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Hygroscopic (adj.) Having the property of absorbing or attracting moisture from the air.
05 October 2001
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.
