Environmental Engineering Reference
In-Depth Information
Table 7.4: Equilibrium constants of organic acids
Acid
K
a
K
b
10
−
5
Acetic
1
.
8
×
10
−
5
Butyric
1
.
5
×
10
−
4
Glycolic
1
.
5
×
1
.
4
×
10
−
4
Lactic
9
.
6
×
10
−
4
2
.
9
×
10
−
5
Tartaric
8
.
7
×
10
−
4
1
.
8
×
10
−
5
Citric
1
.
4
×
10
−
3
8
.
0
×
10
−
7
Malonic
6
.
5
×
10
−
2
6
.
1
×
10
−
5
Oxalic
Initially, the complex formation with metallic cation in the solution will involve anion A. In
this case, it is assumed that metallic ions entered the solution by partial dissolution of solid.
Thus, according to the
K
a
value (
Table 7.4
), the concentration of anion A will be in significant
access compared with anion B. In the case of Ni
2+
, the complex formation may involve the
following tentative scheme:
{
7.2
}
Complex 1 in this scheme represents a transition state before the final product (complex 2) is
formed. It is believed that in the transition state the O H bond of the non-ionized carboxylic
group is weakened considerably. This results in the enhancement in the acidity of the group,
i.e., H
+
is released from this group more readily. Therefore, as the
K
b
value (
Table 7.4
)
indicates, the initial contribution of the completely ionized acid to the complex formation will
be minor, although it may enhanced via formation of complex 2 from complex 1. Two anions
A (Reaction
{7.1}
) can also be involved during complexation with metallic cations in
solution, e.g.,
2(HOOCCOO
−
)
Ni
+
⇒
Ni(
−
OOCCOOH)
2
+
{7.3}
The removal of anions via Reaction
{7.2}
shifts the equilibrium
{7.1}
to the right.
Consequently, additional acid is ionized.
Table 7.4
indicates the differences in the equilibrium
constants of several acids. According to these results, the availability of anions A required for
