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Water Softening & Ion Exchange Flashcards

7 cards from real CWS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Water Softening & Ion Exchange flashcards as text

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

    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.

  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:

    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⁺.

  3. A mixed-bed deionizer combines which two resin types in a single vessel?

    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.

  4. Weak acid cation (WAC) resin is most effective for treating water that contains:

    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.

  5. What is the Donnan exclusion effect in ion exchange?

    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.

  6. For a typical strong acid cation resin in sodium (softening) service, the correct order of selectivity from highest to lowest is:

    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.

  7. Crosslinkage percentage (DVB content) of ion exchange resin primarily affects which resin property?

    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.