EXERCISE 16. CRYSTALLIZED BARIUM THIOSULFATE,BaS2O2,H2O
Sometimes it is desired to obtain a small quantity of a sparingly soluble salt in well-crystallized condition rather than in the form of a fine
precipitate. This may be accomplished by allowing the double decomposition to proceed slowly by diffusion.
Obtain a large beaker of at least 2 1. capacity or a battery jar or a rather deep crystallizing dish of about 25 cm. diameter, also two small
crystallizing dishes (or beakers without lips) of about 75 cc. capacity (Fig.5). Fill the one small dish three fourths full with a saturated solution
of sodium thiosulfate, adding some crystals of the solid salt. Similarly, add to the second dish a saturated solution and crystals of barium chloride
or nitrate. Place the two dishes 3 or 4 cm. apart in the large dish and carefully fill them with water without stirring. By means of a rubber tube
connected with the supply of distilled water, flow water into the large vessel to a height of at least 4 cm. above the tops of the small beakers. The
whole idea is to avoid the premature mixing of the two reagents. Pour a layer of melted paraffin over the surface of the water to protect the contents
of the vessel from disturbances that might produce convection currents and set the vessel away in a place where it can remain undisturbed for several
weeks at as nearly constant temperature as possible. slowly, as diffusion brings the reagents into contact. When a sufficient crop has been secured,
carefully remove the paraffin, siphon off the solution as well as possible, and lift out the small beakers with their contents. Collect the crystals
and dry them on paper. The place where they will form will depend on the relative rates of diffusion of the two ions concerned in the precipitation.
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