Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

NITROGEN DIOXIDE. 61

dioxide becomes apparent. From time to time drive the sub-

limate over into the receiver by fanning the top and neck of the

retort with the flame of a Bunsen burner.

When, at the end of two or three hours, X ~Q

nothing further passes over, decant off the C/

sulphuric acid which has distilled into the Fig- 13.

receiver and recrystallize the phthalic

anhydride, from 75 to 100 c.c. of water. The nitrate from the

first crop yields more crystals on evaporation.

Dry the phthalic acid thus obtained, and distil it in a test-

tube held nearly horizontally, the closed end of which is bent
downward at a slight angle, as shown in Fig. 13. Water is split

off from the molecule during the sublimation, and beautiful

needles of phthalic anhydride (melting-point 128° C.) are formed.

By another crystallization from water, 2 to 3 g. of pure, perfectly
white phthalic acid are obtained.

For the characterization and identification of the phthalic

acid, mix a little of it with equal amounts of resorcinol and an-

hydrous zinc chloride. Heat this mixture slowly in a small,

dry test-tube over a small flame until it sinters and then melts.

After heating a minute longer, cool the brownish-red fusion and

dissolve it in a little alcohol. Pour the solution thus obtained

into a large beaker containing distilled water, and add a few drops

of caustic soda solution. A deep-yellow solution is obtained

which, by reflected light, shows a beautiful green fluorescence

(synthesis of fluorescein).

29. Nitrogen Dioxide.
    Place 40 g. of coarse lumps of arsenic trioxide in a flask, add
    50 g. of concentrated nitric acid (sp. gr. 1.4), and heat the mix-
    ture moderately upon a sand bath (or Babo boiling funnel).
    Lead the gases evolved successively through an empty wash-
    bottle, a U-tube containing glass wool, a second empty wash-
    bottle, and finally into a third wash-bottle (surrounded with ice)
    in which the oxides of nitrogen are condensed. A mixture of
    nitrogen dioxide, nitrogen trioxide,
    1
    and nitric oxide is obtained.

(^1) Nitrogen trioxide N
2 O 3 can exist only in the liquid condition. It is un-
stable, and when vaporized dissociates into NO and NO2 (or N 2 O 4 ). v. Wit-
torff, Z. anorg. Chem. 41, 85 (1904).