Here are some additional hands-on details on Zygoat's prep of the evil one.
Step 1 : Reacting Hg with conc. H2SO4 to form HgSO4
Hg + 2H2SO4 = HgSO4 + SO2 + 2H2O
About 4ml conc. H2SO4 was reacted with 1.355 g Hg obtained from a sacrificial thermometer. Reaction only occurred when sufficient temp reached. The advantage of working with small amounts is that a SO2 vent system is not really necessary (unless you're working in a closet or something).
After ~25 min all the Hg was reacted and white crystalline needles of HgSO4 were formed. After it had cooled, most of the unreacted conc. H2SO4 carefully decanted. The remaining wet crystals were then dissolved in 49 ml hot dH2O and filtered to remove any insolubles. Note: HgSO4 is also poisonous.
Step 2 : Converting HgSO4 to HgO
HgSO4 + Na2CO3 = HgO + CO2 + Na2SO4
An interesting thing about HgO is that crystals < 2µm are yellow, > 8µm are red, and obviously orange in between.
Initially when the sat'd Na2CO3 soln was added to the aqueous HgSO4, the ppte was yellow due to the rapid formation of small crystals. Later the ppte became red-brown. I assume this from the previous crystals acting as sites for new growth so that eventually most of them have grown greater than 8µm.
About 15 ml sat'd Na2CO3 soln was required until effervesscence stopped. This quantity will depend on how much acid was decanted in step 1.
The HgO was filtered off. It was a bit tricky to then get all of it off the paper into a beaker. Eventually it was washed off with about 120 ml dH2O in which HgO is quite insoluble. (In retrospect it may have been easier to dry it in an oven first and then scrape off the solid.)
Afterwards most of the H2O was decanted leaving wet HgO in the beaker.
Step 3 : Converting HgO to HgCl2
HgO + 2HCl = HgCl2 + H2O
This seems to be the trickiest step due to the likelihood of over-acidifying if you're not careful. Therefore instead of suspending the HgO in water first as Z says, a more dilute soln of HCl was used (~ 6ml conc. HCl in 40 ml dH2O), and this was added slowly to the wet HgO with a dropper. After adding about 20 ml of the HCl soln, the HgO disappeared and a small amount of white HgCl2 ppte formed. Most of the HgCl2 was apparently dissolved in the H2O.
After evaporating off most of the H2O by warming (not boiling 'cos HgCl2 is at least slightly volatile), and allowing to cool, significantly more HgCl2 crystallized out.
Filtering, slow oven-drying and bottling yielded 1.15 g HgCl2.
Interesting, side note on HgCl2 toxicity,
Ullman's Encyclopedia of Industrial Chemistry claims:
'A total of 0.2g [HgCl2] introduced into the vagina can be fatal.'
So girls, you have been warned...
