It
comes down in shaggy tufts which are difficult to wash and easy to
oxidise.
VOLUMETRIC ASSAY.
There are two methods which have been proposed; one based on the
precipitation as chromate and the estimation of the chromic acid; and
the other on the precipitation as oxalate and subsequent titration with
permanganate of potash. These offer little advantage over the easy
gravimetric determination.
COLORIMETRIC METHOD.
Bismuth iodide dissolves in excess of potassium iodide, forming a
yellow-coloured solution, indistinguishable in colour from that given by
iodine. The colour, however, is not removed by boiling or by sulphurous
acid. Since none of the commoner metals give such a colour, and free
iodine is easily separated by boiling, this method is specially suited
for small determinations of bismuth.
It requires a _solution of bismuth_, made by dissolving 0.1 gram of
bismuth in a drop or so of nitric acid, evaporating with a little
sulphuric acid and diluting with water to 1 litre. 1 c.c. will contain
0.1 milligram of bismuth. And a _solution of sulphurous acid_, made by
diluting 10 c.c. of the commercial acid to 1 litre with water.
The determination is made in the usual way: 50 c.c. of the prepared
solution, which should not carry more than 0.75 milligram nor less than
0.01 milligram of bismuth, are placed in a Nessler tube and the colour
compared with that observed in a similar tube containing water and
potassium iodide on adding the standard solution of bismuth.
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