Mar. i, 1924 
Movement of Water in Irrigated Soils 
643 
color. According to this explanation the reactions of the carbonate 
titration would be indicated thus: 
{4) 2 NaOH + H 2 S0 4 = Na 2 S0 4 + 2 H 2 0 
It is assumed that only a part of the sodium carbonate is dissociated 
and hydrolized in the orginal solution, but that as titration proceeds 
these reactions go on until the supply of the dissolved carbonate is 
exhausted. The acid used in titrating the solution until the phenol- 
phthalein color disappears accounts for only one half the original 
quantity of carbonate. According to the equation ( 2 ), the first step of the 
titration reaction is to form sodium sulphate and sodium bicarbonate. 
In making carbonate titration the burette reading for the phenolphthalein 
reaction is doubled before computing the carbonate content of the solu¬ 
tion. 
Upon completing the titration with phenolphthalein, a few drops of 
methyl orange is added to the solution and the addition of acid contin¬ 
ued until the solution changes color from lemon yellow to orange. By 
this titration the total quantity of bicarbonate in the solution is deter¬ 
mined. The reaction may be indicated as follows: 
( 5 ) 2 NaHC 0 3 + H 2 S0 4 = Na 2 S0 4 + H 2 0 + C0 2 
As has been indicated above, the bicarbonate content of the original 
solution has been increased when carbonate was present through the 
conversion of the carbonate to the bicarbonate. Thus in computing 
the carbonate and bicarbonate equivalents of the acid used in titrating a 
solution it is necessary to deduct from the burette reading for the 
methyl orange titration the equivalent of the burette reading for the 
phenolphthalein titration. The remainder may be taken as the acid 
equivalent of the bicarbonate originally in the solution. 
The correct interpretation of the results of the titrations for carbonates 
and carbonates in soil solutions is beset with still other complications. 
One of these has to do with the fact that the silicate radical (Si0 3 ) when 
it occurs in solution gives the same color reaction with phenolphthalein 
as the carbonate. This may lead to some confusion in dealing with 
certain strongly alkaline solutions. Not only does it give too high 
values for carbonates but it upsets the determination of bicarbonate 
also. From what has been said above concerning the titration for car¬ 
bonate it will be evident that if a solution contained carbonate but no 
bicarbonate its complete titration would require equal quantities of 
acid to neutralize the phenolphthalein and the methyl orange. With a 
solution containing silicate this is not the case. It takes much more 
acid to neutralize the phenolphthalein color than it does to complete the 
titration with methyl orange. It sometimes happens in laboratory work 
that solutions are obtained that show an abnormally high proportion of 
carbonate as compared with the bicarbonate. With such solutions it is 
advisable to test for silica. 
Another source of error in determining the carbonate-bicarbonate 
constituents of solutions has to do with the evolution or escape of carbon 
dioxid between the time the sample is taken and the time the titration 
is made. On one hand, if the solution contains organic matter either 
suspended or dissolved this may be broken down with the consequent 
liberation of carbon dioxid. If the solution is stored in a closed container 
this carbon dioxid may combine with the normal carbonate to form 
