Make Your Own Sodium
by Norman F. Stanley
Metallic
sodium has many uses in the chemical industry, principally as a reducing
agent in the production of organic chemicals and in the production of
titanium and zirconium, and so is manufactured in large quantities, up
to tank car lots. For laboratory use it may be had in one pound
bricks shipped in hermetically sealed cans. Once opened, the metal
is stored under oil to prevent its reacting with moisture in the air.
For the amateur who would like to experiment with this reactive metal
a pound is a rather large quantity to keep on hand or dispose of safely.
Some suppliers may sell smaller quantities under oil in sealed containers.
In these days, the amateur is likely to find it difficult to locate a
supplier willing to sell sodium to him, while the cost of shipping
as a hazardous material may far exceed the actual cost of the product.
There is hope, though: You can make small quantities in your lab from
a readily available starting material.
On a commercial scale sodium is prepared by electrolysis of molten sodium
chloride in a Downs cell using graphite anodes and an iron cathode.
The cell is lined with refractory material to resist the high temperature
of the molten salt. To reduce the stress on the refractory, the
sodium chloride is mixed with 58-59% calcium chloride. This depresses
the melting point from that of pure NaCl (800 C) to 575-585 C. The
sodium, which is lighter than the salt bath leaves the cell through a
riser. Since the electrolysis produces metallic calcium as well
as sodium, the cell is constructed so that the calcium (m.p. 842.5 C)
precipitates out from the liquid sodium (m.p. 97.5 C) and settles back
into the bath where it dissolves in the molten salts. The resulting
sodium metal is 99.8% pure.
Fortunately, we don't have to construct a Downs cell or work at such high
temperatures in our home lab. Sodium hydroxide, which you can purchase
at the supermarket under the label, "Concentrated Lye", melts
at 318 C. A further advantage is that the product liberated at the
anode is oxygen rather than stinky chlorine.
A few notes of caution at this point should be borne in mind:
DON'T use the product sometimes sold for freeing clogged drains and which
contains a mixture of sodium hydroxide and granular aluminum. This
liberates hydrogen when mixed with water, fine for freeing drains but
potentially explosive. What would happen to hot aluminum in molten
NaOH and in contact with oxygen from the electrolysis is best left to
the imagination.
REMEMBER at all times that NaOH is called "caustic soda" with
good reason. It's extremely irritating to the skin, and getting
it in an eye is a very serious matter, calling for immediate on-the-spot
treatment. Flush with copious amounts of water and head for the
nearest E.R.
Sodium hydroxide is extremely hygroscopic. Keep the can of lye closed
when not in use to prevent its taking up moisture and dissolving or hardening
into a solid mass.
In the simple electrolytic cell to be described, the liberated oxygen
will carry finely divided caustic with it into the work area. Wear
a face shield, gloves, sleeve protectors and a rubber apron
to avoid contact of this mist with skin or clothing. Work in a well-ventilated
area or fume hood. With common sense precautions and awareness of
the hazards the experiment will go smoothly.
For the electrolytic cell use a 100 ml iron crucible. A 4 oz. "tin"
can will do in lieu of a crucible. Support the crucible on a ringstand,
using a suitably sized ring and wire gauze pad. The crucible can
serve as the anode of the cell, or a carbon electrode from a discarded
"A" cell can be used. This may have an advantage in that
the evolution of oxygen will be localized to its vicinity. For the
cathode take an 8 inch length of 16 or 18 gauge iron wire and form a loop
about 5 mm in diameter at one end. The other end can be grasped
with pliers or fitted with a wooden handle. DC power is supplied
from a 12 volt supply or storage battery. A low ohmage power rheostat
may be used in series with the supply and cell to regulate the current.
An ammeter is optional. Connections to the cell are made with alligator
clips or similar connectors. Take care that the setup is stable
and will resist tipping over.
To the crucible add enough NaOH to fill it about 2/3 full when melted.
Heat with a Bunsen burner until melted. Turn on the power and start
the current flow by dipping the wire loop into the surface of the melt.
If the current is sufficient, silvery globules of metallic sodium should
immediately form within and around the loop. These may separate
and skate about the surface of the NaOH where they will ignite and burn
in the presence of the oxygen evolved at the anode. Note the yellow
color of the flame, radiating at the twin sodium spectral lines
at 588.9 and 589.6 nM. To recover the metal, lift the loop
from the melt. Surface tension will hold the globule of liquid metal
in place. Quickly transfer it to a jar of kerosene.
Since the melting point of sodium is so low, it may remain liquid long
enough to be dislodged by shaking the loop and letting the metal drop
into the oil. If it solidifies, use tweezers to break it loose from
the wire. Making the loop of thicker wire (e.g., 16 gauge)
will slow heat loss.
Although only a few milligrams of sodium are recovered at a time using
this rather crude setup, it should be possible to accumulate gram
quantities by repeatedly dipping the loop into the molten NaOH
The recovered globules can be consolidated by heating the oil above 100
C to melt them.
Preparing larger quantities would require some means of confining the
liquid metal to the vicinity of the cathode so as to keep it out of contact
with oxygen. I once tried using a porcelain tube (a lead-in insulator
for old-style knob and tube house wiring) to enclose the cathode, but
the caustic chewed it up in short order. I invite suggestions as
to a suitably resistant material.
At the conclusion of the electrolysis, allow the crucible to cool and
the NaOH solidify. The crucible and NaOH can be stored in a desiccator
for future runs. Otherwise drop it into a bucket of water and allow
the NaOH to dissolve. The dilute caustic can be safely disposed
of down the drain.
The chemistry involved in the electrolysis is simple. Sodium ions
are reduced (gain electrons) to metallic sodium while hydroxyl ions are
oxidized (lose electrons) to form oxygen and water:
Na+
+ e- —>
Na
4OH-
-4e- —> 2H2O
+ O2
You can make metallic potassium by the same process by substituting
potassium hydroxide (m.p 360 C). for NaOH. Potassium is more reactive
than sodium, and so will be more likely to ignite as soon as liberated
at the cathode.
Sodium reacts energetically with water to produce sodium hydroxide and
hydrogen:
2Na
+ 2H2O —>
2Na+ + 2OH- + H2
It's been told of the legendary American physicist, Robert W.
Wood, that on rainy days he would open the window of his lab, fish out
a good sized lump from a jar of sodium, and toss it into the street where
it would skitter across the wet pavement, sputtering and flaming, in the
way of passing carriages. Don't try that one at home, kiddies, unless
you want a visit by the Bomb Squad! You can demonstrate this reaction
safely by submerging the sodium in water and collecting the hydrogen in
test tubes. For this you need a sodium spoon to keep the metal from
floating to the surface. These are obtainable from laboratory suppliers,
but you can make your own at negligible cost:
Cut a piece of 1/4-inch copper tubing about 1-1/2 inches long. Close
off one end by flattening in a vise, leaving about 1 inch open for receiving
the sodium. For a handle solder one end of an eight- or ten-inch
length of 16 gauge copper or brass wire to the side of the tube.
If desired, attach a wooden handle to the other end.
Set up a small tank (a "pneumatic trough") holding about
two gallons of water. A glass fish tank is ideal for demonstration
to an audience. Submerge a few test tubes in the tank for use in
collecting the hydrogen. A rack for holding the water-filled tubes
in the tank is a convenience here. For a pretty effect, add 1 or
2 mL of phenolphthalein indicator solution to the water. Cut
a small piece of sodium and dry with a tissue to remove adhering oil.
Cut this into smaller pellets and pack these into the spoon, using a metal
rod to ram the sodium tightly into the tube. Sodium is very soft
and will pack easily.
Dip the spoon into the water, holding the open end upward. Bubbles
of hydrogen will immediately start streaming from the spoon as the water
reacts with the sodium. If phenolphthalein has been added, red streamers
will show the diffusion of the alkaline NaOH into the water. Hold
an inverted, water-filled test tube over the spoon to collect the evolved
hydrogen. When the gas has displaced the water from the tube, remove
it, still held inverted, and bring it to a flame. The hydrogen will
ignite and burn with a characteristic high-pitched "pop".
Several tubes of gas can be collected in this manner before the sodium
is entirely consumed. 
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