Mercuric Chloride Synthesis Success?

I finally decided to prepare mercuric chloride (HgCl2) since this compound is hard to get by an amateur. I'd like to share the process here for a critical review.

The procedure has been already discussed here and performed by Nile Red. So there goes the credits. Here I just added photos and comments about the procedure. The general procedure goes like this:

Hg ->(nitric acid)-> Hg(NO₃)₂ ->(heat or base)-> HgO ->(hydrochloric acid)-> HgCl

I did one modification to the procedure, which is obtaining mercuric oxide by precipitation from solution instead of heat decomposition. This makes the procedure more messy, but it is quicker and I believe also removes the guesswork of mercuric nitrate being fully decomposed to oxide.

First, I dissolved about 50.2 grams (0.25 mol) of elemental mercury in 1.14 mol nitric acid (I used 90 mL, 59% HNO3):



Note the tissue paper added on top to prevent liquid droplets escaping the flask. Unfortunately, the reaction became too vigorous and I had to quickly transfer everything to a 250 mL Erlenmeyer flask (the 150 mL was too small). It would be also safer to add the mercury by parts and not all at once to prevent frothing and thermal runaway. Maybe cooling the flask would help (I am used to dissolve silver which does not react as readily as mercury).

When everything dissolved, I started adding 25% sodium hydroxide to precipitate mercuric oxide (HgO). At first, a tan-brown precipitate apperead which turned to pumpkin colour and more precipitattion happened as I added excess hydroxide:





Now for the filtration part. At first I tried a sintered funnel but the HgO powder is so fine it went through. I had a hard time later to clean the sinter (using hot nitric acid), so using the sintered funnel for this is a no no:



I then used Buchner funnel with two stacked filter papers with great success, though small amount of solids still went through (might be solved by re-filtering over the bed of solids from the first filtration). I washed the beaker several times with water to remove all the mercuric oxide from it.

One messy part is to get all the HgO off the filter into a clean container. I used standard cellulose filter paper but finer type of filter would work better, such as glass fiber filter.

I washed the solids on filter with distilled water to remove any sodium hydroxide in it.

I then transferred most of the material to a beaker (using spatula) and poured about 52 mL of 36.5% HCl over it. The solids blackened first (is it mercury(I) oxide?) but then dissolved when stirred:



I must note the solution heated up a lot so it would be safer to use more dilute acid, cool the container in an ice bath or do the addition more slowly. Cooling might be preferred as there will be less water to evaporate. Also use as little acid as possible to minimize amount of corrosive fumes.

Once everything dissolved, I filtered the supposed mercuric chloride solution once more (optional) and started evaporating the water. Here is the setup I used:





There is a wire gauze added between the beaker and the hotplate to avoid overheating and smooth out the temperature changes. I then stuck a thermocouple probe between the beaker and the ceramic plate to monitor actual temperature under the beaker and never allowed it to exceed 150 °C. The probe is all metal and protected by glass fibers so it is safe to use it that way.

Finally, the solution was slowly magnetically stirred and a fan was blowing air over the beaker to promote evaporation. The fume hood was being vented at maximum speed.

This process took several hours and I ended up with a moist powder resebling scrambled eggs mixed with sand:



I continued heating but this time ran at lower temperature (around 100 °C) to prevent the dry mercuric chloride to vaporise (I found in text it sublimes at elevated temperatures since it does not form a tightly held ionic structure like most salts).

I also monitored the HCl vapours by a pH paper:



I stopped the drying process once the paper had not turned substantially red even when held over the powder for prolonged time.

Here is the result, a free flowing bright yellow powder:



The yield was 95.6% (about 65 grams of HgCl2) which makes sense provided there were losses during the filtration. It could be greatly improved with technique though.

However, I doubt the purity of my mercuric chloride provided it is yellowish and not white (presence of oxides?). There could be metal impurities from the original mercury sample (not much but enough to color the product perhaps).

Cleanup

I washed all the glassware twice with conc. nitric acid (dilute might be sufficient) then once with deionised water.

All the washings were collected in a large beaker and there is about 1 100 mL of water containing mostly nitric acid, mercuric nitrate and mercuric chloride.

I plan to precipitate mercury as sulfide (using sodium sulfide) and perform more cleanup of the liquid part if necessary. I can use mercury test strips for the liquid to see whether the liquid is safe to discard.

The solid mercury sulfide will be stored for making elemental mercury later.

[Edited on 20-5-2019 by nimgoldman]

[Edited on 20-5-2019 by nimgoldman]

Quote: Originally posted by Keras

Did you try passing your mercury chloride over activated charcoal?