Experiment Station Record, Volume 94, January-June, 1946 Page: 16
xi, 975 p. ; 24 cm.View a full description of this book.
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16 EXPERIMENT STATION RECORD [Vol.'94
Theories of base-exchange equilibriums, L. E. DAVIS. (Univ. Calif.). (Soil
Sci., 59 (1945), No. 5, pp. 379-395).-The author reviews briefly theories of various
investigators with respect to base-exchange equilibria, and discusses these topics:
Simple statistics of base-exchange equilibria; ions of equal charge (Donnan equilibria
and ionic concentrations, Donnan equilibria and ionic activities, and approximate
character of base-exchange equations for ions of the same charge); and
ions of unequal charge.
The significance of the concept of base-exchange equilibria is critically analyzed.
Although the relation between the ions assumed to be on the colloid and those not
on the colloid (in the soil solution or in a filtrate) is usually supposed to involve a
chemical equilibrium which can be expressed by an equation of the mass-action
law type, the author presents evidence to show that "these ideas are incompletely
defined and [the] concept of base-exchange equilibrium as a true thermodynamic
equilibrium is not entirely valid." Equilibria between colloidal suspensions containing
salts of two cations and filtrates obtained from such suspensions are discussed
theoretically. The equilibria are shown as Donnan equilibria. The treatment
is based upon the application of Boltzmann's principle to equilibria between
very small volume elements in the suspension and in the filtrate. The equilibrium
relations between the two complete phases are then derived. On theoretical grounds,
mass-action law equations for the Donnan equilibria should not yield equilibrium
values that are constant when the variables are concentrations, but solely when the
variables are activities. These Donnan systems involve true equilibria between two
phases, the preparation of which has involved a base exchange. However, these
equilibria are not identical with the usual concept of base-exchange equilibria. The
author gives reasons for expecting that "equilibrium constants," with approximately
constant values, may be found, theoretically, for equations which are formally of
the mass-action law type and which express the base-exchange relation between
ionic concentrations for ions of the same charge. Four base-exchange equations
for the case of ions of unequal charge are discussed. The validity of each equation,
as judged by the constancy of the equilibrium constant, varies with concentration
of electrolyte and colloid in the suspension. A theoretical explanation of these
facts is developed. The significance of this explanation is indicated by calculations
based on data presented by various authors.
Base-exchange-pH relationships in semiarid soils, W. T. MCGEORGE. (Ariz.
Expt. Sta.). (Soil Sci., 59 (1945), No. 4, pp. 271-275, illus. 2).-Since adsorbed
sodium contributes most to the high pH values in semiarid soils, the investigation
was confined to this base.
The pH of the soil was found to increase with increase in the percentage sodium
in the complex, the relation between the two values being significant and linear.
It was observed further that when the base-adsorbing complex of semiarid soils
is completely saturated with sodium, all soils have nearly the same hydrolytic pTH
at the 1 : 10 soil : water ratio regardless of the milliequivalents adsorption capacity
per 100 gm. of soil. When the pH determinations are made on a soil paste, at the
moisture equivalent, the pH decreases with increase in adsorbing capacity, and the
correlation is highly significant and linear. Adsorbed sodium was increased by
treating the soils with solutions of sodium salts in which the pH is increased, but
all the sodium-saturated soils, regardless of whether they were saturated at pH 7,
9, or 10, had the same pH value at the 1 : 10 soil : water ratio.
The determination and interpretation of soil pH values, W. T. McGEORGE
(Arizona Sta. Tech. Bul. 104 (1944), pp. 367-426, illus. 21).-Calcareous semiarid
soils have pH values higher than those of noncalcareous soils, but there was no relation
between the percentage of CaCOs and the pH in the calcareous soils. Soil pH
values were depressed by increasing salinity, but there was no correlation between
total salinity and pH values when the analyses of a large number of soils were corm
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U.S. Department of Agriculture. Agricultural Research Administration. Office of Experiment Stations. Experiment Station Record, Volume 94, January-June, 1946, book, 1947; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc5062/m1/27/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.