BULLETIN No. 90a. 
11 
weight of water, the proper amount is poured on 5 to 10 grams of 
tobacco, and thoroughly mixed in a mortar; work the mass lightly 
to remove the sap, then mix with a small quantity of pumice stone ; 
transfer the whole to a Soxhlet extractor, aud subject it to a continu¬ 
ous extraction with ether for at least 15 hours. The operation is con¬ 
sidered complete when a few drops, caught from the outlet of the 
extractor, evaporate readily, showing that there is no acid to hinder the 
evaporation. The ether removes neither the nitric, hydrochloric, or 
sulphuric acids, but completely all traces of phosphoric acid. 
Evaporate the ether until there is but a few c. c. left in the flask, 
then add a few c. c. of water, and mix very slowly with the etherial 
fluid to insure the complete solution of the acids. Finally transfer 
to a beaker, and wash thoroughly. Unite the wash-water with the 
solution in the beaker, and evaporate the ether at the lowest possible 
temperature. The solution is now ready for the determination of 
the organic acids. 
Oxalic Acid .—Neutralize with ammonia, make slightly acid with 
acetic acid, and precipitate the oxalic acid with a dilute solution of 
acetate of lime, avoiding an excess, since it would interfere with the 
other determinations. After the oxalic acid is completely separated 
filter, wash thoroughly, ignite, and weigh. The percentage of oxalic 
acid is calculated from the lime thus obtained. 
Citric Acid .—The filtrate from the preceding determination is 
made very slightly acid with acetic acid. Now add slowly, with 
constant stirring, a dilute solution of acetate of lead until a perma¬ 
nent precipitate appears. When the supernatant liquid is clear 
draw off a few c. c. by means of a pipette, and transfer to a small 
beaker. To this solution add a few drops of acetic acid and a small 
amount of a verv dilute solution of acetate of lead, and observe 
from this portion of the liquid whether a permanent precipitate can 
' be obtained, or whether it dissolves again in the acetic acid. In the 
first case we must continue to add acetate of lead to the original 
solution and again test a small portion, and so continue until we get 
a sample in which the precipitate redissolves in acetic acid. At 
this moment the citric acid is completely precipitated The samples 
taken out to test must be neutralized with ammonia, and washed 
back into the original solution. Let stand for a few hours, filter on 
a tared filter, w 7 ash first with water to which a few drops of acetate 
of lead and acetic acid have been added, then with 80 per cent, 
alcohol, and conclude the washing with a mixture of equal parts of 
water and alcohol. Dry the precipitate, weigh, ignite at a low heat 
in porcelain crucible, and weigh again. The loss in weight is counted 
as citric acid. In the filtrate, under the influence of the alcohol, the 
small portion of citrates still in solution separates and falls with the 
malates. These are filtered out, washed with alcohol, dried, ignited, 
weighed, and calculated as half citrate and half malate of lead. 
The conjoint weight of both acids is so small that the error that 
might arise from this source is very small. 
