Dry the
residue, ignite strongly, and weigh as silica, SiO_{2}. In certain
exceptional cases this may contain some unaltered cassiterite, which is
easily recognised by its appearance.
~Tungsten.~--The ammonic carbonate filtrate from the silica is
evaporated to dryness, ignited strongly over the blowpipe, and weighed.
The residue is tungstic acid, WO_{3}. The tungsten may be conveniently
reported in this form, although it is probably present as a lower
oxide.
~Tin.~--The acid filtrate from the silica and tungstic acid is treated
with sulphuretted hydrogen. The sulphide of tin is filtered off. Since
the percentage of tin has been already determined, this precipitate may
be neglected; or may be treated in the same way as the previous one, so
as to check the result. Since some stannic chloride will have been lost
in the evaporation, a low result may be expected. The tin should be
reported as stannous oxide; and is calculated by multiplying the
percentage of tin by 1.136.
The filtrate from the tin is boiled rapidly down to remove sulphuretted
hydrogen; and then peroxidised with 1 or 2 c.c. of nitric acid. It is
cooled, transferred to a graduated flask, and diluted with water to 200
c.c.
~Ferrous Oxide and Alumina.~--Half the filtrate from the tin (that is,
100 c.c.) is taken, nearly neutralised with soda, and treated with
sodium acetate.
Pages:
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546