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GLUCOSE. 215
salt is dissolved as possible. If a glucose solution \s treated with about
tV vol., or with a larger quantity of the solution when large quantities
of sugar are present, and boiled for a few minutes, the solution becomes
first yellow, then yellowish-brown, and finally nearly black, and after a
time a black deposit of bismuth (?) settles.
The property that glucose has of reducing an alkaline solution of
mercury on boiling is the basis of Knapp’s reaction with alkaline mercuric
cyanide, and of Sachsse’s reaction witli an alkaline potassium-mercuric
iodide solution.
On heating with phenylhydrazine acetate a glucose solution
gives a precipitate consisting of fine yellow crystalline needles which are
almost insoluble in water, but soluble in boiling alcohol, and which separate
again, on treating the alcoholic solution with water. The crystalline
precipitate consists of phenylglucosazone (see page 203). This com-
pound melts when pure at 205° C. It must be borne in mind that
the melting-point of this and other osazones is somewhat variable, depend-
ing upon the rapidity of the heating, the diameter of the tube and the
thickness of the sides of the tube. 1
The osazone dissolves readily in
pyridine (0.25 gram in 1 gram), and precipitates again from this solu-
tion as crystals on the addition of benzene, ligroin, or ether. According
to Neuberg 2
this behavior can be used in the purification of the osazone.
The diphenylhydrazone and the methyl phenylhydrazone are also of
interest.
Glucose is not precipitated by a lead-acetate solution, but is almost
completely precipitated by a solution of ammoniacal basic lead acetate.
On warming, the precipitate becomes flesh-color or rose-red (Rubner’s
reaction 3
)
.
If a watery solution of glucose is treated with benzoylchloride and
an excess of caustic soda, and shaken until the odor of benzoylchloride
has disappeared, a precipitate of benzoic-acid ester of glucose will be
produced which is insoluble in water or alkali (Baumann 4
)
.
If \-\ cc. of a dilute watery solution of glucose is treated with a few
drops of a 10 per cent alcoholic solution (free from acetone) of a-nayhthol
,
on the slow addition of 1-2 cc. of concentrated sulphuric acid a beautiful
reddish-violet ring forms at the juncture of the liquids, or on shaking, the
entire mixture becomes a beautiful reddish-violet color (Molisch 5
).
1
See E. Fischer, Ber. d. d. ehem. Gesellsch., 41.
2
Ber. d. d. chim. Gesellsch., 32, 3384.
3
Zeitschr. f. Biologie, 20.
4
Ber. d. deutsch. chem. Gesellsch., 19; also Kueny, Zeitschr. f. physiol. Chem., 14,
and Skraup, ^Vien. Sitzungsber., 98 (1888).
5
Molisch, Monatshefte f . Chem., 7, and Centralbl. f. d. med. Wissensch., 1887,
pp. 34 and 49.
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