642 
Journal of Agricultural Research voi. xxvii. No. 9 
CHEORIN 
Chlorin is determined by titration with a standard solution of silver 
nitrate, using potassium chromate as an indicator. The first reaction 
is the formation of insoluble silver chlorid. When all the chlorin has 
been combined the silver reacts with the chromate to produce a change of 
color. 
CARBONATE AND BICARBONATE 
The conventional method for the determination of these weak acids is 
to titrate the solution with a standard solution of a stronger acid, such as 
sulphuric, using phenolphthalein as the indicator for carbonate and methyl 
orange as the indicator for bicarbonate. The reaction involved in these 
titrations and the interpretation of their results call for a more detailed 
statement than has been made concerning the other constituents of the 
soil solution. This may be justified in part because the carbonates are 
generally regarded as the most troublesome substances in the solution, 
both in relation to the soil and the plants, and partly because of some 
uncertainty as to the significance of the results of the titrations. 
In the first place, it should be kept in mind that the carbonate- 
bicarbonate complex is very unstable. That is to say, a change in the 
quantity of carbon dioxid in the solution immediately causes a change in 
the proportion of carbonate to bicarbonate. This change may be illus¬ 
trated by the following equation: 
( 1 ) Na 3 C0 3 + H0 2 -hC0 2 = 2 NaHC 0 8 
This equation indicates the reaction that takes place when carbon 
dioxid is added to a solution of sodium carbonate in water, which reaction 
is in the direction of converting the carbonate to the bicarbonate. The 
removal of carbon dioxid from the solution, which may be done by aera¬ 
tion or boiling, causes the reaction to take place in the other direction or to 
change the bicarbonate into the carbonate. The quantity of carbon 
dioxid in solution is extremely variable, depending upon conditions which 
are constantly changing, so that there is comparatively little significance 
to be attached to the distinction between carbonates and bicarbonates 
in irrigation and drainage waters. 
In making a titration for carbonate in a solution, a few drops of phenol¬ 
phthalein is first added. If the solution takes on a pink or rose color 
this is assumed to indicate the presence of the normal carbonate (C0 3 ), 
and a solution of standard sulphuric acid is added until the color disap¬ 
pears. 
The course of the reaction may be illustrated as follows : 
( 2 ) 2 Na 2 C0 3 4- H 2 S0 4 = Na,S0 4 + 2 NaHCO g 
This might be taken to indicate that the pink color of the phenolphtha¬ 
lein indicates the presence of the carbonate radical, but there is another 
explanation that appears to serve better. Sodium carbonate is an exam¬ 
ple of a strong base united with a weak acid. When this salt is dissolved 
in water it is partially dissociated and hydrolized as follows: 
( 3 ) NajCOg + H 2 0 = NaOH + NaHCO, 
It is not assumed that this reaction is complete, but rather that in a 
solution of sodium carbonate there exists a certain quantity of free hy¬ 
droxyl ions (OH) and that these cause the phenolphthalein to show pink 
