Determination of Anion Exchange Resin Capacity

Anion exchange resins are characterized by two main capacities:

• Total (Theoretical) Capacity – maximum number of exchange sites
• Operating (Working/Breakthrough) Capacity – usable capacity in real conditions

1. Total Exchange Capacity of Anion Exchange Resin

Standard Method for Strong Base Anion (SBA) Resins (Type I & Type II)

Principle

Convert resin to Cl⁻ form → displace Cl⁻ with excess Na₂SO₄ or NaNO₃ → titrate liberated Cl⁻ with AgNO₃.

Reagents

• 1 M NaOH
• 1 M HCl
• 5–10% Na₂SO₄ or NaNO₃ solution
• 0.1 M AgNO₃
• Potassium chromate indicator (K₂CrO₄)

Procedure

1. Weigh accurately 1.0 g of wet resin (or 0.5 g dry) in OH⁻ form.
2. Convert completely to Cl⁻ form:
   • Pass 200–300 mL of 5% NaCl or 1 M HCl slowly through a small column.
   • Wash with distilled water until effluent is chloride-free and neutral.
3. Regenerate to displace chloride:
   • Pass 200 mL of 10% Na₂SO₄ (or 1 M NaNO₃) very slowly (2–4 mL/min).
   • Collect effluent + washings in a 500 mL volumetric flask.
   • Wash with distilled water until total volume ≈ 500 mL.
4. Dilute to exactly 500 mL.
5. Take 50.0 mL aliquot and titrate with 0.1 M AgNO₃ using 5% K₂CrO₄ indicator (Mohr method) until brick-red end point.

Calculation

Total Capacity (meq/g) = (V × N × 10) / W

Where:
V = mL of AgNO₃ used
N = normality of AgNO₃
10 = dilution factor (500/50)
W = weight of resin (g)

Alternative for OH⁻ form resins: Convert resin to OH⁻ form with 1 M NaOH → pass excess 1 M NaCl → titrate the liberated OH⁻ in effluent with 0.1 M HCl using phenolphthalein.

For Weak Base Anion (WBA) Resins

Use acid titration:

1. Convert resin to free base (FB) form with 1 M NaOH and wash.
2. Pass excess 1 M HCl → collect effluent.
3. Titrate excess HCl with 0.1 M NaOH to find how much acid was consumed by the resin.

Capacity (meq/g) = (Initial HCl − Excess HCl) / weight of resin

2. Operating (Breakthrough) Capacity

Measured exactly like cation resins but with anions (e.g., chloride, sulfate, silica, or real water).

Typical Procedure

1. Pack 100 mL of resin (in Cl⁻ or OH⁻ form) in a column.
2. Pass feed water with known anion concentration (e.g., 500 mg/L as CaCO₃ alkalinity + chloride) at constant flow rate.
3. Monitor effluent for the target anion until breakthrough (usually when effluent reaches 1–5% of influent).
4. Continue until complete exhaustion.

Calculation

Operating Capacity (eq/L) = (C₀ × Vₑ) / Vᵣ

C₀ = influent anion concentration (eq/L)
Vₑ = volume treated until chosen endpoint (L)
Vᵣ = resin volume (L)

Summary of Typical Values

Resin Type	Total Capacity (eq/L wet)	Typical Operating Capacity (eq/L)
Strong Base Type I (gel)	1.2 – 1.4	0.6 – 1.0
Strong Base Type II (gel)	1.3 – 1.5	0.8 – 1.1
Strong Base Macroporous	0.9 – 1.2	0.6 – 0.9
Weak Base (gel/macroporous)	1.5 – 2.2	1.0 – 1.8

Important: Always specify the form (Cl⁻ or OH⁻) and whether capacity is reported on wet volume (eq/L), dry weight (meq/g dry), or as-received weight when comparing resins.

Related Topics:
Determination of Cation Exchange Resin Capacity
Mixed Bed Ion Exchange Resin Capacity
Mixed-Bed Deionization