CWS Water Softening & Ion Exchange 3 — Questions and Answers
Question 1: Strong acid cation (SAC) exchange resin contains which functional group responsible for ion exchange?
- Carboxylic acid groups (–COOH)
- Quaternary ammonium groups (–N⁺R₃)
- Sulfonic acid groups (–SO₃H) (Correct answer)
- Phosphonic acid groups (–PO₃H₂)
Correct answer: Sulfonic acid groups (–SO₃H)
SAC resin carries sulfonic acid functional groups that remain fully ionized across the entire pH range, enabling cation exchange under all water conditions.
Question 2: In ion exchange selectivity, divalent cations such as calcium (Ca²⁺) are generally preferred over monovalent cations such as sodium (Na⁺) on strong acid cation resin because:
- Divalent ions are larger and physically fit the resin pores better
- Divalent ions form stronger electrostatic bonds with the negatively charged resin matrix (Correct answer)
- Monovalent ions react chemically with the resin functional groups
- Divalent ions have lower hydration energy and are less mobile in solution
Correct answer: Divalent ions form stronger electrostatic bonds with the negatively charged resin matrix
The higher charge density of divalent ions creates stronger electrostatic attraction to the fixed anionic sites on the resin, giving Ca²⁺ and Mg²⁺ higher selectivity coefficients than Na⁺.
Question 3: A mixed-bed deionizer combines which two resin types in a single vessel?
- Strong acid cation resin and weak acid cation resin
- Strong acid cation resin and strong base anion resin (Correct answer)
- Weak acid cation resin and weak base anion resin
- Strong base anion resin and activated carbon
Correct answer: Strong acid cation resin and strong base anion resin
Mixed-bed units blend SAC (hydrogen form) and SBA (hydroxide form) resins to simultaneously remove all cations and anions, producing very high-purity water.
Question 4: Weak acid cation (WAC) resin is most effective for treating water that contains:
- High sodium bicarbonate alkalinity with associated hardness (Correct answer)
- High chloride salinity with minimal alkalinity
- Significant silica and dissolved organics
- High iron and manganese with low hardness
Correct answer: High sodium bicarbonate alkalinity with associated hardness
WAC resin exchanges only cations associated with alkalinity (bicarbonate-balanced hardness), making it ideal for dealkalization while consuming less regenerant acid than SAC resin.
Question 5: What is the Donnan exclusion effect in ion exchange?
- The tendency of resin beads to exclude ions of the same charge as the resin's fixed groups (Correct answer)
- The physical exclusion of large molecules from entering resin pores
- The preferential removal of monovalent ions over divalent ions during regeneration
- The increase in resin capacity when operating at elevated temperatures
Correct answer: The tendency of resin beads to exclude ions of the same charge as the resin's fixed groups
Donnan exclusion causes co-ions (same charge as the fixed resin groups) to be repelled from the resin phase, improving selectivity and reducing leakage of unwanted ions.
Question 6: For a typical strong acid cation resin in sodium (softening) service, the correct order of selectivity from highest to lowest is:
- Na⁺ > Ca²⁺ > Mg²⁺ > H⁺
- Ca²⁺ > Mg²⁺ > Na⁺ > H⁺ (Correct answer)
- H⁺ > Ca²⁺ > Mg²⁺ > Na⁺
- Mg²⁺ > Ca²⁺ > H⁺ > Na⁺
Correct answer: Ca²⁺ > Mg²⁺ > Na⁺ > H⁺
SAC resin in the sodium cycle prefers divalent cations over monovalent ones, with Ca²⁺ > Mg²⁺ > Na⁺ > H⁺ being the standard selectivity sequence.
Question 7: Crosslinkage percentage (DVB content) of ion exchange resin primarily affects which resin property?
- The type of ions the resin will exchange
- Physical stability, swelling behavior, and resistance to osmotic shock (Correct answer)
- The color of the resin beads during regeneration
- The pH range over which the resin remains active
Correct answer: Physical stability, swelling behavior, and resistance to osmotic shock
Higher DVB crosslinkage produces a more rigid resin matrix that is more resistant to osmotic shock and physical attrition but has slightly lower exchange capacity.
CWS Water Softening & Ion Exchange 3
Strong acid cation (SAC) exchange resin contains which functional group responsible for ion exchange?