# Sorption of Organic Electrolytes and Surfactants from Natural Waters by Heterogeneous Membranes
## 2 Materials and Methods
### The Total Ion Exchange Capacity of Membranes  
The potentiometric titration-determination method measures the concentration of the analyte in a solution by monitoring a change in the potential of the indicator electrode located in equilibrium with the determined ion. The analyzed solution is titrated, measuring the electrode potential during titration, which changes with a change in the proportion of reacted substances, and most noticeably in point of equivalence. In potentiometric titration, the indicator is used as a potentiometer for electrodes immersed in the titrated solution, in which electrodes sensitive to titrated ions are used. During titration, changes in the concentration of ions, are recorded on the scale measuring instrument potentiometer. The potentiometer readings are recorded in units of pH or mV, their dependence on the volume of titrant (titration curve) is plotted, and the equivalence point and titrant volume are determined, to establish what has been consumed during titration.  
The equivalence point is determined by: building the dependence of the potential change on the volume of added titrant (integral curve titration) and finding the midpoint of the site, corresponding to the vertical rise of the curve (Figure 7).  
Potentiometric acid-base titration curves show the change in the pH of the solution during titration. Titration curve shape, the position of the equivalence point, and the magnitude of the potential jump (pH) on the titration curve depend on the strength of the acid and base and the composition of the solution. As an indicator a glass electrode was used.  
Figures 8 and 9 show titration curves of cationic and anionic membranes.  
Figure 6: Dependence of electrical conductivity of solutions on concentration for NaNS and NaBS.  
Figure 7: The equivalence point.  
The exchange capacity \(g\) (_mg-equiv/g_) was determined by the difference in the abscissa of the points lying on two curves at the same pH value and was calculated by the formula:  
\[g=C\,(V_{1}-V_{0})/m \tag{1}\]  
where:  
1. \(V_{1}\) is the volume of HCl (NaOH) (cm\({}^{3}\)) when titrated with a membrane;
2. \(V_{0}\) is the volume of HCl (MNaOH) (cm\({}^{3}\)) when titrated without a membrane;
3. \(C\) is the concentration of HCl (NaOH) solution (g-equiv/l);
4. \(m\) is the weight of the membrane, \(g\).  
Sorption of organic ions was determined in the MK-40-CPCI, the MA-40-NaBS and the MA-40-NaNS systems under static conditions using two methods: by decreasing the concentration of organic ions in solutions contacting the MK-40 and MA-40 membranes, and by the potentiometric titration method for the MA-40.  
During the sorption, the solutions were periodically stirred (once a day for 2-3 h). The sorption was studied by analyzing individual samples taken at specific time intervals for a long period of time  
Figure 8: Potentiometric titration curves of free solution (0.06 N MgCl\({}_{2}\)) (1) and a solution with MK-40 membrane (2).  
Figure 9: Potentiometric titration curves of free solution (0.03 N Na\({}_{2}\)SO\({}_{4}\)) (1) and a solution with MA-40 membrane (2).  
(up to 35 days). All measurements were carried out at room temperature (\(\approx\)18 \({}^{\circ}\)C). The concentration of the study substances was determined spectrophotometrically at absorption bands \(\lambda\) = 260 nm for CPCI (\(2.5\times 10^{-4}\) M), \(\lambda\) = 275 nm for NaNS (\(2.5\times 10^{-4}\) M), \(\lambda\) = 263 nm for NaBS (\(2.5\times 10^{-3}\) M) according to the calibration curves of the initial solutions of the same substances with known concentrations. The calibration curve for CPCI is shown in Figure 10 as the dependence of absorption coefficient (\(\varepsilon\)) on the solution concentration. Similar curves were plotted for NaNS and NaBS.