166 QUANTITATIVE AGRICULTURAL AXALYMti
In the analytical method, furfural is produced by hydrolysis
and distillation with hydrochloric acid. Phloroglucin, an aro-
matic alcohol, CoH3(OH)3, is added and this precipitates furfural
phloroglucide:
C5H402 + C6H8(OH)3 -> Ci,H»(>4 + H20
From the weight of furfural phlorogludde the corresponding
weight of pentosans, may be found by referring to Krober's
table, found on page 167.
Determination of Pentosans and Allied Substances. —Prepare the follow-
ing reagents:
Phlorof/lutin.—Test the purity of the laboratory supply !>y dissolving
a small amount in a few drops of acetic anhydrides healing almost, to boiling
and adding a few drops of concentrated sulphuric acid. If more thiin a
faint violet color appears the phlorogluein contains diresorcin and it muHt
then be purified. 'For this purpose heat 11 gin of phlorogluein with .'I(K) cc
of 12-per cent hydrochloric acid (specific gravity l.(MJ), adding the phloro-
gluciri very gradually. Continue heating and stirring until solut ion in nearly
complete. Pour the hot solution into 1200 cc of hydrochloric acid of the
same concentration. Allow to stand for several days, to permit the dire-
sorciri to crystallize. Filter just before using.
Aniline Acetate Paper.—Th'w is prepared by mixing aniline* and water in
equal volumes, then adding glacial acetic acid until the mixture is clear.
Moisten filter paper with the solution.
Place 2 to 5 #m of food in a 250-ce. distilling flunk which in fitted with a
scparatory funnel and which in connected with a condenser. Add 100 cc
of 12-per cent hydrochloric acid (1.06 specific gravity) and wvcral pieeenof
pumice stone, dropped in while hot. Heat over a win* gaiix<* at such it rate
that about 30 cc will distill over in 10 minutes, passing the. distillate through
a Hinall filter paper into a 500»ee volumetric' flank. Add «'!0 cc of 12-per
cent hydrochloric acid to the flank through t he separatory funnel. ('ontintie
this process of distilling and replacing the distillate hy hydrochloric arid until
the distillate amounts to about 300 cc and until a few drop« give no red or
pink color to aniline acetate paper.
Gradually add to the total distillate an amount of pure phUiroghtein about
double the furfural estimated to 1m present. (Commit the instructor.)
It will bo observed there an? several color changes taking place, the flotation
becoming yellow, then green and finally an almost black precipitate uppeitw.
The solution is diluted to 400 cc with 12-per cent hydrochloric acid and
allowed to stand for 12 hours. Test, the solution with aniline acetate paper
to see if precipitation of furfural has been complete, a red color developing
if any furfural remains in solution. Kilter the precipitate through a dried
and weighed Gooeh crucible. Wash with 150 cc of water (retaining «>me
water in the Gooeh crucible until the last, during f lie wanhing) and dry lor
4 hours at 100°. Cover the crucible, cool and weigh rapidly.