Dec. 10, 1913 
Selective Adsorption by Soils 
181 
Certain inactive solid substances presenting large surfaces have the 
power of taking salts from solution—that is, what is known as absorbing 
or adsorbing them, a phenomenon most logically explained at present as 
a concentrating of the solute at the surface of the adsorbing material. 
Qualitatively, it is known that certain of these inactive solid substances 
not only have the power of adsorbing a neutral salt from its solution as 
a whole, but may adsorb one ion more than the other, or selectively 
adsorb. In so doing, a partial hydrolysis of otherwise practically unhy¬ 
drolyzed salts is brought about, since the removal of one ion of the 
salt more or at a greater rate than the other takes an equivalent number 
of ions of opposite charge from the water and thus leaves an excess of 
either hydrogen or hydroxyl ions in the solution. That such is the case 
can be shown by the use of common indicators, after shaking solutions 
of neutral salts with or percolating them through certain of these inac¬ 
tive solid substances. 
These cases are so numerous that only a few of the best known and 
more convincing ones will be here recalled. 
A silver-nitrate solution shaken with animal charcoal and the super¬ 
natant liquid filtered and tested with methyl orange or litmus gives a 
distinct color of acid reaction. 
A potassium chlorid or nitrate solution shaken with cane-sugar char¬ 
coal and the supernatant liquid filtered and tested with phenolphthalein 
gives a strong red color of alkaline reaction. 
An interesting case of selective adsorption is to be found in our com¬ 
mon indicator, Congo red, and absorbent cotton. If the base of a column 
of absorbent cotton is immersed in a solution of Congo red made very 
slightly acid, in a very few minutes the cotton immediately above the 
solution is colored blue (acid reaction), while above the blue color for 
about an inch in height is seen the red color of neutral or alkaline reac¬ 
tion; above the red the cotton is wet with water. 
The soil possesses all the essential properties of these adsorbing mate¬ 
rials; but that it has the power of selectively adsorbing to any appre¬ 
ciable extent has for a long time been a question of dispute. The fact 
that a solution of a neutral salt after contact with a soil is as a rule dis¬ 
tinctly acid to indicators supports this hypothesis. 
If a soil in contact with a solution of potassium chlorid adsorbs potas¬ 
sium ions at a much greater rate or in greater proportion than chlorin ions, 
thereby (since an equivalent number of hydroxyl ions are also removed 
with the potassium ions) causing a partial hydrolysis of the solution 
(KC 1 + H 0 H=(K 0 H) adsorbed+ HC 1 ), then free hydrochloric acid will 
be left in the solution. 
It is not unreasonable to assume that the uncombined acid might dis¬ 
solve an almost equivalent amount of bases from the soil particles. On this 
assumption, by using a solution of a salt of potassium with a weaker acid 
than hydrochloric, there should be a greater adsorption of potassium ions, 
