238
FEDOTOV et al.
PD, mV
faced with a computer, using ion-selective electrodes.
Graphical information was continuously displayed.
The ion activities varied appreciably during 5–15 min
of contact. Then, the system reached equilibrium and
the curve was saturated. The error in the chloride ion
activity was 10%. The error in the potassium ion activ-
ity was 10% for high salt concentrations and 20% for
low salt concentrations.
The test soil specimens were prepared by adding salt
solutions of different concentrations to air-dry soils
until the water content of the soil was 0.8–0.9 of the
least water capacity and by exposing the watered soils
for several weeks.
27
18
9
1, 2
3
4
5
0
20 40 60 80 100 120 140 160
Time, s
To confirm our suggestion, we measured the potas-
sium and chloride ion activities in various soils as a
function of potassium chloride concentration (Fig. 2).
One can see from Fig. 2 that the chloride ion activity is,
in all cases, higher than the potassium ion activity. The
actual difference should be even more significant since
the reference electrode, positioned in a soil, has a
higher negative potential than the potential it would
have had if positioned in the solution (the Pallmann–
Loosjes effect [13]). As a consequence, the anion activ-
ities determined in this manner are lower than actual
values, and the cation activities, higher.
When water moves through such a system, ions with
increased activities, i.e., anions, should be displaced to
a greater extent to the lower soil layers, which is appar-
ently the reason for the appearance of the percolation
potential of the colloidal solution. In fact, we are deal-
ing with the potential that appears because the anions
and cations have different activities in front of and
behind the moving liquid front.
This mechanism interprets the experimental results
on the watering of soils by salt solutions. When the
solution concentration increases for salts built of an
alkali cation and a singly charged anion that does not
form insoluble compounds with soil cations, the OMG
is contracted, with the salts leaving the OMG for the
solution. The cation and anion activities are equalized
as a result. In addition, the existence of salts in the solu-
tion per se is a factor equalizing the activities. As a
result, the power of the excess anion activities is
reduced and vanishes with increasing salt concentra-
tion. In this case, the potential difference appearing
upon percolation is only due to the flow potential [3].
When the concentration increases for a salt built of
an alkaline-earth cation and a singly charged cation that
does not form insoluble compounds with the soil cat-
ions, the OMG is contracted with salt expulsion but,
because of the high adsorption capacity of the cations,
the excess anion activities dominate over the flow
potential [3].
Fig. 1. Potential difference (PD) between electrodes as a
function of time elapsed from watering of the greenhouse
substrate by distilled water depending on the burial depth of
the electrodes in the soil: (1) 5–30, (2) 5–20, (3) 5–15,
(4) 5–10, and (5) 15–30 cm.
soil surface by means of standard silver/silver chloride
electrodes that were connected to the soil samples
through agar-thickened bridges filled with a saturated
potassium chloride solution. A Mastech M890 digital
multimeter with an internal resistance of 10 MΩ was
used to measure the potential difference between the
electrodes. The error was no higher than 10% of the
value measured.
The experiments performed on the greenhouse sub-
strate (Fig. 1) did not confirm the decisive influence of
peptization on the appearance of electric fields upon
unsteady-state percolation. The potential jump between
the electrodes is observed even when the moving water
front has not yet reached the lower electrode. There-
fore, the charge separation occurs at the moving liquid
front.
The last mechanism to suggest is based on the dis-
placement of negatively charged particles in the water
movement direction. In view of the enormous differ-
ence between the mobilities of colloidal particles and
ions, it is pertinent to first dwell on anions.
This contradicts the common approach since soils
are negatively charged colloidal systems with cations as
counterions, which should be captured by the moving
water, thus creating the flow potential. However,
because soils cannot be regarded as ideal colloidal sys-
tems, this is only a seeming contradiction. Soils contain
nonleachable salts, as opposed to the classical
approaches employed to study the flow potential [12].
A soil is a far more complex colloidal entity in which
single colloidal particles interact with each other form-
ing a gel structure that can absorb salts.
We may suggest that, due to negative adsorption
[10], the anions in salt-containing soils should have
higher activities than the cations.
When the concentration increases for a salt built of an
alkali cation and a singly charged anion that forms insol-
uble compounds with soil cations, the anions decrease
The potassium and chloride ion activities in the soils their activities dramatically and their effect vanishes very
were measured on an Akvilon I-500 ionometer, inter- rapidly, changing to the flow potential [2, 3].
DOKLADY CHEMISTRY Vol. 399 Part 2 2004
Gazizov
