The residue from sewages and waters high in organic matter may be
ignited to burn off the organic matter, which, with some volatile
inorganic matter, constitutes the loss on ignition.
_Procedure._—Ignite the residue in the platinum dish at a low red heat.
If great accuracy is desired this should be done in an electric muffle
furnace or in a radiator, which consists of a platinum or a nickel dish
large enough to allow an air space of about half an inch between it and
the dish within it, the inner dish being supported by a triangle of
platinum wire laid on the bottom of the outer dish. A disc of platinum
or nickel foil large enough to cover the outer dish is suspended over
the inner dish to radiate the heat into it. The larger dish is heated to
bright redness until the residue is white or nearly so. Allow the dish
to cool, and moisten the residue with a few drops of distilled water.
Dry the residue in the oven, cool in a desiccator, and weigh. The fixed
residue on evaporation is the difference between this weight and the
weight of the dish.
The loss on ignition is the difference between the total residue on
evaporation and the fixed residue on evaporation.
If the odor and color on ignition of some residues give helpful clues to
the character of the organic matter record them.