Mercury(II) chlorite : a
beautiful lab curiosity
The compound, prepared in this experiment, is mercury(II) chlorite. It has a beautiful orange/red color, but it is very toxic and also easily decomposes, more or less explosively. I had an unexpected quick decomposition of solid mercury(II) chlorite, which resulted in the formation of a cloud of mercury(II) chloride, which is very toxic. 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!
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.
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
all mercury has dissolved, dilute the liquid with appr.
10 ml of water.
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.
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.
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.
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
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.
Isolation of mercury(II)
Finally, the mercury chlorite can be
isolated. After decanting the (nearly) colorless liquid,
a small volume of liquid, containing the orange
precipitate, is left.
the liquid with the precipitate into a petri dish and
allow this to dry on a warm (but not hot) place, free of
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!
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.
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.
Discussion of resultsIn the first step, mercury metal is dissolved in hot fairly concentrated nitric acid. This results in the following reaction:
Hg + 4 HNO3 → Hg2+ + 4 NO3 – + 2 H2O + 2 NO2
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:
+ 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+
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
There also is a side reaction.
Chlorite ion reacts with acid to form chlorous acid,
which is unstable and quickly decomposes. The net
5 ClO2– + 4 H+ → 4 ClO2 + Cl– + 2 H2O
This explains the
yellow color of the liquid above the red/orange
The compound Hg(ClO2)2
is unstable and easily decomposes. Slight heat
or other provocation causes it to decompose as
Hg(ClO2)2 → HgCl2 + 2 O2
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.