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

AMMONIUM TRIBROMIDE. 107

about 40 g. On long standing, the crystals become lighter colored,

due to decomposition. Determinations of ammonia and sulphur
confirm the above formula.

(c) Polyhalides.
Polysulphides and polyhalides are very closely related. The latter likewise
occur chiefly as salts of the alkali and alkaline earth metals and as the free
halogen acids. The stability of the compounds increases very rapidly in the
series of the alkali metals with the increase in the atomic weight of the metal.
From caesium and rubidium a large number of both simple and mixed polyha-
lides of the type MHa 3 and MHa 5 (Ha = Cl, Br, or I) have been obtained. As
regards the stability of its polyhalogen compounds, ammonium comes next to
the higher alkali metals; still more stable are the substituted ammonium salts,
namely, the polyhalides of the complicated alkaloids, such as coniin, nicotin,
atropin, narkotin, and of the diazonium salts.

69. Ammonium Tribromide, NH 4 [Br 3 ].

Add 8 g. of bromine * to a lukewarm solution of 10 g. ammonium

bromide^2 in 12 g. of water, whereupon the temperature of the

mixture rises a little. Allow the solution to stand over sul-

phuric acid in a vacuum desiccator which, if possible, should be

placed in an ice-chest. Prismatic crystals, or lamellar aggre-

gates, of the color of potassium pyrochromate separate from the

solution which still contains free bromine. After one or two days

drain the crystals and dry them in a vacuum desiccator con-

taining a small dish of bromine in addition to sulphuric acid (the

preparation would lose bromine to an atmosphere free from that

element). The yield is about 10 g. Analyze the product as

(^1) This is only half the calculated amount. The use of more bromine does
not improve the yield.
(^2) Ammonium bromide may be prepared either by the neutralization of
hydrobromic acid (No. 35) with ammonia or by the action of bromine on
ammonia. According to the latter method, allow 13 g. bromine to flow drop
by drop from a dropping-funnel into 130 c.c. of 2-normal ammonia in a flask,
which is surrounded by ice and is constantly shaken.
8 NH 3 + 3 Br 2 = 6 NH 4 Br + N 2.
Evaporate the solution on the water bath, and if desired powder the residue
and dry it in the steam closet. The salt may be purified by recrystallization.
Dissolve it in its own weight of boiling water, cool the solution with ice, and add
an equal volume of alcohol; recover the rest of the salt from the mother-liquor
by evaporating and crystallizing in a similar manner.