Ion exchange resins are porous materials, typically used in the form of small beads, with surface fu — Analytical Chemistry Chemistry Question
Ion exchange capacity of a cation exchange resin
Ion exchange resins are porous materials, typically used in the form of small beads, with surface functional groups capable of ion exchange. As the binding of ions from a solution occurs, other ions are released from the resin. For example, the binding of cations from a sample of sea water is accompanied by the release of the corresponding amount of hydrogen ions originally bound to the sulfonyl acid groups on the surface of a cation exchange resin.
Let us have a look at the following cation exchange resin – catex A.
Since n is very large, the terminal hydrogen atoms can be neglected in the following calculations.
Calculate the mass percentage of sulfur and carbon.
Model Answer
The molecular formula of one unit of the catex polymer is C17H16O5S1, which corresponds to the molecular weight of 332.369 g mol−1. Mass percentage of an atom wx is
where ax and Ax are the number of atoms and the atomic weight of an atom X, respectively. M is the molecular weight of one unit of the catex polymer.
For sulfur (aS = 1, AS = 32.06 g mol−1) and carbon (aC = 17, AC = 12.011 g mol−1), the mass percentage is wS = 9.65% and wC = 61.43%, respectively.
Calculate the theoretical ion exchange capacities Qm given separately by SO3H groups (a strong catex) and COOH groups (a weak catex) in mmol g-1 of the dry catex.
Model Answer
The theoretical ion exchange capacity is the amount of exchange groups in one unit of the catex polymer per mass of the unit, i.e.
For -SO3H (one ion exchange group, aSO3H = 1) and -COOH (one ion exchange group aCOOH = 1) we get Qm,SO3H = Qm,COOH = 3.01 mmol g−1.
Calculate the total theoretical ion exchange capacity, Qm,total, in mmol g−1.
Model Answer
The total ion exchange capacity is a sum of individual strong and weak exchange capacities. For Qm,SO3H = Qm,COOH = 3.01 mmol g−1 we get Qm,total = 6.02 mmol g−1.
Frequently, ion-exchange resins become swollen when hydrated, i.e. the volume of the beads changes significantly due to the hydration of highly polar ion-exchange functional groups.
Calculate the total ion exchange capacity, QV,total, in mmol cm−3 of a swollen resin. The ratio of void volume to the total volume is ε = 0.48, the density of the swollen resin is ρ = 1.28 g cm-3 and the mass ratio of water bound to a resin is w = 0.45.
Model Answer
The total ion exchange capacity in mmol cm−3 of a swollen resin QV,total is
QV,total = Qm,total (1 − ε) ρ (1 − w)
where ε and ρ are porosity and density, respectively, of a swollen resin and w is the mass ratio of water bound to the resin. For Qm,total = 6.02 mmol g−1, ε = 0.48, ρ = 1.28 g cm−3, and w = 0.45 we get QV,total = 6.02 × (1 − 0.48) × 1.28 × (1 − 0.45) = 2.20 mmol cm−3.