Allow to cool; moisten
with strong hydrochloric acid; warm with a little water and test for the
metals in the solution by the ordinary methods. Since the quantities of
the metals likely to be present may be given in milligrams the work must
be carefully performed. It may be worth while to determine the
proportions of lime and magnesia as well as those of the metals proper.
Or the 20 c.c. of cyanide liquor may be evaporated with 5 c.c. of strong
nitric acid to dryness and gently ignited and the residue taken up with
2 or 3 c.c. of strong hydrochloric acid.
Copper, iron, and zinc can be rapidly determined in such a solution, as
follows. Dilute with water to 10 or 15 c.c., add an excess of ammonia,
and filter. The precipitate will contain the iron as ferric hydrate;
dissolve it in a little hot dilute sulphuric acid: reduce with
sulphuretted hydrogen; boil off the excess of gas, cool and titrate with
standard potassium permanganate (p. 236). Determine the copper in the
filtrate colorimetrically (p. 203); but avoid further dilution. Then add
dilute hydrochloric acid, so as to have an excess of 4 or 5 c.c. after
neutralising the ammonia; add some clean strips of lead foil, and boil
until the solution has for some time become colourless. Titrate with
standard potassium ferrocyanide (p. 263) without further dilution, and
bearing in mind that at most only one or two c.
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