Page 83 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

Applications of Acid-Base Titrations

Titration curves for polyprotic (polybasic) acids
• A polyprotic (polybasic) acid can release more than one
+
H ion in successive steps.
• A polybasic acid may be treated as a mixture of acids;
each stage of dissociation is treated as a separate
monobasic acid.

✓ A polyprotic acid can be titrated stepwise with good
end-point breaks (inflections) for each proton if:
the difference in pK values is at least 4 [i.e. (pK ̶ pK ) ≥ 4 ]
a a2 a1
4 4
or the ratio of K values is at least 10 [i.e. " " /" " ≥ 10 ]
a
of each two successive protons.

- more than Less than
+
H A ⇌ H + H A ………………….. K , pK
3 2 a1 a1
- + 2-
H A ⇌ H + HA ………………….. K , pK
2 a2 a2
2- + 3-
HA ⇌ H + A ……………………. K , pK
a3 a3

Example#1: Sulfuric acid (H SO ):
2 4

H SO is a strong diprotic acid. Its ionization can be expressed as follows:
2 4
+ -
H SO → H + HSO [complete ionization, pK is too small (supposed 0)]
2 4 4 a1
- + 2-
HSO ⇌ H + SO …………………..….. pK = 2.1
4 4 a2
✓ The difference between the two pK values is < 4 so stepwise
a

titration of each proton is not possible and only one inflection

(one equivalence point) is obtained.

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