Mercury(II) chlorite : a beautiful lab curiosity

So, beware, when you follow the directions of this webpage, there is a very real risk of uncontrolled violent decomposition of the material. For this reason, do not scale up the experiment, do not store mercury(II) chlorite in a small, tightly sealed container, and be prepared of possible production of a very toxic cloud of mercury(II) chloride!

Required chemicals:

• mercury metal
• concentrated nitric acid (home made acid of any concentration between 50% to 70% is suitable)
  
• sodium hydroxide
• sodium chlorite
• concentrated hydrochloric acid

Required equipment:

• small beaker or erlenmeyer
• test tubes
  
• petri dish
  
• plastic spatula
  

Safety:

• Mercury metal is toxic and one must be very careful not to spill any metal on the floor.
• Nitric acid is very corrosive. Avoid contact with skin and eyes.
• Sodium hydroxide is corrosive. Avoid contact with skin and eyes.
• Sodium chlorite is a strong oxidizer. Mixes with reducing agents are quite sensitive and may ignite by friction.
• Concentrated hydrochloric acid is corrosive and gives off corrosive and choking fumes.
• The chemical, made in this experiment, is very unstable and for this reason the experiment should not be scaled up. Unexpected violent decomposition may occur, which may have dire consequences if this occurs with large quantities.
  

Disposal:

• The waste of this experiment may not be flushed down the drain! Mercury waste is very toxic and has severe long-term environmental consequences for aquatic life. Bring the waste to a proper municipal waste processing facility.
  

Preparation of a weakly acidic solution of mercury nitrate

For this experiment, a small mercury switch is used, which can be purchased on eBay. A lot of 100 pieces can be bought for 15 to 20 euros, including the cost of shipping. The switches look like this:

These switches have a diameter of 5 or 6 mm and the length of the glass tube is appr. 1 cm. From such a switch, the top can be taken off, using a metal clamp, without shattering the entire glass tube. After removing the top, the blob of mercury must be transferred to a test tube.
Whatever method is used for getting the mercury out of the switch, do this in a plastic tub, such that if the switch shatters and the mercury is spilled, it does not spill on the workbench or on the floor. A drop of mercury easily breaks apart into a bazillion of small droplets, which can be very hard to remove and may get in cracks of the floor, workbench or wall.
The particular switch, which I used, had a globule of 172 mg of mercury. The actual amount in such switches may vary, any amount between 150 mg and 250 mg is possible.

In another test tube, approximately 1 ml of nitric acid must be heated till it is close to boiling. The acid must be fairly concentrated. The acid, used in this experiment, was home made by means of distillation, and diluted with water to get a concentration of 60% or so. Any concentration between 50% and 70% by weight is suitable, it is not very critical. Too low a concentration must be avoided, because then part of the mercury may be converted to mercury(I) and too high a concentrated may lead to problems in getting the mercury dissolved easily.
The blob of mercury must be added to the hot acid. The mercury quickly dissolves in the acid, it takes one minute or so to get it dissolved completely. The resulting liquid is yellow, due to some dissolved nitrogen dioxide. It is best to put a piece of paper tissue in the open end of the test tube, while dissolving the mercury in the acid. The dissolving of the metal is accompanied by strong bubbling and this brings an aerosol, containing mercury ions, into the air. If a piece of paper tissue is put in the open end of the test tube, then any gas (mainly air and nitrogen dioxide) can easily escape, while droplets, containing mercury, are absorbed by the paper tissue.
In the hot concentrated acid, all mercury metal is oxidized to mercury(II).

Once all mercury has dissolved, dilute the liquid with appr. 10 ml of water.

The solution with mercury(II) ions is too acidic for the remaining part of the experiment. It must be converted to a much less acidic solution. The easiest way to do that is first adding an excess amount of a dilute solution of sodium hydroxide. This will precipitate all mercury as yellow mercury(II) oxide.

The yellow precipitate should be allowed to settle. This takes a few hours. After that, decant as much of the colorless solution as possible. This colorless solution only contains trace amounts of mercury and can be flushed down the drain (assure that no yellow precipitate is decanted). Next, add another 25 to 30 ml of distilled water and stir. Allow the yellow precipitate to settle again and decant again. After this treatment, most sodium hydroxide is removed, but having a little sodium hydroxide remaining in solution is not a problem.

Next, add 1 ml of concentrated nitric acid to 10 ml of water and carefully add this dilute acid to the wet yellow precipitate, drop after drop, until all yellow precipitate just dissolves. After each drop, wait a few tens of seconds to allow part of the precipitate to dissolve. Keep adding drops of dilute acid, until the liquid is only slightly opalescent. After that, put it aside. After a few tens of minutes, the opalescence is gone and one has a completely clear colorless solution. This solution is weakly acidic, and that is exactly what is needed for the further steps in this experiment.

Preparation of mercury(II) chlorite

In the next step of the experiment, mercury(II) chlorite is prepared by adding a large excess of sodium chlorite to the colorless solution of mercury(II) nitrate.

Prepare a fairly concentrated solution of sodium chlorite by dissolving 1 gram or so of solid sodium chlorite in 10 ml of water (the commercial product usually contains approximately 80% of sodium chlorite).

Slowly, while stirring, add the solution of sodium chlorite to the weakly acidic solution of mercury nitrate. When this is done, a beautiful bright orange/red precipitate is formed. This bright orange/red compound is mercury(II) chlorite, Hg(ClO₂)₂. After formation of the precipitate, add some additional water and stir to mix the liquid and the precipitate well.

Allow the orange compound to settle at the bottom. The liquid above the orange solid is yellow. This is due to the presence of chlorine dioxide, which is formed from the excess amount of chlorite ions in the solution, together with the small amount of acid in the solution of mercury(II) nitrate.

This yellow color is a good indication. It means that the solution really was somewhat acidic, but not too much (otherwise the liquid would have an intense yellow color). The weak acidity is important. If the solution is not acidic, then the precipitate will not be pure mercury(II) chlorite, but it will be a basic precipitate, containing chlorite, but also oxide and/or hydroxide. The color of such a basic precipitate is less brilliant, it is more brown/orange instead of red/orange.

Once the orange precipitate has settled, decant the yellow liquid and add 30 ml of distilled water and stir. Let the precipitate settle again. After this second step, the liquid will be nearly colorless, or maybe very pale yellow, depending on how well the liquid was decanted after the first step. Again decant the (nearly) colorless liquid.
The liquids can be decanted into the sink, they hardly contain any mercury. Just be careful not to decant some of the orange precipitate into the sink with the liquid.

Isolation of mercury(II) chlorite

Finally, the mercury chlorite can be isolated. After decanting the (nearly) colorless liquid, a small volume of liquid, containing the orange precipitate, is left.

Pour the liquid with the precipitate into a petri dish and allow this to dry on a warm (but not hot) place, free of dust.

One day later, after drying, the solid looks as follows:

With a plastic spatula, the solid can be scraped from the glass without any problems. This scraping must be done carefully. If it is done too wildly, then the material decomposes already while scraping it from the glass. This is accompanied with tiny high-pitched crackling noises. Do not collect all of the solid on a single heap!

UNEXPECTED EXPLOSION!

The next step is collecting the solid into a small vial. Only appr. 200 mg of solid is obtained, so only a small vial is necessary. This is where this experiment had an unexpected turn. After scraping nearly all orange solid from the glass, the solid was collected on one side of the petri dish and a small funnel was used to collect the solid into the small vial. When nearly all solid was in the vial (appr. 3 mm layer of orange powder) it suddenly exploded, giving a big cloud of white smoke and a high pitched, not very loud bang. The explosion was not violent, it was at the border of being a very fast decomposition and being a true explosion.

Below follow pictures of the vial after the explosion. There is no picture of the vial with the orange powder, it exploded before a picture could be made. The explosion occurred while the funnel was still above the vial and was tapped a little to tap off adhering orange powder into the vial.

The pictures show the white solid, which covers the inside and the rim of the little vial and one can also see some remains of the orange solid.

After the white cloud was produced, I immediately kept my breath, opened the window, started a fan at full speed, closed the door and left the home lab for a while. The white smoke almost certainly is mercury(II) chloride, very toxic, not something you want to inhale!

The vial was not hot after the explosion (I had it in my hand, together with the funnel, while the explosion occurred and with the other hand I tapped the funnel). Apparently not much heat is produced in the decomposition reaction, but a big plume of white smoke was ejected from the vial. The funnel also was covered by a white layer of 'frost'.

After this failure, some concentrated hydrochloric acid was added to the frosty material in the vial. This results in immediate dissolving of the white solid and also of the orange specks. The solution is yellow, due to formation of chlorine dioxide.

            

In the petri dish a tiny amount of orange material was left over. In the recorded video it is demonstrated how easy it is to decompose the orange material. The spatula, which touches the orange powder is heated somewhat (close to 100 °C, most likely it cooled down to a little less before it hit the orange powder) and as soon as it touches the orange powder, it decomposes, giving white smoke.

Discussion of results

This reaction is carried out in a large excess amount of acid. The mercury(II) ions are precipitated with hydroxide ions and after that, the resulting oxide is redissolved again with a small excess amount of acid:

   Hg²⁺  +  2 OH^–  →  HgO  +  H₂O

The HgO precipitates as a yellow solid. Probably it is in a hydrous form. With a little acid, the yellow HgO is redissolved again to mercury(II) ions in solution:

   HgO  +  2 H⁺  →  Hg²⁺  +  H₂O

At this point, there is a weakly acidic solution of mercury(II) nitrate. This is treated with a solution of sodium chlorite. The main reaction is formation of the bright orange/red precipitate:

    Hg²⁺  +  2 ClO₂^–   →  Hg(ClO₂)₂

The nitrate ions and sodium ions are spectator ions and remain in solution. Because of the acidity, this indeed is the main reaction, which occurs. Without the acidity, a basic precipitate is formed, which besides chlorite also contains oxide and/or hydroxide.

There also is a side reaction. Chlorite ion reacts with acid to form chlorous acid, which is unstable and quickly decomposes. The net reaction is:

    5 ClO₂^–  +  4 H⁺  →  4 ClO₂  +  Cl^–  +  2 H₂O

This explains the yellow color of the liquid above the red/orange precipitate.

The compound Hg(ClO₂)₂ is unstable and easily decomposes. Slight heat or other provocation causes it to decompose as follows:

    Hg(ClO₂)₂  →  HgCl₂  +  2 O₂

With this decomposition a lot of gas is produced and this explains why so much smoke is expelled. The mercury(II) chloride is expelled with the oxygen gas as a white smoke and it settles on nearby surfaces as a white frosty solid.

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