Mercury (II) Chloride

2010-05-22 21:35 by Ian

In an endeavor to make higher-purity mercury, I decided to reduce it from one of its salts. But first, I have to make and purify the salt. I selected HgCl2 as the salt from which to reconstitute Hg0.

Hg is somewhat reluctant to give up electrons, so I cannot easily go directly from Hg0 to HgCl2. Moreover, even if I could, it would be harder to isolate the mercury salts from those of impurities I know to be present in the Hg.

I went via the route of Hg(NO3)2. The idea is to digest the Hg0 in concentrated nitric acid to yield a mix of Hg(NO3)2 and HgNO3, as well as the nitrates of any contaminants (primarily lead).

The lead nitrate is insoluble in nitric acid (by Le Chatelier’s principle via the nitrate anion), and is filtered out while the Hg salts stay in solution.

The acid was retained and the filter discarded. The next step was to convert the HgNO3 to Hg(NO3)2 by forcing this reaction to the right (using heat):

2Hg2(NO3)2 → Hg0 + Hg(NO3)2

The acid was heated to about 120C and left at that temperate for several hours until NOx ceased forming in the flask. The flask was then removed from heat and chilled to recrystallize the salt.

I had to do this twice, because I didn’t force the above reaction far enough right the first time. The picture below was taken on the first chill. Note the yellow crystals (HgNO3) floating on the surface-tension of the water. There are also yellow crystals at the bottom of the flask. I conclude that the HgNO3 is less-soluble in H2O than is Hg(NO3)2. The yellow crystals formed first, followed by the white forest that dominates the picture (Hg(NO3)2).

Here is some video of the first recrystallization in progress:

The Hg(NO3)2 was dessicated over a steam bath. The crystals were recovered and re-dissolved in fresh distilled water. This solution was poured into HCl (muriatic acid) at 29% and the Hg(NO3)2 fell out of solution. It was then slowly attacked by the chloride ions free in solution….

Hg2(NO3)2 + 2HCl → H2O + NO2 + HgCl2

As expected, a noxious brown gas was released from the flask as the above reaction proceeded to the right and the freshly-precipitated salt was consumed.

The final step is to dessicate the HgCl2 over a steam bath. Do not omit the steam bath, this particular compound sublimates at about 300C. The steam bath will hold a temperature sufficient to boil off any remaining solvent and stay below the temperature that would send a nasty mercury gas into the air.

At the end of the day, 48g of HgCl2 was collected and bottled. This salt will then be electrolyzed back to Hg0 in degassed distilled water and under an argon atmosphere.

Waste was made safe by dilution with water and addition of Al0. HgCl2 was reduced to Hg0 and amalgamated with the aluminum. The reaction was left to sit away from ignition sources until all Al0 was consumed and left as a gray, muddy mix of AlCl3 and Al(OH)3. A small puddle of Hg was isolated and returned to stock.

In working through this process, I couldn’t find a KSP value for HgCl2. I have the 87th edition of the CRC Handbook, and on page 4-76, it states that 7.31g of HgCl2 will dissolve in 100g of water. If this is true, I calculate that the KSP for HgCl2 is 7.81×10-2.