CWS Water Softening & Ion Exchange 3

Question 1

Strong acid cation (SAC) exchange resin contains which functional group responsible for ion exchange?

Accepted Answer

Sulfonic acid groups (–SO₃H)

Answer

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:

Accepted Answer

Divalent ions form stronger electrostatic bonds with the negatively charged resin matrix

Answer

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?

Accepted Answer

Strong acid cation resin and strong base anion resin

Answer

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:

Accepted Answer

High sodium bicarbonate alkalinity with associated hardness

Answer

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?

Accepted Answer

The tendency of resin beads to exclude ions of the same charge as the resin's fixed groups

Answer

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.

(CWS) Certified Water Specialist Practice Test

(CWS) Certified Water Specialist Practice Test

CWS Water Softening & Ion Exchange 3