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

68 4 Water Permeability
potentials were highest, but did not quite reach the Nernst potential of 17.8 mV,
hence they were not perfectly permselective for cations. Under natural condition
the pH at the surfaces of the cuticles will be lower, and exclusion of anions will
be far from complete. This means that diffusion of salt can take place, and this
has been confirmed experimentally. Self-diffusion of NaBr across Citrus MX membranes
was studied at pH 3 and 8.5 at 25 ◦ C, and Na + and Br − permeances were
calculated (Schönherr and Huber 1977). The concentration of NaBr and pH was the
same on both sides of the membrane (4 × 10 −3 moll −1 ), which implies that there
was no net driving force. Radio-labelled ions were used ( 24 Na + and 82 Br − ), and
the fluxes of the ions were not coupled and both ions could diffuse independently.
At pH 3 the ratio of the permeances of Na + /Br − ranged from 0.58 to 0.69, showing
that permeance for Br − was higher because the membranes carried a net positive
charge and co-ion (Na + ) diffusion was reduced by relative Donnan exclusion. At pH
8.5 the ratio of permeances ranged from 3.83 to 4.39, because the membranes were
negatively charged and now Br − experienced Donnan exclusion. Thus, depending
on pH, cuticles are either cation or anion exchangers. The nature and concentration
of the fixed charges will be dealt with next.
4.3 Ion Exchange Capacity
The concentration of charges covalently bound to the polymer matrix can be determined
by potentiometric titration, as can be done with soluble electrolytes. The
main difference is the fact that it takes longer to obtain equilibrium, because ions
must diffuse into the polymer matrix, where diffusion coefficients are much smaller
than in water and mixing is absent. The progressive batch method can be employed
(Schönherr and Bukovac 1973). Small amounts (200 mg) of isolated CM or MX
are weighed into glass tubes, and a constant volume of degassed water or salt solutions
are added. To determine cation exchange capacity, increasing amounts of base
is added under a stream on nitrogen, the vessels are closed airtight and agitated at
constant temperature (25◦C). After 1–4 days, equilibrium is obtained. The pH of an
aliquot from the supernatant is determined under a stream of nitrogen. If pH deviates
significantly from 7 the supernatant is titrated back to pH 7 with standard acid
or base, to determine the amount of H + released or base that was not used up in
ion exchange. The reaction is stoichiometric and the equilibrium condition can be
written as
RH+ NaOH ⇄ RNa+H + + OH − ,
H + + OH − (4.4)
⇄ H2O,
where R − is the polyanion and barred quantities refer to the polymer phase. The
exchange capacity at a given pH is calculated from the amount of base added, minus
the amount of base left at equilibrium. If a solution of a neutral salt is added to the
cuticle, some ion exchange takes place and the supernatant becomes acidic:
RH + NaCl ⇄ RNa + H + + Cl − . (4.5)