Control of Ion exchange by pH Changing the pH can eliminate the charge of the column if the column is weak, or eliminate the charge on the ion if ion is weak. Either way, the retention is reduced. Strong ion - Weak Exchanger
Exchange Capacity of Anion Exchanges Exchange Capacity: Number of functional groups per unit weight of resin
Exchange Capacity of Anion Exchanges pH has no effect on capacity of strong cation exchanges. Weak cation exchanges change dramatically with pH.
Control of Ion exchange by Ionic Strength Sample ZoneBGE Zone A - + As the concentration of the eluent ion increases, retention tends to decrease
Control of Ion exchange by Eluent Ion cations anions
The equilibrium constant At pH=pKa 50% is ionised and 50% is neutral At pH=pKa +1 90% is ionised At pH=pKa -1 10% is ionised
Effect of pH Compare this separation at pH=4.55 and 0.05 in the previous example. Increasing pH reduced retention.
Effect of Temperature Increasing temperature increases efficiency, decreases k’, and may affect . This is due to improved mass transfer.
UV Detection UV detection. Direct detectionUV transparent eluent eg, bromide, nitrate, nitrite, thiocyanate, @214nm Indirect Detection UV absorbing electrolyte. Anions are detected via “vacancies” in background absorbance universal detection usually used when other modes of detection are unavailable. Very specific