The Students' Guide in Quantitative Analysis - Free Ebooks Library

NOTES TO THE PRECEDING SCHEME. 6f
Note 3. Removal of the fused mass,— Let the crucible
cool until just below red heat, then chill it suddenly by
plunging it into cold water contained in a porcelain cas-
serole, lay the crucible on its side and digest with boiling
water. The fused mass will generally become detached
from the crucible and come out in a cake. Then remove
the crucible, wash it, treat in a small beaker with a little
cone. HCl to remove any adhering particles of the mass,
and add this solution to that of the Insoluble Residue
{2). Should any portion of the fused mass, thicker than a
film, obstinately resist solution in the hot water, it ought to
be removed only by patience and long boiling; and no
attempt should be made either to dig it out or to dissolve
it in IICl ; lest by the formation of Aqtia Regla or free CI
(in the presence of NaNOj, or Mn^OJ the crucible be
attacked and injured.
Note 5. Sefakat!on of SiO^.— In order to render the
SiOj entirely insoluble, it must be perfectly dehydrated.
Tfce evaporation should be carried to dryness, the residue
heated until odors of HCl can no longer be detected, and
the mass is hard and crumbly. Since the residue is to be
re-fused with Residue b, the drying may be completed, at a
temperature somewhat higher than 100° C, in an air-bath.
Note 8. Precipitation of BaSO^.—Avoid the addition
of a large excess of BaCI^ solution. Add only 5 c.c. at first,
and then after complete subsidence of precipitate, add a
few drops to determine if any HjS04 remains unprecipi-
tated, etc. Then proceed as in Fres., § 132, I, i. After
decanting the clear supernatant liquid, boil the precipitate
with water, allow to subside, decant, filter, and wash with
liOt water. These precautions are necessary to dissolve
out any other salts of barium, which are always carried
Jown on the first precipitation. If the precipitate of BaSO^
«s dark colored after ignition, dissolve in the crucible in