posted 12-05-1999 08:29 PM         

86. (p69) Mercuric Chloride (HgCl2) by double decomposition

When two salts are dissolved together in water, the solution contains all four types of ion, and , if none of the possible ionic combinations leads to the production of an insoluble salt, the solution may be regarded as containing the four possible salts, all of which can be obtained from it on evapostion ----

---Pulverize 20 g. of mercuric sulphate with an equal weight of fused sodium chloride. Mix the salts , add to the mixture 2 g. of manganese dioxide (to oxidize any mercurous salts which may be present) and place in a small porcelain basin. Cover the baisn with an inverted filter funnel whose rim is of slightly smaller diameter than that of the dish and which thus rests on the side of the dish.

Slip the stem of the funnel through one hold of a two hole rubber stopper, the other hold of which carries a short , straight piece of tubing.

Now fit the stopper in a conical flask (Erlenmeyer) which must be clamper in position (inverted above the porcelain dish).

Support the porcelain dish on a clay triangle and heat, at first gradually and then more strongly until white fumes are seen to rise from the dish and condense on the walls of the funnel. (CAUTION Mercury is poinson!!!!)

Continue heating until volatilisation cerases.

When the apparatus is cool, scape the white crystals of mercuric chloride from the funnel in a bottle. Very little of sublimed salt should have penetrated into the flask.

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Preparation of Mercuric Sulphate

Treat about 20g. of mercury with jusst over half its weight of concentrated S2HO4 until half of the metal is converted to the white solid salt. Pour off the mercury, wash the product with cold dilute H2SO4 and dry on a porous plate.