C - Lublin

PRACTICAL REMARKS
The titration curve of a soild, and especially of the soil, may include free
acidic ions adsorbed on the surface (exchange acidity), which are not surface acidic
groups. Therefore there exist a danger of interpretation of exchange acidity in
terms of variable charge. To avoid the latter prior to the titration, the soil suspensions
should be exchange acidity depleted. This can be done by standard neutral
salt washing. We use fivefold centrifuging with excess of 1 mole dm -3 NaCl solution
of pH=3 that provides the lack of Al (ICP AES) in the supernatants. The
washed sample can be directly used for the titration, provided its mass is known.
Next 1 mole dm -3 NaCl suspension containing around 1.00g (dry mass) of the
sodium homoionic form of the solid is kept at pH≈2.9 by additions of small increments
of 1 mole dm -3 HCl for half an hour of mixing. During additional 3 min.
mixing the pH is controlled, and if changed by more than 0.02 unit, the suspension
is centrifuged and the pretreatment repeated. From the final suspension, half of the
supernatant (weighing) and the remaining suspension are titrated with 0.100 mole
dm -3 NaOH in 1 mole dm -3 NaCl solution with the rate of 30 µl/min. The amount of
the titer consumed between pH 3 to 9 is recorded with a step of 0.2 pH unit. The
measurements are performed in four replicates. Usually the deviation of the titration
curves is lower for the suspensions (around 5%) and higher for the supernatants
(around 15%). If washing of the solid and the pretreatment for the titration
are properly performed, the titration curves of the supernatants are similar to a titration
curve of 1 mole dm -3 NaCl pH=3 solution, which indicates also that the dissolution
of the solid phase under experimental conditions is negligible. This is important
to note that in the above procedure the equilibrium conditions are not reached,
so the titration curves can be used rather for comparative purposes. 24 hours equilibrium
titration of a soil consumes up to twice as much titer as the continuous
titration described here and a high dissolution of the solid phase occurs as is indicated
by high deviation of the titration curves of the supernatants from the titration
curve of NaCl solution (Jozefaciuk and Shin, 1996a).
Applying constant and high concentration of neural salt during titration has a
few advantages. It minimizes adsorption of exchange protons at low initial pH
value and their further titration (replacing exchange protons by neutral salt cations),
dilution effects (changes of variable charge with salt concentration), dissolution of
solid phase, and allows for a better development of variable surface charge (surface
groups dissociation is less hindered by electrostatic effects at high ionic strengths).
Some results of the application of the back-titration method in analysis of
various materials are presented in Józefaciuk (2002) for minerals, Józefaciuk and
Szatanik-Kloc (2002) for plant roots, Jozefaciuk et. al. (2002) for soils.
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