference of acetic acid because the proton (H ) of HCl suppresses the ionization
of acetic acid by common-ion effect as shown in the following equations:
+ -
HCl → H + Cl

+ -
CH COOH ⇌ H + CH COO
3 3

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.

2) While during the determination of total HCl & CH COOH (E.P ),
3 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. At this point, the pH

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

3) The color change of M.O. at the first E.P. is not very sharp as in case of titrating HCl

alone against NaOH. This is due to the formation of acetate buffer (CH COOH &
3

CH COONa) after finishing the reaction of the titrant (NaOH) with HCl and starting
3
the reaction with CH COOH. The formed buffer resists the change in pH and so resists
3
color change of M.O.

▪ The principle of this determination can be applied to the determination of many

(Strong acid/Weak acid) mixtures where:
➢ E.P ≡ Strong acid
M.O.
➢ E.P ≡ Total (Strong acid + Weak acid)
ph.ph.
Examples of strong acids: HCl, H SO
2 4
Examples of weak acids: several organic acids like acetic acid, butyric acid,

phthalic acid, ….. etc.

88