Q4(a). Ion-exchange capacity of an anion-exchange resin

R-N+(CH₃)₃ OH⁻ + HCl → R-N+(CH₃)₃ Cl⁻ + H₂O

1. Conditioning: wash the strong-base resin (R–N⁺Cl⁻) in a column with 1 M NaOH until the effluent has no Cl⁻ (AgNO₃ test), then with distilled CO₂-free water until neutral (pH 7). Dry at 105°C and weigh 1.000 g (or work with the drained resin and find its dry mass).
2. Exchange: add 50.0 mL of 0.1000 M HCl (excess) in a stoppered flask and shake or stir for 24 h so that every OH⁻ site is replaced by Cl⁻.
3. Titration: filter; titrate an aliquot (e.g. 10.0 mL) of the filtrate with 0.1000 M NaOH, methyl red or phenolphthalein indicator.
4. Calculation (specimen figures): 10.0 mL of filtrate needs 4.20 mL of NaOH, so unused HCl =5×4.20×0.1000=2.10 mmol; used =5.00-2.10=2.90 mmol; capacity =2.90 meq per 1.000 g =2.9 meq g⁻¹.
5. Alternative (column method): load the resin in the Cl⁻ form, elute all Cl⁻ with 1 M NaNO₃ (or Na₂SO₄), titrate the Cl⁻ in the eluate with AgNO₃ (Mohr). Capacity =n(Cl⁻)/m.

typical capacity: polystyrene–DVB strong-base resin 3–4 meq g⁻¹ (dry); the capacity fixes how much sample can be loaded on a column before breakthrough.