Sciencemadness Discussion Board

HgI2-->HgCl2 question

Justin - 30-10-2010 at 14:41

Hi, i just ordered some HgI2 and i would like to convert it too the soluble HgCl2 for amalgams, and recover the I2 in the conversion process. However HgI2 is totally insoluble in water, on the order of 6mg/100mL. So metathesis is out of the question. Will treatment with aq HCl work??

Also i am quite aware of the dangers of soluble mercury salts and have the PPE and resources to deal with it.

Thanks, Justin

not_important - 30-10-2010 at 15:29

Treatment with Cl2 would release I2, while using HCl would yield HI. The low solubility of HgI2 would tend to drive a mixture to the HgI2+HCl side, unless the chloro complex is so much more stable as to reverse that - and you'd still be faced with separating the HI from the mercuric solution.

You'd likely want an oxidising attack HNO3 to give I2 and mercuric nitrate, or HCl plus an oxidiser to give I2 plus HgCl2 (or chloro complex); the I2 eing removed by filtration through fritted glass (hot oxidising acidic solutions and not friends of paper) or by distilling it off along with water.

Thus while I can't give a specific answer, I do think what you appeared to be hoping to do isn't going to be real simple although it may be reasonable easy.

Justin - 31-10-2010 at 02:21

Im thinking of using HNO3 to oxidize the iodide to iodine and vacuum filtering the I2 thru my glass fritted filter funnel and decomposing the formed nitrate too HgO and then treatment with aq HCl, thats the usual way to HgCl2 from Hg correct??

Jor - 31-10-2010 at 03:08

Yes, I would oxidise it with HNO3. This also works with PbI2, giving I2 and a lead nitrate solution.
You can filter it, distill it off, or extract multiple times with an organic solvent to remove the I2.
Then add excess NaOH, filter the HgO, and dissolve in HCl.

smuv - 31-10-2010 at 08:49

HgI2 can be used for amalgumations...just use it as a suspension. Initiation might take a little longer, but it for sure works.

Even then, if your aim is to get a soluble Hg salt, there is no point making HgCl2. Make the nitrate as described previously and use that.

[Edited on 10-31-2010 by smuv]

not_important - 31-10-2010 at 09:41

Indeed, the nitrate will work fine. If you really want the chloride then simply evaporate to a mushy state, add 20% HCl in several-fold molar excess evaporate once again, repeat until all the nitrate has been decomposed - no more Cl2 & NOCl are formed during evaporation. It's an or trad wet analysis method for converting betwixt chlorides and nitrates, or from the salt of a volatile acid to the sulfate or phosphate.

Justin - 31-10-2010 at 13:09

Quote: Originally posted by smuv  
HgI2 can be used for amalgumations...just use it as a suspension. Initiation might take a little longer, but it for sure works.

Even then, if your aim is to get a soluble Hg salt, there is no point making HgCl2. Make the nitrate as described previously and use that.

[Edited on 10-31-2010 by smuv]


I might use it as is, however it would be nice to recover the iodine since i got quite a large bit of HgI2(100g). I do have plenty of iodine however, and its probably safer working with an insoluble salt rather than a soluble one I'll try it as is and see how well it works, my intention is to use it for amalgamating Al for aluminum isopropoxide and other Al/Hg reductions. But that 55g of iodine sitting in that 100g HgI2 sure is tempting to get out considering im in the states.

mr.crow - 31-10-2010 at 17:26

There are much easier ways to make both I2 and HgCl2. I don't recommend it, just let the pretty bottle sit on your shelf.

You could use the standard HCl and H2O2 method for iodine. This may be really slow or not work at all due to the poor solubility. Separating the two products would be annoying since HgCl2 also sublimes at high temperatures.

Justin - 31-10-2010 at 20:39

Quote: Originally posted by mr.crow  
There are much easier ways to make both I2 and HgCl2. I don't recommend it, just let the pretty bottle sit on your shelf.

You could use the standard HCl and H2O2 method for iodine. This may be really slow or not work at all due to the poor solubility. Separating the two products would be annoying since HgCl2 also sublimes at high temperatures.


Im aware that there are much easier ways to I2 and HgCl2, I just happen to have got 100g of HgI2 for a very good price, i've already got 200g 99.8%+ I2 from mallinckrodt, so im in no need for I2 but why not recover it?? Insolubility is gonna be a problem anyway for my intentions, the nitrate salt would be much better. It is a pretty bottle though, 99%+ pfaltz and bauer.

madscientist - 31-10-2010 at 21:15

Nitric acid should oxidize HI to I2 as it's formed...

Jor - 1-11-2010 at 03:16

Quote: Originally posted by not_important  
Indeed, the nitrate will work fine. If you really want the chloride then simply evaporate to a mushy state, add 20% HCl in several-fold molar excess evaporate once again, repeat until all the nitrate has been decomposed - no more Cl2 & NOCl are formed during evaporation. It's an or trad wet analysis method for converting betwixt chlorides and nitrates, or from the salt of a volatile acid to the sulfate or phosphate.


Actually, although this method works, I would not recommend this. When boiling solution, there are always some small aerosols created, wich in this case contain mercuric-ions. These could be inhaled or contaminate the area.
I have some purple spots on the bench in my fume hood. This was caused by boiling Au/Ag solutions vigorously. Or just try boiling a silver nitrate solution , and keep your hand over the mouth of the flask. You will soon have black stains on your hand!

This can ofcourse be avoided if you use distilling apparatus.

But I would still go the HgO route, seems safer in my opinion.

not_important - 1-11-2010 at 13:55

Generally done in Erlenmeyer flasks with a loose tuft of glasswool in the top. Same problem with just about any aqueous solutions of mercury salts being boiled take precautions.