Ion-exchange resins
The concentration of unreactive cations such as Na+ and K+ cannot be determined directly by titration.
One method for determining the total concentration of cation(s) in a solution is to pass the solution through the beads of a cation-exchange resin (see at right). This results in the cations being taken up by the resin by exchanging for H3O+, which can then be titrated.
One type of cation exchange resin is cross-linked polystyrene with sulfonic acid groups attached.
While the resin is not soluble in water, it is permeable to water. Sulfonic acids are strong acids, and release H3O+ from the resin to the water surrounding the beads.
Re-SO3H + H2O → Res-SO3– + H3O+
The H3O+ may move in the solution around the resin and may be replaced by another cation, provided charge balance is maintained (M2+ gives 2H3O+).
One method for determining the total concentration of cation(s) in a solution is to pass the solution through the beads of a cation-exchange resin (see at right). This results in the cations being taken up by the resin by exchanging for H3O+, which can then be titrated.
One type of cation exchange resin is cross-linked polystyrene with sulfonic acid groups attached.
While the resin is not soluble in water, it is permeable to water. Sulfonic acids are strong acids, and release H3O+ from the resin to the water surrounding the beads.
Re-SO3H + H2O → Res-SO3– + H3O+
The H3O+ may move in the solution around the resin and may be replaced by another cation, provided charge balance is maintained (M2+ gives 2H3O+).
| Res-SO3– H3O+ resin | + | M+ solution | M+(aq) solution  regenerate with H+(aq) | Res-SO3– M+ resin | + | H3O+ solution |