Preparation of Mercury Salts

Mercuric Chloride (HgCl2)

By ZyGoat

ZyGoat has seen several Hive posts relating to the use of Hg2Cl2 as a substitute for HgCl2 in the Al/Hg reductive amination procedure. Hg2Cl2 is inferior to HgCl2 due to its insolubility. Making HgCl2 is not difficult - here's the procedure:

You will need:

  • Mercury (thermometers, mercury switches, old barometers, suppliers to watch and clock repairers (ever heard of a mercury pendulum?), blood pressure measuring device from medical supplies)
  • Concentrated sulphuric acid (boiled battery acid, plumbing supplies, drain cleaner)
  • Sodium carbonate (washing soda) or sodium bicarbonate
  • Hydrochloric acid (pool supplies, if you have sulphuric acid you can make hydrochloric with salt)

[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.

Mercurous Chloride (Hg2Cl2)

by AtomicDog

Mercurous Chloride is a simple mercury salt with the form of Hg2Cl2. The reaction described herin was thought to produce Mercuric Chloride (HgCl2) instead, which is used for making amalgams. But it turns out that Mercurous chloride does a perfectly fine job for amalgams and is thus a good substitute. Hg2Cl2 is insoluble in alcohol and only slightly soluble in water but it still manages to coat aluminum in a sol'n of either solvent. It is suggested that Hg2Cl2 amalgams be made in mildly acidic water (pH 5-6) because it is at least somewhat soluble in water and acidity increases that solubility. Apparently even plain Hg in acidic water can work but Atomic feels that this is very difficult to measure out plain mercury and thus get the same results. This reaction is super easy and takes only a few hours at most.

On the plus side, Mercurous Chloride isn't quite as dangerous as HgCl2. That insolubility makes it difficult for your body to absorb, the Merck even says that if ingested, a saline laxative should be administered to prevent mercury poisoning, try that with HgCl2! (joking, don't try that). Hg2Cl2 is a white, odorless, tasteless powder. Protect it from sunlight or it will slowly degrade into HgCl2 and Hg. Hg2Cl2 sublimes at 400-500C without melting. Please note that any solution of mercury should be considered EXTREMELY dangerous and poisonous and be handled using vynal gloves with extreme care.

We'll be making Hg2Cl2 today by passing chlorine gas over elemental mercury at room temperature and pressure. You'll need elemental mercury from an old thermometer or thermostat, and muriatic acid (31.5% HCl), and sodium hypochlorite sol'n (Clorox bleach). Atomic has found that a 2:1 ratio of bleach to acid is about right or you could use liquid chlorine (stronger stuff) from the same pool store you got the acid at go 1:1. Chlorine gas is a nasty green gas that will drive you out of the lab in very small amounts and is so corrosive that in time it will turn steel into dust. You don't want any leaks in your setup for gas to escape! 200ml of acid and 400ml of bleach will give you 15-20 min of gas and that should do it for you. In an ideal setup, your Sodium Hypochorite sol'n drips from an addition funnel into a flask filled with your acid and is stirring away. If you don't have mag stirring then you'll have to swirl the flask occasionally to get everything to react. A tube leads the gas to another where it bubbles through water to clean the gas and then another filled with CaCl2 to dry it, and then to another where the mercury waits. Atomics experience is that the water and drying aren't really necessary because not much else will react with the Hg and since Hg2Cl2 isn't really H2O soluble or nightmarishly dangerous that drying the gas is no big either. But be warned! The best way to go is clean and dry. If you use water to clean the gas be aware that for a few minutes it will absorb your gas until it becomes saturated and lets it pass through, be patient. When the green gas does hit the mercury you'll se it right away: Your fast moving, T2 looking blob will turn a dull dark gray instantly and slow down or freeze up entirely. This is a hard layer of Hg2Cl2 forming on the outside of your mercury, protecting it from the gas all around it. You have too pick up the flask and shake that baby to break out the Hg inside and spread it all around the flask so it can react, expect to be doing a lot of this.

And try to use only as much Hg as you think will coat your flask or you'll have a hard time getting it all to react. At this point Atomic should point out how nice it is to use surgical tubing for this reaction instead of regular chem tubing. Surgical tubing is very flexible and allows you to really pick-up and shake that flask without pulling your setup apart. Once you have that stuff spread all around it will probably be dark grey with white at any tips. If you can see that the flask is full of green chlorine gas then you can stop the gas for now and wait. You need to wait for the mercury to react, you'll be done when it's almost completely white and even looks reacted from underneath. You'll probably never get it 100% reacted unless you take the flask out of the setup (hold your breath!) and use a spatula to 'scratch' around in there and break-up any remaining clumps and unreacted patches. This is especially true if you use too much Hg for the size flask you're using. Once finished you can use that spatula to scrap out all you can and put it in a suitable container. Then pour some water in the flask and swirl that baby to lift the remaining Hg2Cl2 off the flask walls and quickly pour it into a Pyrex dish. When the water evaporates you can scoop the rest of your Hg2Cl2 into your container.

There you have it, the new and improved doc, now go forth and bee fruitful!