Water and Solute Permeability of Plant Cuticles: Measurement and ...

4.3 Ion Exchange Capacity 69
The higher the affinity between R − and the cation, the lower is the resulting pH.
For instance, the affinity of the MX for Ca 2+ ions is very high, and the pH obtained
with CaCl2 is much lower as with NaCl.
The high affinity for Ca 2+ can be utilised to determine cation exchange capacity
(Schönherr and Huber 1977). A known amount of isolated CM is equilibrated in
buffered solutions containing 45 CaCl2 until equilibrium is obtained. Then, the CM
pieces are washed repeatedly with deionised water to remove adhering solution and
sorbed electrolyte (CaCl2). With Citrus MX four washes sufficed, and radioactivity
contained in cuticles no longer decreased when washing was continued. When
cuticles are dropped in 1 N HCl, the exchangeable Ca 2+ is released and can be
determined by scintillation counting. This method is simple and very accurate, and
it can be used with very small amounts of CM (