Potentiometric Study on the Interactions between Divalent Cations and Sodium Carboxylates in Aqueous Solution - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 25

resistances, measured by a conductivity cell with a constant of 0.1178 cm -1 , uncer-
tainty 0.02% [40]. Cell constant was determined from electrical resistance measure-
ments with KCl (reagent grade, recrystallized, and dried) using the procedure and data
of Barthel et al. [41]. Measurements were taken at 25.00ºC (±0.02ºC) in a Thermo
Scienti¿c Phoenix II B5 thermostat bath. Solutions were always prepared immediately
before the experiments. In a typical experiment, 20 ml of metal divalent solution 1
mM were placed in the conductivity cell then aliquots of the surfactant solution were
added at 4 min intervals by a Gilson Pipetman micropipette. The speci¿ c conductance
of the solution was measured after each addition and corresponds to the average of
three ionic conductances (uncertainty less than 0.2%), determined using homemade
software. The speci¿c electrical conductance of the solutions, ț, is calculated from the
experimental speci¿c conductance, ț exp , and corrected for the speci¿c conductance of
water, ț 0 :ț = ț exp í ț 0.
Potentiometric measurements were carried out with a pH Radiometer PHM 240.
For Pb(II), the metal ion concentration present in solution was measured using a lead
selective electrode (WTW, Pb 500) and an Ag/AgCl reference electrode (Ingold). The
pH measurements were carried out with a pH conjugated electrode (Ingold U457-K7),
the pH was measured on fresh solutions, and the electrode was calibrated immediately
before each experimental set of solutions using IUPAC-recommended pH 4 and 7
buffers. In a typical experiment, using a Gilson Pipetman micropipette, aliquots of the
surfactant solution were added to 20 ml of metallic divalent solution. In both cases, the
electrode potential was recorded after signal stabilization, and all measurements were
carried out at 25.00ºC (±0.02ºC).
3.3 DISCUSSION AND RESULTS
Figure 1 shows the effect of adding sodium carboxylates (C n H 2n+1 COO í , which for
simplicity we will designate as C n COO í ) on the specific conductance of aqueous solu-
tions of lead and calcium nitrate at surfactant concentrations below their critical mi-
celle concentration (cmcs: C 7 COO í 0.34 M; C 9 COO í 94 mM, C 11 COO í 24 mM [42]).
With the aqueous Pb 2+ solution, addition of sodium carboxylate leads to a decrease in
the specific conductance until around 2 mM carboxylate. Since the specific conduc-
tance gives a measure of the ion concentration in solution, this observation indicates a
strong interaction between the ions of the two salts, with the consequent charge neu-
tralization [16]. We can also conclude that the change in the k = f(c) behavior occurs
for a molar ratio [C n COO í ]/[Pb 2+ ] of 2, indicating an interaction stoichiometry is 2:1
(C n COO í :Pb 2+ ), consistent with the charge neutralization. We can also see that in the
case of Pb 2+ , the effect of alkyl chain length is not significant for the solutions behav-
ior [32]. For molar ratios above 2, the specific conductance increases with increasing
concentration of carboxylate. This behavior can readily be understood since after all
the Pb 2+ has been “consumed” by the carboxylate, the excess of surfactant produces
an increase in the solution conductivity, similar to what occurs in the absence of the
divalent ions.
However, the carboxylate behavior, in presence of Ca 2+ is signi¿ cantly different from
that observed for Pb 2+ . With calcium(II), it is possible to distinguish three different zones
of k as a function of the surfactant concentration, depending on the alkyl chain length.