Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

326 FLUOKBSCBIN
ice and 15 c.c. of concentrated hydrochloric acid are added. After
diluting the solution thus obtained to about 50 c.c. the liberated iodine
is titrated with 0-1 iV-thiosulphate solution, prepared with sufficient
accuracy by dissolving 6-2 g. of pure sodium thiosulphate (Na2S2O3 +
5 H2O) in water and diluting in a measuring flask to 250 c.c. 1 c.c. of
0-1 iV r -thiosulphate=0-012 g. of PbO2.
5. FLUORESCEIN AND EOSIN 1
Phthalic anhydride (15 g.) is intimately ground in a mortar with
22 g. of resorcinol, and the mixture is heated in an oil bath to 180°.
For this purpose an internally glazed meat-extract jar is convenient;
it is suspended in the bath by means of a wire triangle fastened
round the neck of the jar. 7 g. of zinc chloride, previously dehydrated
by fusing and then powdered, are stirred with a glass rod into
the molten mass during the course of ten minutes. The temperature
is then raised to 210° and the heating is continued until the mass,
which becomes progressively more viscous, has completely solidified ;
for this one to two hours are required. By means of a sharp instrument,
preferably a chisel, the brittle melt obtained on cooling is
chipped out of the jar, powdered finely, and boiled for ten minutes in
a porcelain basin with 200 c.c. of water and 10 c.c. of concentrated
hydrochloric acid. In this way the unchanged starting materials and
the basic zinc salt are dissolved. The fluorescein is then separated
from the aqueous liquid by filtration, washed with water until the
filtrate is no longer acid, and dried on the water bath. Yield,
almost quantitative. Dissolve a particle of the preparation in a little
ammonia and dilute in a beaker with one litre of water.
Eosin.—Into a flask containing 16-5 g. (0-05 mole) of fluorescein
under 80 c.c. of alcohol, 36 g. (12 c.c.) of bromine are dropped with
shaking from a tap funnel during the course of twenty minutes.
When half the bromine has been added, and the fluorescein has been
converted into dibromofluorescein, all the solid material disappears
temporarily, since dibromofluorescein is soluble in alcohol, but later
the sparingly soluble eosin begins to crystallise.
After standing for two hours the mixture is filtered and the
precipitate, after being washed several times with alcohol, is dried
on the water bath. The material so obtained still contains alcohol
of crystallisation, which is removed by drying at 110°, when the
colour becomes lighter. Observe the magnificent fluorescence
1 Bacyer, Caro, Annalen, 1876, 183, 1.