Boffis - 15-6-2017 at 08:49
Many years ago I was given a jar of mercurous iodide, it originally contain 1lb (about 450g) of Hg2I2 and although is very very old little appears to
have been used. I stumbled across it recently and wondered if it can be converted into anything useful, which to me means mercuric nitrate or
thiocyanate. I would also like to recover the iodine if possible.
I did a little experiment with sodium hydroxide solution, I mixed 10g of the iodide with 15.3ml of 2M NaOH (1:2 M ration) and added another 10ml of
water. This was based on the idea that the iodide will decompose into sodium iodide solution and mercurous hydroxide which is very unstable and almost
immediately decomposes to mercuric oxide, mercury and water. As expected the iodide (greenish yellow) turned black and remained undissolved. However I
discovered that a significant amount of mercury ends up in solution, presumably as an iodomercurate complex anion. This suggests a reaction along the
lines:
2Hg<sub>2</sub>I<sub>2</sub> + 2NaOH → Na<sub>2</sub>HgI<sub>4</sub> + 2Hg + HgO +
H<sub>2</sub>O
(I am not sure about the composition of the iodomercurate ion)
So where next? My idea was to dissolve the black oxide/mercury mixture in hot, strong nitric acid and vacuum evaporate to form a concentrated solution
of mercuric nitrate while the sodium iodide solution would be simply evaporated down to crystals. The problem is the mercury content of the sodium
iodide.
Any suggestion? I thought about bubbling H2S through the sodium iodide solution but would this precipitate all of the mercury? Mercuric sulphide is
extremely insoluble so it should dcompose the iodomercurate ion.
Alternatively I thought about using sulphuric acid (or nitric acid) and hydrogen peroxide and extracting the iodine with a chlorinated solvent and
then recovering the Hg2+ from the aqueous residue in some form.
j_sum1 - 15-6-2017 at 15:45
Chemplayer did something similar with AgI. https://www.youtube.com/watch?v=_DYFjJFHWsg
There is little reason why you could not recover both the mercury and the iodine.
(Disclaimer -- it is some months since I last watched this video and do not recall the exact procedure used. So, direct appropriation of the method
might not be sensible.)
Melgar - 15-6-2017 at 15:53
Your best bet seems to be a dissolving metal reduction, giving you the iodide salt of the more reactive metal, and mercury metal. Zinc seems like it
might be convenient, and zinc is very commonly used as a means to precipitate out nobler metals. You'd probably want to separate zinc and iodine via
electrolysis, which should be very easy, since neither are especially reactive with water, and zinc iodide is water-soluble, hygroscopic even. This
also is pretty foolproof and gets the mercury into its elemental form immediately, limiting your use of mercury salts to as little as possible.
[Edited on 6/16/17 by Melgar]