Chemistry 222 - Lecture Notes (Part 6)

Professor Ralph Allen

Chromatography -- heterogeneous equilibria -- 4 types

• 1) solubility
• 2) adsorption on a solid
• 3) ion exchange
• 4) diffusion and pore penetration

Ex-Na+ + H+ = Ex-H+ + Na+

       [Na+]s         DNa+
DNa = ________   B = ______
       [Na+]Ex        DH+

separation factor

divinylbenzene (cross linker) CH - CH2 - CH - CH2 - CH - CH2
polystyrene copolymer R Ch2 - CH - CH2 R R

2ExCl- + SO4-2 = Ex2SO4-2 + 2 Cl- anions

Ion exchanges occur naturally - clay and soil - zeolites

Developed synthetic resins which could be controlled and resistant to chemical attack. (cellulose, dextron)

• 1) Strongly acid cationic - resembles a strong acid (i.e., completely ionized over a large pH range) - capacity is not a function of pH.

2ExSO3-H+ + CaCl2 = (ExSO3-)2Ca+2 + 2HCl

• 2) Weakly acid cationic - Carboxylic acid - COOH, phenolic hydroxyl groups - OH, phosphonic (stronger) - PO(OH)2

(pH > 7) - Exchange rate and capacity increase with pH

2EX - COOH + CaCl2 ---> no reaction (salt of strong acid)

2EXCOOH + Ca(OAc)2 (salt of weak acid)

Can also have exchanges with particular preferences for some metal which forms strong chelated complexes.

• 3) Weak base anionic - Prim., sec., tert. amino groups

• 4) Strong base anionic - quaternary ammonium groups

Example: ExH+ + Na+ = ExNa+ + H+

Position of equil. and rate of reaction depend on various factors.

Resin is like a drop of concentrated electrolyte - we get diffusion

• 1) diffusion of ion in soln. up to exchanger surface
• 2) diffusion of ion into bead
• 3) exchange reaction (usually fast)
• 4) diffusion of exchanged ion away from interface

				    (aExNa)(aH+)
Equil. constant for reaction   K = _______________
				    (aNa+)(aExH+)

Besides using as we are it is possible to separate ions from each on the basis of how strongly they interact.

SELECTIVITY -- hard to define as it varies with resin

• 1) Ionic size - absorption strength increases with size of hydrated ion if charge same.

Al3+ < Sc3+ < Y3+ < Eu3+ < Sm3+ < La3+

• 2) Cationic charge - affinity increases with charge +1 < +2 < +3
• 3) Dissociation - incomplete ionization of salt decreases affinity

Amino acids contain acid and base groups so depending upon pH will interact.

• a) Cross linked polymer - 4 different types based on functional groups (use anion for metals)
• b) Extent of reaction depends upon conc. gradients but there is also a kinetic effect.
• c) How things move through column - retention does depend upon flow rate - diffusion into pores more efficient with slower, at least to a point (could go backwards)
• d) Separation of components -- selectivity depends upon size, charge, degree of ionization (reaction). Regeneration of resin.
• e) Uncharged or opp. charged species are also retained to some extent.

• a) Solid support - usually don't want it to interact by adsorption
• b) liquid phase large choice - usually sep. depend upon relative polarity
• c) speed - if you try to go too fast -- diffusion into pores

• a) New controlled surface area beads. Basis for HPLC
• b) Inject ---> pump Column ---> detector

• c) detectors --- Refractive Index or UV absorption

Ca+2aq + 2H+res = Ca+2res + 2H+aq

Under many situations one ion predominates in both phases

[Ca]aq < < [H+]aq [Ca]res < < [H+]res in dilute but acidic soln.

therefore [H+] is constant in both phases 

     [H+]aq^2 [Ca+2]res
K = ____________________
     [H+]res^2 [Ca+2]aq

	        [Ca+2]res 
therefore KD = ___________ = K
	        [Ca+2]aq

which is analogous to extraction equilibrium 

Sulfonated cation exchange resins KD decreases in order

Ag+ > Ca+ > Rb+ > K+ > NH4+ > Na+, H+ > Li+ > Ba+2 > Pb+2 > Sr+2 > Ca+2 > Ni+2 > Cd+2 > Cu+2 > Co+2 > Zn+2 > Mg+2 > UO2+2

End Section 6 of Professor Allen's Notes