Oct. ao, 1933 
Soil Reaction in Relation to Calcium Adsorption 
85 
for which indicators are unsuited. With soils electrometric titration 
has not come into extensive use because of certain inherent difficulties 
that have been pointed out by those who have used the method (26, 41). 
Some of these will be discussed later. 
RELATION OF INTENSITY AND QUANTITY OF ACIDITY 
The P H value obtained by thejiydrogen electrode on a soil suspension 
gives a measure of the actual amount of hydrogen ions present under the 
conditions of the experiment. This is often designated as “intensity 
of acidity” (14) and is very different from the total quantity of hydrogen 
ions which may be produced by the gradual introduction of hydroxyl 
ions. This point may be illustrated with the following example: A 
certain solution gives a reaction equivalent to P H 5. This means that 
there is present 0.00001 gm. hydrogen ions per liter of solution. Sup¬ 
pose that in this case it takes five cc. of a 0.04N hydroxid to bring 1,000 
cc. of this solution to P H 7. Five cc. of 0.04N hydroxid solution con¬ 
tain enough hydroxyl ions to react with 0.0002 gm. hydrogen ions. In 
other words, if a liter is titrated, the figure which represents the total 
acidity is 20 times greater than the figure which represents the intensity 
of acidity. If it takes 5 cc. of 0.04N hydroxid to change 100 cc. from 
Ph 5 to P H 7, the total acidity is 200 times greater than the figure which 
represents the intensity. 
DIFFICULTY IN THE USE OF THE HYDROGEN ELECTRODE WITH SOIL 
In speaking of the electrometric titration of soil with standard 
Ca(OH) 2 , Sharp and Hoagland remark (41 ): “Such a method is logically 
adapted to obtain the information necessary for the proper adjustment 
of the soil reaction by the addition of lime. There are, however, certain 
difficulties met with in its application to soils. One of the chief difficul¬ 
ties is due to the relative insolubility of the acid-forming constituents of 
soil which prevent a rapid attainment of equilibrium.” Data given by 
these investigators show that the time necessary to attain equilibrium 
varied from 3 to no hours. This time factor and other difficulties are 
also discussed by Knight ( 26 ). That the direct electrometric titration 
of a soil suspension is a tedious and difficult operation is known to any¬ 
one who has tried the method. The direct titration was used in a former 
investigation (44 ), and it was noted that if observations were made soon 
after the introduction of the Ca(OH) 2 solution into a soil suspension it 
would usually be found that the voltage reading was greatly increased. 
If readings were taken at, say, five-minute or longer intervals, it would be 
found that the reading decreased, or the H-ion concentration gradually 
increased. This means that H ions are produced by more of the acid¬ 
forming constitutents going into solution. This change may continue 
until a P H value of, say, 6 or 5 is obtained, showing that if neutrality is 
desired more hydroxid must be added. As soon as this is added the 
voltage reading again suddenly goes up, numerically, probably beyond 
the equivalent of P H 7. This does not necessarily mean that too much 
or even enough hydroxid has been added. The readings will again 
gradually decrease, showing that hydroxid must again be added if 
neutrality is to be attained. It is necessary to repeat this process 
several times, until equilibrium is established at P H 7, or any other 
