Ion Exchange Calculations

Ion Exchange Equilibrium

Simple stoichiometric reaction between sequestered and presaturant ions

Selectivity

Expression showing preference for ions based on concentration

Exchange Capacity

Total chemical equivalents available for exchange per unit weight of resin

Strong Acid Cationic Resin

Resin dissociates completely in ionic form at any pH

Weak Acid Cationic Resin

Resin with pKa 4-5, proton donation at pH >6

Strong Base Anion Type 1 Resin

Resin with low pKb, readily gives up hydroxide ion

Strong Base Anion Type 2 Resin

Resin with low pKb, readily gives up hydroxide ion

Weak Base Anion Resin

Resin with pKb values implying easy dissociation of hydroxide ion

Separation Factor

Measure of preference for one ion over another during ion exchange

Binary Component Mixtures

Equilibrium expressed in equivalent fractions for ion exchange

Sequestered Ion

Ion attached to the resin in ion exchange

Presaturant Ion

Ion in the resin before exchange in ion exchange

Mol Frac

Molar fraction of ions in aqueous or resin phase

Concentration

Amount of ions in aqueous or resin phase

Separation Factor of Cation/Anion

Measure of preference for known ions during ion exchange

Binary Component Mixture Example

Calculation involving removal of nitrate from water with high chloride content

Maximum Volume of Water Treated

Estimate based on resin capacity for ion exchange

Ion Exchange Equilibrium

Simplified stoichiometric reaction involving valence, ionic group, sequestered ion, and presaturant ion.

Selectivity

Expression determining ion preference based on concentration, not activities.

Ion Exchange Selectivity

Manufacturer-provided coefficients influencing ion preference for resin.

Exchange Capacity

Total chemical equivalents available for exchange per unit weight or volume of resin.

Strong Acid Cationic Resin

Resin dissociates completely in ionic form over a wide pH range.

Weak Acid Cationic Resin

Resin with pKa values requiring pH greater than 6 to give up a proton.

Strong Base Anion Resin

Resin with low pKb values giving up hydroxide ions easily at pH < 13.

Weak Base Anion Resin

Resin with pKb values requiring higher pH to give up hydroxide ions.

Separation Factor

Measure of ion preference during ion exchange, expressed as equivalent fractions.

Binary Component Mixtures

Equilibrium expressed in equivalent fractions for ion exchange preference.

Sequestration Factor

Amount of sequestered ion in resin phase compared to aqueous phase.

Presaturation Factor

Amount of presaturant ion in resin phase compared to aqueous phase.

Maximum Volume of Water Treated

Estimation based on resin total capacity for ion removal from water.

Cations

positively charged ions (e.g. Na+ or Mg2+)

Ions

are electrically charged (positive or negative) atoms or molecules. Ions can be made of one atom only or several atoms linked together like molecules.

Anions

negatively charged ions (e.g. Cl- or NO3-)

Salts

crystallized substances with fixed proportion of cations and anions.

Counterion

Ion in solution that can exchange with an ion attached to a stationary functional group.

Ion exchange

Process in which ions attached to a stationary functional group exchange for ions in a solution. Ions are exchanged on an equivalence basis.

Presaturant ion

Ion that comes attached to the virgin resin or is exchanged onto the resin during the regeneration process.

Selectivity

Preference of one ion over another for exchange onto an ion exchange site on a resin.

Strong acid cation resin

Ion exchange resin that will readily give up a proton over a wide pH range.

Separation factor

Quantitative description of the preference of one ion over another for a given ion exchange resin.

Strong base anion resin

Ion exchange resin that will readily give up a hydroxide ion if the pH is less 13.

Total ion exchange capacity

Total amount of chemical equivalents available for exchange per unit weight or unit volume of resin.

Dark lines

polymeric skeleton of the resin bead: it is porous and contains water.