Some precipitates must be dried at a temperature not higher than
100° C. These are placed in the water-oven (fig. 20), and, when
apparently dry, they are taken from the funnel, placed between glasses,
and then left in the oven till they cease to lose weight. Such
precipitates are collected on tared filters. Those precipitates which
will stand a higher temperature are dried in the hot-air oven at a
temperature of from 120° to 150°. The drying is continued until they
appear to be free from moisture, and until the precipitate ceases to
adhere to the filter. In drying sulphides the heat must not be raised to
the melting point of sulphur, since, if there is any free sulphur
present, it fuses and filters through.
[Illustration: FIG. 20.]
The precipitate, having been dried, is transferred to a watch-glass. The
filter-paper is opened out over a sheet of note-paper, and, with a
camel-hair brush, the precipitate is gently brought into the glass. Most
precipitates come away easily, and the transfer can be made without
apparent loss. The watch-glass is covered by the funnel, and the
filter-paper (folded into a quadrant) held by the tweezers and set fire
to with the flame of a Bunsen burner. It is allowed to burn over the
crucible, into which the black bulky ash is allowed to drop, and two or
three drops of nitric acid are then added.
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