Oct. 16, 1916 
Acidity and Adsorption in Soils 
135 
When 0.1 mgm. of OH as calcium hydroxid is added to 50 c. c. of 
distilled water, there is produced an OH-ion concentration of 3.8 X io“ 5 
gram molecules per liter. JPo reach this same concentration of OH ions 
in the presence of the soil requires the addition of a much greater quan¬ 
tity of hydrate, whether of sodium, calcium, barium, or potassium. The 
difference between the quantity of hydrate necessary to add to distilled 
water to obtain the concentration fixed upon and that required when the 
soil is present gives an index of the amount of hydroxyl ions removed 
from the ionic equilibrium. In the case of acid soils it is probable that 
until the neutral point is reached the removal of OH ions can be accounted 
for by the reaction between the added OH ions and H ions derived from 
the soil acids. Beyond the neutral point, however, another type of 
reaction must necessarily be involved. For example, soil 16 required the 
addition of 1.0 mgm. of OH as calcium hydroxid per gram of soil to give 
the suspension an OH-ion concentration of 0.6 X io“ 7 , while the second 
point, representing a concentration of 4.2 X io~ 5 gram molecules .of OH ions 
per liter, required the further addition of 3.4 mgm, of OH per gram of 
soil. By subtracting the quantity of OH required to reach the same 
point when added to distilled water, it is evident that 3.3 mgm. of OH 
have been removed by some mechanism not associated with the neutrali¬ 
zation of acid. The nature of the latter type of reaction has been the 
subject of much conjecture; but before entering upon a detailed discus¬ 
sion of this matter it is desirable to point out certain other relations 
which may be derived from a further study of the data presented above. 
These are concerned with the comparison of the amounts of OH ions 
removed from solutions of the various bases, when added in combination 
with different positive ions. In general, it may be said that to bring 
about this higher OH-ion concentration in the soil suspension requires 
a larger equivalent of calcium hydroxid and barium hydroxid than of 
sodium hydroxid or potassium hydroxid. As a striking instance of this, 
soil 19 may be cited. In order to assure a reasonable degree of validity 
for these comparisons, they have been made, in accordance with Han¬ 
ley’s (14) suggestions, under conditions such that the suspended soil 
particles were in equilibrium with solutions of the same concentration. 
It is difficult to draw definite conclusions in regard to the relative quan¬ 
tities of bases required to produce neutrality, but it may be said that the 
equivalent quantities are of approximately the same magnitude. Exact 
equivalents would hardly be expected on account of various side reac¬ 
tions involving the interchange of bases. 
The interesting observation recorded in Table VII that barium chlorid 
when added to soils brings about a slightly greater acidity than when 
sodium or potassium chlorid is added may possibly be correlated with 
the fact that a greater quantity of OH is removed by the soil when 
added as barium hydroxid than when added as sodium or potassium 
hydrate. This agrees with certain contentions of Parker (24). 
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