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

106 POLYSULPHIDES.

flask. Amalgamate the surface of about 0.5 g. of zinc turnings

by treating with a solution of mercuric chloride, and then add

the zinc a little at a time to the contents of the flask. On

shaking vigorously, the zinc dissolves completely in the mercury

and then becomes oxidized according to the equation given

above. To show the presence of hydrogen peroxide, treat

samples of the resulting turbid liquid with an acidified iodide-

starch solution containing a drop of ferrous sulphate, or with a

titanium sulphate solution.

(b) Poly sulphides.

68. Ammonium Pentasulphide, (NH 4 ) 2 S 5.
    The sulphur which is bound directly to the metal in metallic sulphides is
    capable of taking on more sulphur to form polysulphides (Berzelius). The
    additional binding power of the sulphur seems to be influenced to a high degree
    by the metal with which it is combined. Polysulphides of the alkali metals
    are numerous and well known, as are also many of the alkaline earth group;
    but for the heavy metals either none exist or they play, at most, a subor-
    dinate role. Polysulphides of the alkali metals can be obtained by fusion
    (sulphur livers), those of caesium and rubidium with the general formula
    M 2 Sn, in which n = 2, 3, 4, 5, and 6, having been identified;' polysulphides can
    also be formed by dissolving sulphur in solutions of the monosulphides and
    thereupon crystallizing or precipitating the product. (See below.) Of the
    polysulphide solutions, those of the tetrasulphides with the ion S 4 are
    especially stable.^2 Besides the types already mentioned, other polysulphides
    have been obtained under varying conditions from such solutions, but the
    individuality of some of them is doubtful.

Saturate 25 c.c. of concentrated ammonia solution with hydro-

gen sulphide in a closed flask, add another 25 c.c. of the ammonia,

and then dissolve in this mixture as much roll sulphur as pos-

sible (about 25 g.) at 30° to 40°. Filter off the excess of sulphur

and to the yellow solution in an Erlenmeyer flask add an equal

volume of 95% alcohol. After the solution has stood over night

in the ice-chest, an abundant crystallization of intensely yellow

needles of ammonium pentasulphide is obtained. Drain the crys-
tals, wash them with alcohol and ether and allow them to dry for
a day in a vacuum desiccator over quicklime upon which a few
drops of concentrated ammonia have been poured. The yield is
1
W. Biltz and Wilke-Doerfurt, Ber. 38, 123 (1905); Z. anorg. Chem. 48,
297; 50, 67 (1906); J. S. Thomas and A. Rule, Chem. Soc. Ill, 1063
(1917).

(^2) Kuster, Z. anorg. Chem. 43, 53 (1904); 44, 431 (1905).