TRANSITION POINT. 37
When sulphur is heated in a test tube above 160° it becomes
dark and so viscous that the tube can be inverted without the
sulphur flowing out.
- Transition Point of Cuprous Mercurilodide Cu 2 [Hgl 4 ] and of
Sliver Mercuriiodide Ag 2 [HglJ.
First prepare mercuric iodide by precipitating a solution of
6.8 g. of mercuric chloride with a solution of 8.3 g. of potassium
iodide. Wash the precipitate once by decantation, and dissolve it
together with 8.3 g. of potassium iodide in 50 cc. of water. Mix
the filtered solution with another filtered, concentrated solution
containing 12 g. of blue vitriol, and pass sulphur dioxide into the
filtrate, in order to reduce the cupric salt. The sulphur dioxide
can be prepared from sodium sulphite and sulphuric acid (cf. foot-
note, page 71). A bright-red precipitate is produced of some-
what the appearance of mercuric iodide. Wash it thoroughly on
the suction filter and dry it in the hot closet. Yield, about 20 g.
Cuprous mercuriiodide is transformed at about 71° into a
black modification. The color change can be observed if a pinch
of the material is gently heated in a test tube over a free flame;
on cooling, the black color changes again to red. To determine
the transition temperature, a sample may be heated slowly in a
dry test tube which is immersed in a beaker of water containing
a thermometer that serves also as a stirrer. By means of several
repetitions, the transition temperature may be obtained in this
way with considerable accuracy.
The silver salt also of hydromercuriiodic acid, H 2 [Hg IJ, pos-
sesses a transition point between 40° and 50°, which is likewise
characterized by a change in color. Below this temperature the
salt is yellow; above, it is red.
Precipitate a few cubic centimeters of mercuric chloride solu-
tion with a solution of potassium iodide, and redissolve the pre-
cipitate in an excess of the precipitant. Then add a few drops of
silver nitrate solution and observe, without filtering, the change in
color of the precipitate on heating and cooling. This transition
point is less sharp than that of the copper salt.
64, 257 (1906). Hoffmann and Rothe, ibid. 55,113 (1906); Smith and Carson,
ibid. 63, 273 (1908). With respect to the influence of different forms of S on the
melting point, see Beckmann, Paul, and Liesch, Z. anorg. Chem, 103,189 (1917),