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⊝ ⊝⊝
Example:
α = 0.50
⊝⊝
⊝ ⊝
⊝ ⊝⊝⊝
⊝
⊝ ⊝⊝ ⊝ ⊝ ⊝
⊝ ⊝
⊝⊝⊝⊝
Figure 4.3.12 Relationship between α, pH and pKa. 4.3.7.
Acid-­‐base titrations of polyelectrolytes: pKa depends on the degree of ionization Polyelectrolytes behave differently from small molecules when it comes to
acid base titration. A simple acid such as acetic acid (CH3COOH) has a welldefined titration curve and a unique pKa (4.76). The Henderson-Hasselbach
plot (pKa as a function of α) is simply a horizontal line:
Consider then a dibasic acid such as oxalic acid:
O
O
O
OH
HO
O
HO
O
H2A
O
O
O
HA-
O
A2-
Figure 4.3.13 Oxalic acid: ionization forms Oxalic acid contains two chemically identical carboxyl groups. Yet, oxalic acid
has two pKa values: 1.25 and 4.14 (Wikipedia). This means the first ionisation
(H2A → HA-) proceeds easily (low pKa), whereas ionisation the second
carboxyl is less favoured (high pKa). This can be understood by the influence
of the charges. Forming a –COO- in close proximity of an existing negative
charge is thermodynamically unfavourable. The titration curve of oxalic acid
clearly reveals the two pKa values.