To
70 cc. of concentrated sulfuric acid (Note
1) in a
1-l. short-necked flask fitted with a
mechanical stirrer and immersed in an
ice-water bath is added, with vigorous stirring,
10 g. (0.145 mole) of solid sodium nitrite in small portions. The addition is made at such a rate that only a very small amount of
nitrogen oxide is evolved. When solution is complete,
20 g. (0.145 mole) of salicylic acid is added in small portions with stirring; about fifteen minutes is required. The mixture is stirred at 20° until all the solid is in solution (Note
2). The mixture should then be light red to brown in color, very viscous, and quite homogeneous. It is surrounded by an
ice-salt bath, and, when the temperature reaches 0°,
5 cc. (0.065 mole) of a 35–40 per cent solution of formaldehyde (formalin) (see
Note 1 on p. 378) is slowly added with extremely vigorous stirring, at such a rate that the temperature does not rise above 5° (Note
3). The reaction is complete a few minutes after all the
formaldehyde has been added. About 100 g. of finely crushed ice is then added, followed by 500 cc. of ice water; the stirring should be vigorous during the addition (Note
4). The contents of the flask are stirred until the
aurin tricarboxylic acid has disintegrated into small pieces.
The solid is washed several times by decantation, using cold water, and finally filtered with suction. It is then dissolved in dilute
ammonia (1 volume of concentrated
ammonia with 2 volumes of water) while it is still on the
filter paper in the
suction funnel (Note
5). The filtrate is evaporated to dryness on a
steam bath. The resulting glassy, light yellowish-brown ammonium salt, which weighs
19–22 g. (
83–96 per cent of the theoretical amount) is sufficiently pure (Note
6) for use as a test for
aluminum.
1