Determination of Cation Exchange Resin Capacity

There are two main types of capacity measured for cation exchange resins:

1. Total (Theoretical) Exchange Capacity

This is the maximum number of exchange sites per unit weight or volume of resin (usually expressed in meq/g or eq/L).

Standard Laboratory Method (Acid-Base Titration)

Reagents Needed:

• 1 M NaOH
• 1 M HCl
• 5% NaCl solution
• Phenolphthalein indicator

Procedure:

1. Weigh accurately about 1.0 g of wet resin (or 0.5–1.0 g dry resin) in H⁺ form.
2. Convert the resin completely to Na⁺ form:
   • Pass 200–300 mL of 5% NaCl solution through the resin column slowly (or soak overnight with stirring).
   • Wash with distilled water until the washings are chloride-free (test with AgNO₃).
3. Regenerate the resin back to H⁺ form:
   • Pass 200 mL of 1 M HCl slowly through the column.
   • Collect all effluent + washings in a 500 mL volumetric flask.
   • Wash the resin with distilled water until neutral (≈300–400 mL total volume).
4. Make up the collected solution to 500 mL with distilled water.
5. Take 50 mL aliquot and titrate with 0.1 M NaOH using phenolphthalein indicator.

Calculation:

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

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

Note: For dry resin, capacity is reported as meq/g dry. For wet resin, convert using moisture content or report as meq/mL settled bed volume.

2. Operating (Breakthrough) Capacity

This is the actual usable capacity under real service conditions.

Method:

1. Pack a known volume of resin (e.g., 100 mL) in a column.
2. Pass a solution containing known concentration of cations (e.g., hard water or CaCl₂ + MgCl₂) at a fixed flow rate.
3. Collect effluent fractions and monitor cation concentration (or hardness) until breakthrough occurs (when effluent concentration reaches ~1–5% of influent).
4. Continue until complete exhaustion (effluent = influent concentration).

Calculation:

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

Where:
C₀ = influent cation concentration (eq/L)
Vₑ = volume of water treated until exhaustion (L)
Vᵣ = resin bed volume (L)

Summary Table

Type	Method		Units	Typical Value (Strong Acid Cation Resin)
Total Capacity	Regeneration with HCl → titration	meq/g or eq/L	4.5–5.5 eq/L (wet)
Operating Capacity	Column breakthrough test	eq/L or kg/m³		1.5–2.5 eq/L (depends on regenerant level)

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