2. Hydriodic acid. A 1 • 5 litre three-necked flask is charged with a
mixture of 480 g. of iodine and 600 ml. of water. The central aperture
is fitted with a stopper carrying an efficient mechanical stirrer leading
almost to the bottom of the flask, and the smaller apertures respectively
with a lead-in tube for hydrogen sulphide extending to well below the
surface of the liquid and with an exit tube attached to an inverted funnel
just dipping into 5 per cent, sodium hydroxide solution. The mixture is
vigorously stirred and a stream of hydrogen sulphide (either from a
freshly-charged Kipp's apparatus or from a cylinder of the gas) passed
in as rapidly as it can be absorbed. After several hours the liquid assumes
a yellow colour (sometimes it is almost colourless) and most of the sulphur
sticks together in the form of a hard lump. The sulphur is removed by
filtration through a funnel plugged with glass wool (or through a sintered
glass funnel), and the filtrate is boiled until the lead acetate paper test for
hydrogen sulphide is negative. The solution is filtered again, if neces-
sary. The hydriodic acid is then distilled from a 500 ml. Claisen flask,
and the fraction b.p. 125-5-126-5°/760 mm. is collected. This is the
constant boiling point hydriodic acid and contains 57 per cent, of hydrogen
iodide. The yield of the constant boiling acid is 785 g. or 90 per cent, of
the theoretical.
H 2 S + I 2 —* 2HI + S.
Note.
The hard lump of sulphur remaining in the flask is best removed by boiling with
concentrated nitric acid in the fume cupboard.
Hydriodic acid, of analytical reagent quality, sp. gr. 1*7 (54-56 per
cent HI) and sp. gr. 1«94 (64-68 per cent. HI) may be purchased.
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Mostly joking here, but it'd be
pretty sweet if we did. Is there a TOR FTP-like protocol