C - Lublin

DESCRIPTION OF VARIABLE CHARGE
FROM BACK TITRATION DATA
Józefaciuk G., Matyka-Sarzyńska D.
Back titration procedure (Duquette and Hendershot 1993, Józefaciuk and Shin
1996a, Nederlof et al., 1991, 1993) has been widely used for characterizing variable
surface charge of minerals, soils and plant roots. The titration is performed
separately for the suspension and for its equilibrium solution (supernatant), starting
from low pH upwards. The idea of this method is based on the assumption that the
hydroxylic ions of the base added to the suspension are consumed by acids present
in the equilibrium solution and by acidic functional groups present on the surface
of the suspended solid. Therefore after subtraction of the titration curve of the equilibrium
solution from this of the whole suspension, one obtains the titration curve
of the solid.
THEORY
The solid phase titration curve i.e. an amount (mole) of base consumed by the
suspension, N susp , minus the amount consumed by the supernatant, N sol , is treated as
an increase of the variable surface charge of the solid phase, ∆Q V (pH), during the
titration:
∆Q V (pH) = N susp - N sol , (1)
The ∆Q V (pH) accounts for an increase of negative charge and a decrease of
positive charge.
The variable charge vs. pH dependence can be interpreted in terms of proton
dissociation/association of surface functional groups using site-heterogeneity theory
(De Wit et al., 1990; Jozefaciuk and Shin, 1996b; Koopal et al., 1987), that is
briefly outlined below.
Assuming that the variable charge origins from dissociation of surface acidic
groups of kind i: SOH i = SO - i + H + s , (H s denotes a proton in a plane of dissociation)
and the (intrinsic) dissociation constants K i :
K i = [SO - i][H + s ]/[SOH i ], (2)
where the brackets denote surface activities, the variable charge at a given pH is:
n
-
Q V (pH) =∑ [SO i ](pH) = Niα i (K i ,pH s ), (3)
i=1
n
∑
i=
1
where α i (K i ,pH s )=[SO - i]/[SOH i ] is the degree of ionization of groups kind i and
N i =[SOH i ] is their amount. For any pH during the titration:
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