Potentiometric Study on the Interactions between Divalent Cations and Sodium Carboxylates in Aqueous Solution - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 29

From analysis of Figure 2(b), we can also conclude that the addition of C 7 COO í
and C 9 COO í to Pb 2+ solutions, leads to an increase in pH values but with different
trends of those obtained in aqueous solutions in the absence of divalent ion.
From the analysis of Figures 1, 2, and 4 it is still possible to obtain the interac-
tion stoichiometry between the divalent ions and different alkanoates. The calculated
values, by using different techniques, are summarized in Table 1. It appears that the
interaction of C n COO í :Pb 2+ and C 11 COO í :Ca 2+ occurs with a stoichiometry of 2:1,
which is consistent with charge neutralization [45-47].
As observed, the interaction between Pb 2+ and the alkanoates studied is indepen-
dent of the size of the alkyl chain, and as such it is relevant to quantify the association
constant. For this purpose, potentiometric measurements were carried out to quan-
tify the concentration of free lead in solutions containing alkanoates (Figure 4). From
analysis of Figure 4, it appears that the concentration of free Pb 2+ in solution decreases
with increasing concentration of alkanoates, reaching a minimum at a concentration
of alkanoates approximately of 2 mM, for an initial 1 mM Pb 2+ solutions, which is
again consistent with the values of stoichiometry association calculated by electrical
conductivity.

TABLE 1 Interaction stoichiometry of divalent ions (Ca 2+ and Pb 2+ ) and sodium carboxylates
obtained by different techniques.

[C n COO - ]/[M 2+ ]
Conductivity

pH measurements Selective electrode

Ca 2+ /C 7 COO í Not detected Not detected -

Ca 2+ /C 9 COO í 2.6 ± 0.2 - -

2.15 ± 0.02
2.26 ± 0.01 d

2.35 ± 0.01 a
2.304 ± 0.009 c

Ca 2+ /C 11 COO í -

2.49 ± 0.03
2.36 ± 0.06 d

Pb 2+ /C 7 COO í 1.85 ± 0.01 b 2.35 ± 0.05

1.88 ± 0.04
1.917 ±0.008 d

2.08 ± 0.05 a
2.15 ± 0.01 c

Pb 2+ /C 9 COO í 1.89 ± 0.15

2.18 ± 0.06
2.08 ± 0.01 c

2.38 ± 0.03 a
2.307 ± 0.009 c

Pb 2+ /C 11 COO í 2.08 ± 0.05

a Boltzmann fit; b Gauss fit; c First derivative; d Second derivative.

It is worth noticing that the interaction between Pb 2+ and alkanoates (Figure 2(b))
is quite different from that observed with, for example, sodium dodecyl sulfate and
trivalent ions [48]. In that case, it is observed that under surfactant excess conditions,
the precipitate (aggregate formed by dodecyl sulfate and the metal ion) redissolves,
due to preferential hydrophobic interactions between the alkyl chains of the surfactant,