
HI Gas
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Hydrogen iodide may be conveniently prepared by allowing a solution of two
parts of iodine in one part of hydriodic acid (density 1.7), to drop onto
an excess of red phosphorous. The reaction takes place in the cold. When
the evolution of gas slackens considerably, the mixture should be gently
warmed.


57% Hydriodic acid
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A 1.5 litre three-necked flask is charged with a mixture of 480g of iodine
anf 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% NaOH. The mixture
is vigorously stirred, and a stream of hydrogen sulphide (*) is passed in
as fast as it is absorbed. After several hours the liquid assumes a yellow
colour (sometimes even colorless) and most of the sulphur sticks together
in the form of a hard lump. The sulfur is removed by filtration through a
funnel plugged with glass wool (or through a sintered glass funnel). The
hydriodic acid is then distilled, and the fraction boiling between
125.5-126.5 deg C is collected as 57 percent hydrogen iodide. The yield is
90% of the theorethical.

                            H2S + I2 => 2 HI + S

* Hydrogen sulfide is either generated by dripping HCl on lumps of FeS
  (made by fusing iron powder with sulfur), or upon sodium bisulfite.

(Ref: Vogel, 3rd Ed.)

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Preparation of 100g 57% HI
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Put 100g H3PO4 and 100g NaI or KI in a flask with a vacuum adaptor in the neck.

Run a tube from the neck of the flask into a beaker or similar containing 42g 
dH2O. (Aquarium bubbler in the end of the tube)

Heat the gas generation flask to around 60C or thereabouts untill gas is 
produced and bubbling commences.

Continue bubbling untill the reciever beaker has gained 57g and the contents 
weighs 100g total.

Notes -
* Suckback is a bastard. Try using a one-way vale avail. from aquarium places.
* If using 81% H3PO4 (ph down from Hydropnics suppliers) decrease the amount of 
  dH2O slightly.
* You may wish to place a balloon or condom over the neck of the receiver flask 
  and apply mag. stirring to minimalise the amount of HI gas lost, and the stink.

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Hydriodic Acid: 

11 parts (by weight) of iodine is placed in a small flask, and 1 part of yellow 
phosphorous, cut into small pieces and dried, is gradually added. Expect a flash 
of light and the contents to turn liquid upon the addition. When all the 
phosphorous has been added, phosphorous tri-iodide is to be separated upon 
cooling. The product is treated with 1 1/2 parts water, heated gently to produce 
hydrogen iodide, which is passed over some red phosphorous, that has been 
moistened with a little water and placed in a U tube. Heating is continued until 
the liquid just becomes colorless, because if heating is continued further, 
phosphine and phosphonium iodide are formed, which can cause a powerful explosion. 
If you require a solution of hydriodic acid (most formulas do), the gas is led
through an inverted funnel into a small quantity of cold water. This
solution if dilute can be coned by distillation. Bp: 127ø.
