Sciencemadness Discussion Board

HgCl2 questions

Pheromone - 21-8-2009 at 18:17

I've some questions. Really it's more about reaction mechanisms. Involving 2 of the HgCl2 syntheses. Anyway, I'm confused as to why you need to use a base in the Hg/H2SO4/HCl method & in the Hg/nitric/HCl method you use no base. But I've also seen where people say you need a base in the nitric method.

e.g. "To get mercury(II) oxide one would need mercury(II) nitrate & add it to an excess of sodium hydroxide which would yield the oxide as a yellow precipitate. At first adding sodium hydroxide in small amounts will form a brown red precipitate 'till a stoichiometric amount is added when it will form a yellow precipitate."

e.g. "NaOH has the advantage of no gas evolution & therefore no toxic spray droplets.The problem would be the strong heat evolution, but this can be overcome by diluting both solutions. The nitric doesn't have to be removed first, because the sodium nitrate stays in solution.

I haven't tried to make HgO, but someone told me that he had tried it & the addition of NaOH to the Hg(NO3)2/HNO3 solution caused the red modification to precipitate. This is strange because precipitations nearly always produce the yellow modification. The yellow HgO is finely divided & has some lattice distortions (& thus higher reactivity: it reacts faster with acids such as HCl). The red modification is produced by carefully heating the yellow modification."

But most the nitric synths I've seen don't need a base to get to the oxide. Why does ZyGoat's method need NaHCO3, Na2CO3, or NaOH. It's the basic sulfate to oxide, then HCl to HgCl2; & the nitric method doesn't. It's nitrate to oxide, then HCl to HgCl2.

I'm interested in the ZyGoat method & the Mr. Anonymnous method.

ZyGoat:
[1] HgSO4

Measure 20g Hg and place in a 100ml conical flask. Do not use a round bottom flask, as all the mercury will not react. Add 60 ml concentrated H2SO4 (should be at least 94%) to the flask and fit a single hole stopper with a tube leading outside. This reaction produces lots of SO2 that will give you chemically induced asthma if breathed. Now slowly heat the flask. Bubbles of SO2 will rise from the acid/mercury interface. Maintain a vigorous bubbling of SO2 by adjusting the heat. A white crystalline deposit of HgSO4 will appear. The mercury should be completely reacted after about 30 minutes. Allow the reaction mixture to cool and pour off the acid. Pour the crystals/acid into 750 ml hot water and filter. Keep the liquid.

[2] HgCl2

Make up 500 ml saturated sodium(bi)carbonate solution. Add this to the HgSO4/acid solution in small quantities until effervesscence upon addition stops and a red-brown percipitate forms. Filter off the percipitate. Add some more (bi)carbonate solution to the filtrate to ensure all the HgSO4 has been reacted. Filter and repeat until no more red-brown percipitate forms. Wash percipitate with DH2O. Place in a 500 ml beaker and create a suspension with 20 ml DH2O. Make up a solution of 20 ml strong hydrochloric acid in 100 ml DH2O. Add this to the suspension in small quantities with good stirring. The solution will change from brown-red to yellow and finally a white curdy percipitate will be present in the liquid. Stop addition of the HCl when the white percipitate appears. It is important that the solution pH should be near neutral as excess hydrochloric acid will prevent the HgCl2 crystallizing in the next stage. Evaporate off most of the water either outside or fit a single hole stopper with a tube leading outside to vent the steam away. Cool, filter the HgCl2 and dry. Yield 19.37g white crystals. For greater purity recrystallize from boiling DH2O.

Be very careful with HgCl2. Ingestion of 1/2 g has been known to be fatal. Don't heat in an open container above 300 C as it will volatise and end up in your lungs. Always wear gloves when handling HgCl2 and its solutions.

Mr. Anonymous:
Mercuric Chloride HgCl2 is a useful, although highly toxic material which may be easily made from metallic mercury. It is imperative this synthesis be done outdoors because of extremely toxic fumes, or in a fume hood, or making some other arrangement for active ventilation which efficiently carries away the quite deadly nitrogen dioxide fumes which are evolved at two stages in the synthesis of the precursor mercuric oxide. The mercury is added to and dissolved in an excess of nitric acid density 1.42, and the mercuric nitrate solution produced is evaporated to dryness, decomposing the unstable mercuric nitrate to form mercuric oxide. The mercuric oxide is then dissolved in a slight excess of the theoretical amount required of concentrated HCl to form an acidic solution of mercuric chloride. The solution is evaporated to yield crystals of stable mercuric chloride.

Experimental :

50.2 grams of mercury is added to 75 ml of nitric acid in a 250 or 500ml flat bottom flask, and set aside for about two hours unattended for the mercury to dissolve. Nitrogen dioxide is evolved from the mixture and the red fumes should be avoided because of the extreme toxicicity. The resulting solution of meruric nitrate is poured in a shallow layer in a large glass bowl placed on a slightly boiling water bath, and the heated solution is evaporated to dryness on the very slightly simmering water bath.

A crock pot with a removable liner is a utensil which I find is ideal for such evaporations. With the crock left uncovered the evaporation to dryness of shallow solutions is easily done. Once a kitchen implement is used for processing toxic chemicals, it should never be used again for any food handling because of toxic residues.

The dry hot mercuric nitrate residue is heated for an additional one hour with occasional stirring of the decomposing mercuric nitrate with a glass rod until a yellowish white residue of mercuric oxide results. Red fumes of nitrogen dioxide are freely evolved during both the dissolving of the mercury in the nitric acid and the subsequent decomposition of the mercuric nitrate, and these fumes are highly toxic. Avoid these fumes, understanding that by "highly toxic", deadly is the accurate description of nitrogen dioxide. It is very bad stuff. Keep distant from the reactions during the time when these dark red fumes are seen being evolved. To the dry residue of mercuric oxide is added 60 ml of 31.45 per cent HCl and the bowl is swirled to form a clear solution of HgCl2. Some fumes from the excess HCl will be evolved from the hot solution and these should be avoided also. The clear solution is again evaporated to dryness on the same slightly boiling hot water bath. The residue of mercuric chloride is scraped from the bowl as much as possible using a plastic (not metal) spoon and stored in a properly labeled (Mercuric Chloride, HgCl2, POISON) glass container with a plastic or glass cover. Use no metal lids and use no metal implements in contact with Mercuric Chloride. The slight residue which cannot be scraped from the bowl can be rinsed from the glass with a small stream of methanol, into a small puddle on the bottom of the tilted bowl. When the methanol evaporates the residue will be collected in a small area and this can be scraped from the bowl and added to the bulk of the mercuric chloride which was scraped from the bowl earlier. Good technique should be practiced with mercury chloride because of its toxicity, keeping account of every gram possible and avoiding its release into the environment. This ethic should apply to all mercury compounds but especially highly soluble mercury salts like mercuric chloride. 67.95 grams would be 100 percent of theory, however the actual yield of 67 grams will be slightly less than the theoretical yield, due to volatility and losses during manipulation.

Caution concerning metals:

On contact with many metals, the mercuric chloride or its solutions, is likely to amalgamate the metal and begin an active corrosive oxidation of the metal, which will gradually reduce the metal to dust. Aluminum and magnesium are especially suceptible to attack and the rate of the oxidation is such that the oxide forming from the metal can be seen extruding from the surface at a visible rate. The process can only be stopped by heating the amalgamated metal hot enough to boil away the mercury from its surface, or to chemically arrest the process with something which reacts with the mercury and converts it to a sulfide (in theory). Free sulfur, or sulfur dissolved in carbon disulfide, or sodium sulfide, or perhaps sodium polysulfide may work to deactivate an amalgam. However I have not done experiments to confirm this countermeasure. If even a microscopic trace of the amalgam remained as a residue, the corrosion would continue. Do not get mercuric chloride on anything made of aluminum which you wish to keep intact.

not_important - 21-8-2009 at 19:08

Because the 'nitric' method uses heat to decompose the mercuric nitrate, forming the (red) oxide. The other precipitates the oxide from a solution of mercuric salt, which can form the yellow, red, or orange (intermediate structure) oxide depending on temperature and other conditions.

Decomposing the nitrate forms nitrogen oxides, which are nasty and require good ventilation to prevent damage to one's lungs and the neighbors' complaining; it also releases some mercury if you heat it too hot. The wet method avoids those problems.

There is an older traditional preparation, 20 parts by weight of HgSO4 is heated with 10 parts NaCl and 1 part MnO2 in a wide necked flask or retort, the neck venting into a bottle that receives the sublimed HgCl2. Strong heating is needed, as well as good ventilation (because you ar subliming the HgCl2) although modern ground glass joints make it a bit easier than in the old days.

Pheromone - 21-8-2009 at 20:31

Heating with H2SO4 doesn't produce deadly sulfur oxides, as compared to nitric oxides? Seems the same to me, just using different acids. So the heat with the H2SO4 doesn't decompose it to HgO? It needs an intermediate basic salt 1st? What exactly is going on with the subsulfate phase to the oxide? How does the base influence this in ZyGoat's method? How come some mention using NaOH in the nitric method & then some don't?

Is this the reaction mechanism for the nitric method:
Mercury (Hg) + Nitric acid (HNO3) -> Mercury(II) nitrate or Mercuric nitrate (Hg(NO3)2 or HgN2O6) + Nitrogen dioxide (NO2) + heat -> Mercury(II) oxide or Mercuric oxide (HgO) + Nitrogen dioxide (NO2) -> HgO + HCl -> HgCl2

What's the reaction mechanism for the ZyGoat method???

What's the wet method?

[Edited on 22-8-2009 by Pheromone]

not_important - 21-8-2009 at 21:10

It's not HNO3 vs H2SO4, it's wet vs dry in the formation of HgO; wet being the precipitation of HgO via addition of base, dry being either decomposing Hg(NO3)2 with heat. Wet = yellow if kept cool, dry = red.

The other wet method (you didn't mention it) doesn't use the oxide, it forms the basic sulfate by boiling HgSO4 with water. The basic sulfate reacts with concentrated hydrochloric acid in a fashion similar to HgO, except that H2SO4 is formed and the HgCl2 is separated from that (remember there is a lot of water in concentrated hydrochloric acid). The other dry method is the reaction of HgSO4 and NaCl at temperature to directly get HgCl2, no HgO intermediate step and no base added.

Heating HgSO4 will decompose it, but at such a high temperature that the HgO decomposes too; it starts to decompose at a lower temperature to give basic sulfates while the nitrate goes fairly easily to the oxide. And yes, SO2 is formed when reacting hot H2SO4 with Hg, but it's not as bad on your lungs as the NOx mix.

Seems you could use some time with some good pre-WWII inorganic chemistry books before messing with using mercury. Just following recipes without understanding isn't very wise.


Pheromone - 21-8-2009 at 21:46

"Just following recipes without understanding isn't very wise."

Which is exactly why I want to know the reaction mechanisms, etc. I want to understand everything that's going on. Not just adding the milk & eggs cuz it says so. I wanna know why? Thanks though, you've helped a great deal.

One more question though if you could? Can these reactions be used similarly to produce other +2 metal chlorides. Like dissolving copper wire for cupric & platnum(II) chloride or palladium(II) chloride, etc.???

not_important - 21-8-2009 at 23:42

Wet routes are easier for copper, unless you're making large amounts of CuCl2 from copper scrape, in which case heating in air and pounding on the hot copper to knock loose the oxide is a cheap way to CuO, then with HCl to CuCl2. The platinum group metals usually are converted to their chlorides through aqua regia solution, which forms chloro complexes in most cases; read up on that group to see more.

Again, download some of the copyright-expired books on inorganic preparations to get examples of making particular compounds, as well as Mellor's or similar.

turd - 22-8-2009 at 00:22

I posted it before, but it cannot harm to post it again: The Zygoat method wastes precious Hg. The offending part is in the innocent looking sentence:
Quote:
Pour the crystals/acid into 750 ml hot water and filter. Keep the liquid.

Adding HgSO4 to water will produce one of the infamous basic mercury salts and this will be filtered off. Of course the losses will depend on how much free acid was still present in the HgSO4/acid mix. Typically, when making a HgSO4 solution, HgSO4 is suspended in water and H2SO4 is added dropwise with stirring until nearly everything goes into solution. This is then filtered. I'm not sure if a solution obtained like this precipitates sulfate-free HgO by addition of a base. Check the literature.

Mercury salts (Hg+ and Hg2+) are acidic in water and that the corresponding basic (hydroxy) salts often have low solubility (with the exception of for example HgCl2 and Hg(AcO)2).
Quote: Originally posted by Pheromone  

Is this the reaction mechanism for the nitric method:
Mercury (Hg) + Nitric acid (HNO3) -> Mercury(II) nitrate or Mercuric nitrate (Hg(NO3)2 or HgN2O6) + Nitrogen dioxide (NO2) + heat -> Mercury(II) oxide or Mercuric oxide (HgO) + Nitrogen dioxide (NO2) -> HgO + HCl -> HgCl2

What's the reaction mechanism for the ZyGoat method???

Reaction mechanisms are a very useful concept in organic and also to a lesser extent in metalorganic chemistry. What you are posting are chemical equations, which are perfectly adequate in inorganic chemistry. Also please don't write HgN2O6 for mercury(II) nitrate.

As not_important already pointed out, you seem to be confused about precipitation vs. roasting reactions. Check the literature on when HgSO4 decomposes into HgO on heating. The brute chemical equation for the precipitation with sodium hydroxide is of course simply:
Hg(2+) + 2OH- --> HgO + H2O

zed - 12-9-2009 at 10:39

Seems like a lot of trouble, if you just want HgCl2 to make amalgams.

HgI2 should work equally well. I am under the impression that HgI2 can be made directly, by the reaction of Hg with I2.