Ion exchangers can be employed to adsorb and separate cations and anions. They can be prepared from — Organic Chemistry Chemistry Question
Ion Exchangers
Ion exchangers can be employed to adsorb and separate cations and anions. They can be prepared from organic or inorganic materials. An organic, cationic ion exchanger can be synthesized by the polymerization of styrene / divinyl benzene followed by sulfonation with H2SO4, as shown in Scheme 1:
Cationic ion exchanger (denoted as R – H+ ) can be employed to adsorb the cations, M + , the chemical reaction and the equilibrium constant Kc as well as the distribution coefficient Kd can be expressed as follows:
R – H+ + M+ = RM + H+ , Kc = [RM][H+] / ([M+][RH]) (1)
Kd = [RM] / [M+] (2)
The cationic ion exchanger R – H+ can be transformed into the ion exchanger R– M+ or R – Mz+ by the reaction of R– H+ with a metal hydroxide (M(OH)z). The approximate equations are:
R – H+ + MOH = R– M+ + H2O (3)
and z R – H+ + M(OH)z = (R– )zM+ + z H2O (4)
A cationic ion exchanger R – Na+ was employed to remove CaCl2 in tap water, Give the chemical equation for the adsorption of Ca 2+ by the cationic ion exchanger R – Na+ .
Model Answer
2 R – Na+ + Ca2+ = 2 R– Ca2+ + 2 Na+
or 2 R – Na+ + CaCl2 = (R– )2Ca2+ + 2 NaCl
Tell how the removal of H+ from a solution of HCl can be achieved with an anionic ion exchanger if another ion exchanger R – H+ is employed instead of R– Na+ . Give the chemical equation for the process.
(a) Give the chemical equation for the adsorption of Ca 2+ by the ion exchanger R – H + .
(b) Tell which ion exchanger, R – H+ or R– Na+ , is suitable for drinking purpose and give the reason.
Model Answer
a) The tap water contains trace HCl after the adsorption of Ca2+ by the ion exchanger R – H+ and the chemical equation of the adsorption is:
2 R – H+ + Ca2+ (R– )2Ca2+ + 2 H+
or 2 R – H+ + CaCl2 (R– )2Ca2+ + 2 HCl
b) R– Na+ is suitable for drinking purpose because the product of the adsorption of Ca2+ by R – Na+ is Na+ or NaCl while the product is H+ or HCl after the adsorption of Ca2+ by R – H+.
An organic, anionic ion exchanger (denoted as R + Cl– ) can also be synthesized by the polymerization of styrene / divinyl benzene followed by the reaction of the resulting polymer, poly (styrene / divinyl benzene), with the Lewis acid AlCl3 and tertiary amine NR3, as shown in Scheme 2:
The anionic ion exchanger R + OH– can be obtained from the chemical reaction of the ion exchanger R + Cl– with a solution of NaOH (c = 3.0 mol dm-3 ) by the equation:
R + Cl – + NaOH R+ OH– + NaCl (5)
Tell how the removal of H + from a solution of HCl can be achieved with an anionic ion exchanger and give the chemical equation for the process.
Model Answer
The removal of H+ can be achieved by using the anionic ion exchanger R+OH– with the equation:
R + OH– + HCl R+ Cl– + H2O
Tell how the amount of SO4 2– in tap water can be estimated by using an anionic ion exchanger R + OH– . Give all of the chemical equations involved in the process.
Model Answer
Firstly, the anionic ion exchanger R + OH– is used to adsorb the SO4 2– ion with the equation :
2 R+OH – + SO4 2– (R+)2SO4 2– + 2 OH–
Secondly, a standard solution of HCl can be used for the titration of the OH – in the solution after adsorption of SO4 2– by the anionic ion exchanger R+OH– .
H + + OH– H2O (acid–base titration)
The capacity (S) of the cationic ion exchanger R – H+ for an adsorbed ion can be expressed in moles of the adsorbed ion per gram of the ion exchanger in 1.0 cm 3 of aqueous solution and can be calculated by using the following equation:
S = ([RM] + [RH]) × 10 –3 (6)
The capacity (S) of the cationic ion exchanger R – H+ for M+ ions in an aqueous solution can be estimated from the equilibrium constant Kc, the distribution coefficient Kd and the concentrations of M + and H+ ions in the aqueous solution.
Show that the relationship between Kd, S, Kc, [M + ] and [H+ ] as shown by the equation:
1 / Kd = [M+] / (S(103)) + [H+] / (S Kc(103)) (7)
Model Answer
R – H+ + M+ RM + H+ , Kc = [RM][H+] / ([M+][RH]) (3.1)
Kd = [RM] / [M+] (3.2)
S = ([RM] + [RH]) × 10 –3 (3.3)
We substitute Equations (3.1) and (3.2) into Equation (3.3) and obtain:
S = (Kd [M+] + [RM][H+] / Kc [M+]) × 10–3
= (Kd [M+] + Kd [M+] [H+] / Kc [M+]) × 10–3
= (Kd [M+] + Kd [H+] / Kc ) × 10–3
S Kc 10 3 = Kd Kc [M+] + Kd [H+]
1 / Kd = [M+] / (S(103)) + [H+] / (S Kc (103)) (3.4)
Ion exchangers can be employed as stationary phase materials in liquid chromato-graphy to adsorb and separate various ions. For example, the anionic ion exchanger R + OH– can be used to separate X– and Y– ions with the eluent NaOH. The chromatogram for separation of X – and Y – ions using a 30 cm of anionic ion exchange column is shown in Figure 1, where t1, t2 and to are the retention times (tR) for X – , Y– and the pure eluent (NaOH) to traverse the column, respectively. 1 and 2 are the peak–widths for X – and Y– .
Figure 1. Liquid Chromatogram for X– and Y– ions
The number of theoretical plates N and the plate height H (height equivalent of the theoretical plates) of the column can be estimated as shown below:
N = 16 (tR / ) 2 (8)
and H = L / N (9)
where L is the length of the column. The resolution (R) of the column and the separation factor () for X – and Y – also can be estimated using the following equations:
R = 2 (t2 – t1) / (1 + 2) (10)
and = (t2 – t0) / (t1 – t0) (11)
Calculate the average number of theoretical plates N of the column.
Model Answer
N1 = 16 (t1 / 1) 2 = 16 (10 / 1.0) 2 = 1600
N2 = 16 (t2 / 2) 2 = 16 (14 / 1.5) 2 = 1394
N = (N1 + N2) / 2 = (1600 + 1394) / 2 = 1497
Calculate the plate height H of the column.
Model Answer
H = L / N = 30 / 1497 = 0.021 cm
Calculate the resolution (R) of the column for X– and Y– ions.
Model Answer
R = 2 (t2 – t1) / (1 + 2) = 2 (14 – 10) / (1.0 + 1.5) = 3.2
Calculate the separation factor () for X– and Y– ions.
Model Answer
= (t2 – t0) / (t1 – t0) = (14 –1) / (10 – 1) = 1.44
Some ion exchangers are derived from inorganic matters. Zeolites [(M z+ )(Al2O3)m / (SiO2)n] (M z+ = Na + , K+ or Ca2+ , Mg2+ ) are the best known examples of inorganic ion exchangers. Some examples of Zeolites are shown in Figure 2.
A Na + –Zeolite (denoted as Z–Na+ ) with a pore size of 13 Å is an important ion exchanger for the removal of Ca 2+ or Mg 2+ ion from tap water. Zeolites with definite pore sizes also behave as highly selective adsorbents for various molecules, e.g. H2O and iso–butane. Thus, the zeolite can be used as a molecular sieve. The zeolite can also be used as a catalyst by adsorption of a petroleum component, e.g. iso–butane, in petroleum resulting in the enhancement of rate of the cracking of the adsorbed component.
Figure 2. Various types of Zeolites
Give the chemical equation for the removal of Ca 2+ ions from tap water with Z–Na + zeolite ion exchange column.
Model Answer
Z–Na+ + Ca2+ = Z–Ca2+ + Na+
Give the chemical equation for the adsorption of K + with Z–Na + zeolite.
Model Answer
Z–Na+ + K+ = Z–K+ + Na+