7.13.3 Ion Exchange

Use of ion exchange in producing de-ionised water and in the determination of the total cation concentration of a solution such as a sea-water.

An ion exchange resin is a synthetic, polymeric material with a 3-dimensional network structure containing ionic units, which can be exchanged with ions in a surrounding aqueous solution. Many common resins are prepared from polystyrene and are usually in the form of small beads.

De-ionisation of tap water

This process can be done either as a two-stage or one-stage process.

Two-stage Process

The tap water is first passed through a column containing a CATION-EXCHANGER, which is a resin containing an acidic group such as –SO₃H or –COOH. Any impurity cations in the water are replaced by H⁺ ions.

2R-H + Ca²⁺ (aq) � R₂-Ca + 2H⁺ (aq)

The high concentration of Ca²⁺ ions in the tap water results in the equilibrium lying well to the right.

The water is next passed through an ANION EXCHANGER, which is a resin containing a basic group such as amino (-NH2) or quaternary ammonium group (R4N+). Any anions in the water are replaced by OH- ions.

R-OH + Cl^- (aq) � R-Cl + OH^- (aq)

Again, the equilibrium lies very much to the right.

The H⁺ and OH^- ions then react to give neutral water molecules.

H⁺ (aq) + OH^- (aq) � H₂O (l)

Highly pure, de-ionised water, comparable with distilled water is produced.

When the resins are exhausted they can be regenerated and used repeatedly. The cation exchanger by treatment with acid and the anion exchanger by treatment with alkali (i.e. the reverse of the above equilibrium reactions).

One-stage Process

The tap water passes down a single column containing a mixture of cation and anion exchangers. Regeneration is more difficult in this case, as the exchangers have to be separated.

Any ideas of how you could make it relatively easy to separate the two exchangers for regeneration?

This is generally facilitated by having the 2 exchangers of different densities.

Total Cation Content of Sea-Water

The total cations in sea-water can be determined by ion exchange using a cation exchange resin.

To obtain a sample suitable for analysis it is advisable to dilute the sea-water by a factor of 10.

20 cm³ of the 10 x diluted sea-water is added to the column and allowed to drain down before being washed through with successive additions of water, using about 100 cm³ in all.

The cations are replaced by H⁺ ions.

nR-H + Catⁿ⁺ R_n-Cat + nH⁺ (aq)

The effluent and washings are collected in a 250 cm3 conical flask and titrated with standard (e.g. 0.1M) sodium carbonate solution using methyl orange as indicator.

2H⁺ (aq) + CO₃^2- (aq) CO₂ (g) + H₂O (l)

From the volume of sodium carbonate solution required the total cation content of the original sea water sample can be calculated.