Structure of catex B (R = H, COOH, SO3H). The total cation exchange capacity of catex B has been det — Analytical Chemistry Chemistry Question
Weak and strong cation exchange resin
Structure of catex B (R = H, COOH, SO3H).
The total cation exchange capacity of catex B has been determined experimentally by the following procedure. The volume of 4 cm3 of swollen catex was rinsed with the excess of sodium chloride solution to get a resin with sodium cations at all cation exchange sites. Afterwards, all unbound sodium cations were removed by rinsing with water. Next, the column was rinsed with a solution of acetic acid and all the effluent was collected in a 1 000 cm3 volumetric flask which was then filled with water to the mark (solution A). During this procedure, H+ ions were bound to all weak exchange sites and to some strong exchange sites. Subsequently, the column was rinsed with water to remove the excess acetic acid. In the next step, the column was rinsed with a neutral solution of MgSO4 and all the effluent was collected in a 500 cm3 volumetric flask which was then filled with water to the mark (solution B). In this case, Mg2+ was bound to all strong exchange sites.
The amount of sodium ions in 100 cm3 of solution A was determined by direct potentiometry with a sodium ion-selective electrode (ISE); the measured potential was E1 = −0.2313 V, whereas the corresponding potentials of the electrode in the solutions of c(Na+) = 10.0 mmol dm−3 and c(Na+) = 0.100 mmol dm−3 were E2 = −0.2283 V and E3 = −0.3466 V, respectively.
The amount of sodium ions in 100 cm3 of solution B was determined by the same procedure as described above. In this case, the measured potential of the electrode in solution B was E4 = −0.2534 V. The concentration of hydrogen ions in 100 cm3 of solution B was determined by alkalimetry. The volume of sodium hydroxide of c = 0.1000 mol dm−3 at the equivalence point was 12.50 cm3.
All potentials were measured at a temperature of 298 K.
Hint: For the determination of sodium ion concentration use the equation E = k + S log10[Na+], where E is the potential of ISE and k and S are constants.
Calculate the ion exchange capacities of the catex, QV, which correspond to sulfonyl and carboxyl ion exchange groups, respectively. Provide the results in mmol cm−3.
Model Answer
At the beginning, all cation exchange sites are occupied with Na+ ions. Weak acetic acid exchanges all the weakly bound Na+ ions (weak cation exchange sites) and some of the strongly bound Na+ ions (strong cation exchange sites). The amount of Na+ in solution A is n1. When the resin is rinsed with a neutral solution of Mg2+ ions, all ions at the strong cation exchange sites are exchanged for Mg2+. Thus, solution B contains n2 moles of Na+ and n3 moles of H+.
The electrode potential is linearly proportional to the logarithm of concentration; i.e. for sodium ion selective electrode E = k + S log10[Na+]. Based on a two-point calibration, we get the following equations
−0.2283 = k + S log(0.0100) and −0.3466 = k + S log(0.00010)
Solving the system of equations, we get k = −0.1100 V and S = 0.05915 V.
The amounts of Na+ ions in solutions A (VA = 1 000 cm3) and B (VB = 500 cm3) are
The alkalimetric titration is based on 1:1 stoichiometry of the reaction of OH− (titration agent) and H+ (titrant). Then amount of H+ ions in solution B (Va is an aliquot of 100 cm3) is
Ion exchange capacities of the strong and weak ion exchange resins (V0 = 4 cm3)
Calculate total ion exchange capacity, QV,total, in mmol cm−3.
Model Answer
The total ion exchange capacity is
QV,total = QV,SO3H + QV,COOH = 2.033 + 0.662 = 2.695 mmol cm-3