Sodium

• 1. Introduction

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  Sodium, the element with the symbol Na, revealed itself in 1860 from the far corner of a 60 cubic metre room. There the chemists Kirchhoff and Bunsen set off 3 milligrams of sodium chlorate mixed with milk sugar, as far as possible from their apparatus. At the apparatus they were watching a colorless flame through a slit. After a while the flame glowed bright yellow, and a strong sodium line stayed visible for ten minutes. From the weight of the salt and the volume of the room, they worked out that air could hold no more than one twenty-millionth of its own weight in sodium.

  That tiny trace was enough to be seen. This documentary asks where the name sodium came from, and why its symbol reads Na. It asks how a silvery metal soft enough to cut with a knife came to be dissolved in the oceans, burning in distant stars and working inside the nerves of animals. And it asks why people who handle the pure metal treat it with such respect.

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• 2. A Headache and a Name

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  In medieval Europe, a sodium compound with the Latin name sodanum was used as a headache remedy. The name sodium is thought to come from the Arabic word suda, which means headache. Sodium carbonate, known as soda, had a reputation for easing headaches in early times, and salt had long been a valuable commodity because of its importance to human health.

  In 1807, Sir Humphry Davy finally isolated the metal itself by passing an electric current through sodium hydroxide. Until then, sodium had been known only locked inside its compounds. Two years later, in 1809, the German physicist and chemist Ludwig Wilhelm Gilbert proposed different names. He suggested Natronium for Davy's sodium and Kalium for Davy's potassium.

  The abbreviation Na came in 1814, when Jöns Jakob Berzelius published it in his system of atomic symbols. It comes from natrium, the Neo-Latin name for the element. That name points back to natron, an Egyptian mineral salt made mainly of hydrated sodium carbonate. Natron once had several important industrial and household uses, which other sodium compounds later eclipsed. The metal Davy freed, meanwhile, turned out to have properties that change completely under extreme squeezing.

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• 3. Soft Silver Metal

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  A plain knife is enough to cut sodium at room temperature. The fresh metal is silvery, but it combines with oxygen in the air to form sodium oxides. For that reason bulk sodium is usually kept under oil or an inert gas. It conducts electricity and heat well.

  Its melting point is 98 degrees Celsius and its boiling point is 883 degrees. Both are lower than lithium's and higher than those of potassium, rubidium and caesium, following the pattern down the alkali metal group. Pressure changes the picture dramatically. At 1.5 megabar, sodium turns from silvery to black. At 1.9 megabar it becomes transparent and red. At 3 megabar it is a clear, transparent solid. All of these high-pressure forms are insulators.

  A sodium atom has 11 electrons, one more than the noble gas neon. Removing that single outer electron takes 495.8 kilojoules per mole, but a second would cost 4562. As a result, sodium almost always forms ionic compounds containing the Na+ cation. It is more reactive than lithium and less reactive than potassium.

  Sodium also mixes with other metals. Combined with potassium, it forms an alloy called NaK, which holds 40-90% potassium and stays liquid at ambient temperature. In liquid form, sodium mixes completely with lead. Dissolved in ammonia, sodium gives deeply colored solutions, and evaporating them leaves a shiny film of the metal.

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• 4. Stars and Seas

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  Twenty isotopes of sodium are known, but only 23Na is stable. Stars make it by carbon burning, when two carbon atoms fuse. That needs temperatures above 600 megakelvins and a star of at least three solar masses. Across the universe, sodium is the 15th most abundant element, at about 20,000 parts per billion, or 0.002% of all atoms.

  The other isotopes are short-lived. Cosmic rays create two radioactive ones through spallation. 22Na has a half-life of 2.6 years, and 24Na has a half-life of 15 hours. Every other isotope lasts less than a minute. The 24Na isotope has a medical use. After a nuclear criticality accident, acute neutron radiation turns some of the stable 23Na in a victim's blood into 24Na. Measuring the ratio of the two reveals the dose the person received.

  On Earth, the crust is 2.27% sodium. That makes it the sixth most abundant element and the fourth most abundant metal, behind aluminium, iron, calcium and magnesium, and ahead of potassium. It never occurs as a pure element. Instead it sits in minerals, some very soluble, such as halite and natron, and others much less so, such as amphibole and zeolite. Cryolite and feldspar resist dissolving because their anions form long polymer chains, and in feldspar that chain is a polysilicate.

  The oceans hold an estimated 10.8 grams of sodium per liter. Over eons, water has leached sodium ions out of the Earth's minerals, because so many sodium salts dissolve readily. As a result, sodium and chlorine are the most common dissolved elements by weight in seawater. Sodium's light, however, is how it is best known from far away.

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• 5. The D Line

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  In 1814, Joseph von Fraunhofer was studying the dark lines in the solar spectrum when he noticed a strong one in the yellow-orange region. He named it the D line. These dark features are now called the Fraunhofer lines. Closer study showed that the D line is really a group of closely spaced lines, split by fine and hyperfine structure.

  The light comes from the electron in the 3s orbital. When it is excited and then falls back from the 3p orbital, it emits a photon at about 589.3 nanometers. Spin-orbit interactions split that line in two, at 589.0 and 589.6 nanometers.

  In the Sun and in many other stars, the D line appears as an absorption line. It is visible in any star whose surface is cool enough for sodium to stay in atomic form, which means stars of roughly F-type and cooler. Some stars seem to show the line only because of gas in the interstellar medium between us and them. Astronomers tell the two apart with high-resolution spectroscopy, since interstellar lines are much narrower than lines broadened by a star's rotation.

  Sodium has also been found in the thin outer atmospheres of Mercury and the Moon. Some comets carry a sodium tail, first detected on Comet Hale-Bopp in 1997. Transit spectroscopy has even spotted it in the atmospheres of some extrasolar planets.

  On city streets, sodium-vapor lamps shine with the same line. Their light ranges from yellow-orange to peach as the pressure rises. Observatories have turned the line into a tool. Lasers tuned to the sodium D line create artificial laser guide stars, which help adaptive optics sharpen the view of land-based visible-light telescopes.

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• 6. Making and Using

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  About 100,000 tonnes of metallic sodium are made every year, for fairly specialized jobs. In the late nineteenth century, producers made it by heating sodium carbonate with carbon at 1100 degrees Celsius. This was the first step of the Deville process for making aluminium. Strong demand for aluminium therefore created demand for sodium. When the Hall-Héroult process arrived, using electrolysis of a molten salt bath, the need for large quantities of sodium ended. A related method based on reducing sodium hydroxide was developed in 1886.

  A process patented in 1924 now supplies the metal. It electrolyzes molten sodium chloride in a Downs cell. Calcium chloride is mixed in to lower the melting point below 700 degrees Celsius, and since calcium is less electropositive than sodium, it does not deposit at the cathode. This is cheaper than the older Castner process, which used sodium hydroxide. Distilling the product once or several times gives high purity. Because storage and shipping are difficult, the market for the metal is volatile.

  Most of the sodium that industry uses comes as compounds. Millions of tons of sodium chloride, hydroxide and carbonate are produced annually. They are central to making glass, paper, soap and textiles. Most soaps are sodium salts of fatty acids, and sodium soaps melt at a higher temperature than potassium soaps, so they seem harder. Sodium chloride spreads on roads as a de-icing agent and works as a preservative. Sodium bicarbonate raises baked goods and powers sodablasting. Many medicines have sodium added to improve their bioavailability, chosen over potassium for its lower price and lighter atomic weight.

  The metal itself goes mainly into sodium borohydride, sodium azide, indigo and triphenylphosphine. It once helped make tetraethyllead and titanium metal, and production fell after 1970 as the world moved away from tetraethyllead and found new titanium methods.

  In sodium-cooled fast reactors, liquid sodium carries heat away. It has high thermal conductivity and a low neutron absorption cross section, which helps achieve a high neutron flux. Its high boiling point lets the reactor run at normal pressure. One drawback is that the liquid is opaque, which hinders visual maintenance. Where a reactor must be shut down often, the sodium-potassium alloy NaK is used because it never solidifies in the pipes. High-performance engines make another use of sodium. Their valve stems are partly filled with it so that each stem works as a heat pipe cooling the valve.

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• 7. Fire and Water

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  Sodium dropped into water reacts exothermically, producing caustic soda and flammable hydrogen gas. High-speed camera images of alkali metals exploding in water have suggested that the blast is a Coulomb explosion. The sodium hydroxide dissolves in the water and exposes more fresh surface, so the reaction feeds itself. Moisture on skin, eyes or mucous membranes can cause severe burns.

  In air the story is milder. Burning sodium forms mostly sodium peroxide with some sodium oxide. Autoignition has been reported when a molten pool reaches about 290 degrees Celsius, and even then the fire is relatively gentle. A massive solid piece slows its own burning by forming a protective layer.

  Putting out a sodium fire needs the right agent. Water-based extinguishers accelerate the fire, and carbon dioxide and bromochlorodifluoromethane should not be used. Sodium burns as a Class D metal fire, but not every Class D extinguisher works. Met-L-X is effective. So is Lith-X, which contains graphite powder and an organophosphate flame retardant. Dry sand also works.

  In nuclear reactors, designers keep sodium away from oxygen with surrounding pipes full of inert gas. Against pool-type fires they use catch pan systems, which collect leaking sodium in a leak-recovery tank where it is isolated from oxygen.

  R. J. Gordon, writing in a technical report for the United States Fire Administration, stressed the danger of the liquid. In his words: "Molten sodium is extremely dangerous because it is much more reactive than a solid mass." Even a minute amount of water can cause a violent explosion inside the pool. Most powdered extinguishing agents sink to the bottom of burning molten sodium while the metal floats up and keeps burning. Soda ash is the exception. Nitrogen or argon can form a protective layer if applied very gently, and a container fire may be smothered by putting a lid on it.

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• 8. Salt in the Body

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  Inside animal cells, a membrane enzyme complex called the sodium-potassium pump works constantly to push sodium ions out. It keeps the concentration of sodium roughly ten times higher outside the cell than inside. In nerve cells, sodium ions rush back in through voltage-gated sodium channels, and that sudden flow carries a nerve impulse, known as the action potential. The same differences in ion concentration support cardiac muscle contraction and long-distance communication between neurons.

  In humans, sodium regulates blood volume, blood pressure, osmotic equilibrium and pH. The minimum physiological requirement is estimated at about 120 milligrams per day in newborns and 500 milligrams per day beyond the age of 10. The kidney's renin-angiotensin system manages the balance. When blood pressure and sodium concentration drop, renin production sets off a chain that has the body reabsorb sodium into the bloodstream. As sodium concentration rises, renin production falls. Blood sodium that is too low or too high is called hyponatremia or hypernatremia. Genetic factors, ageing, or prolonged vomiting or diarrhea can cause these conditions.

  Table salt is the main dietary source, used as seasoning and as a preservative in pickled preserves and jerky. For Americans, most of it comes from processed foods. Other sources include monosodium glutamate, sodium nitrite, sodium saccharin, baking soda and sodium benzoate. The U.S. Institute of Medicine set the tolerable upper intake at 2.3 grams per day, yet the average American eats 3.4 grams. The American Heart Association recommends no more than 1.5 grams. A National Academies committee found too little evidence to set a Recommended Dietary Allowance, so it established Adequate Intake levels instead. For adults the figure is 1,500 milligrams per day, and for infants up to six months it is 110 milligrams per day.

  Studies show a strong correlation between sodium intake and blood pressure. Lowering intake by 2 grams per day tends to lower systolic pressure by about two to four millimeters of mercury. That drop is estimated to mean 9-17% fewer cases of hypertension. Hypertension causes 7.6 million premature deaths worldwide each year. The evidence is not all one-directional. One scientific review found that people excreting less than 3 grams of sodium per day had a higher risk of death, stroke or heart attack than those excreting 4 to 5 grams. Levels of 7 grams or more were linked to higher mortality in people with hypertension, but not in people without it.

  Plants have their own relationship with sodium. In C4 plants it is a micronutrient that helps metabolism, and in other plants it can stand in for potassium. Too much in the soil limits water uptake and can cause wilting. Some plants respond by limiting uptake at the roots, storing the excess in cell vacuoles, or parking it in old tissue. Halophytes go further and flourish in sodium-rich ground.

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