CONTROL OF SAPONIFIED SOLUTIONS 7 
colorless. But when cresol also is present, the final change is only 
from violet to blue and is too indistinct for observation in the titra- 
tion flask. Moreover, the colors fade rather rapidly in the presence 
of cresol. These difficulties led to the adoption of the following 
method : 
About 10 grams of the sample are weighed by difference into a 200 
cubic centimeter Erlenmeyer flask and dissolved in 100 cubic cen- 
timeters of 95 per cent alcohol previously rendered neutral to either 
phenolphthalein or Poirrier blue. A supply of clean and dry test 
tubes or vials is provided, measuring about 60 by 12 millimeters, also 
a 0.5 per cent alcoholic solution of Poirrier blue C 4 B and a 3 per 
cent aqueous solution of acetic acid, with droppers for each. Between 
0.5 and 1 cubic centimeter of the alcoholic solution of the sample 
is poured into one of the test tubes and mixed with sufficient indi- 
cator drop by drop, to produce a pronounced color; then, with the 
tube held against the light, 1 drop of the dilute acetic acid is added 
and a change in color is looked for. If no change occurs, the sample 
contains no excess alkali. If treatment with acetic acid effects a 
change toward blue, half -normal acid is run into the flask from a 
burette, in quantity depending upon the effect produced by acetic 
acid in the test, but not restricted by fear of somewhat overrunning 
the end point. After mixing, 1 cubic centimeter is tested as before. 
The end point is reached when a drop of the acetic acid leaves the 
color of the test unchanged ; that is, it is approached from the alka- 
line side, but if overrun, the solution may be brought back to alka- 
line reaction with half-normal alkali from a burette and once more 
titrated with acid. From the net consumption of standard acid is 
calculated the percentage of excess alkali, expressed as sodium 
hydroxide. Tar bases do not react alkaline toward the indicator. 
It will be evident that contamination of the throat of the flask by 
drops of the standard acid or alkali must be avoided, else such might 
be washed into the test tube and vitiate the test. A clean, dry tube 
should be used for each test, and the contents should be immediately 
discarded after the test, not returned to the flask or held for later 
observation. The blue given by a strictly neutral solution may be- 
come more intense through copious addition of acid, particularly of a 
highly ionized acid. Accordingly a 15 per cent alcoholic solution of 
oleic acid may be preferable to the 3 per cent aqueous solution of 
acetic acid, but the latter is entirely satisfactory if only 1 drop is 
used. 
EXPERIMENT 1 
Oleic acid. 2 cubic centimeters, was dissolved in 100 cubic centi- 
meters of 95 per cent alcohol, phenolphthalein was added, then 10 
per cent aqueous sodium hydroxide in excess, and finally half -normal 
hydrochloric acid until 1 drop discharged the pink color. After the 
addition of 5 cubic centimeters of redistilled commercial cresol 1 cubic 
centimeter of the solution appeared to be acid when tested with 
Poirrier blue. Accordingly 0.5 cubic centimeter of half-normal 
sodium hydroxide was added, and then the solution gave a reddish 
violet test which underwent a very pronounced change to blue upon 
the addition of 1 drop of 5 per cent acetic acid. Titration from that, 
point with half -normal hydrochloric acid then produced the follow- 
ing effect : After 0.2 cubic centimeter, a dull violet, very much modi- 
