52 | The History and Use of Our Earth’s Chemical Elements useful sodium salts are sodium bicarbonate (baking soda), sodium carbonate (soda), sodium

52 | The History and Use of Our Earth’s Chemical Elements useful sodium salts are sodium bicarbonate (baking soda), sodium carbonate (soda), sodium

chloride (rock salt), sodium borate (borax), and sodium sulfate (used in paper and photo industries). Mineral springs have a variety of sodium salts, as well as other trace elements, that give the water its “fresh” taste.

Sodium is used in both low-pressure and high-pressure sodium vapor lamps. The low-pres- sure arc uses just a small amount of Na along with some neon for a starter. The lamp is eco- nomical and bright. The illumination with its single yellow color (electromagnetic frequency) makes it difficult for us to recognize other colors. In addition to sodium, the high-pressure lamp uses mercury, which provides a more natural color rendition of light. The very bright light of sodium-mercury lamps makes them ideal for use in sports stadiums and highways.

Because the melting point of sodium metal is about 98° C (a bit lower than the boiling point of water), it is heated into a liquid phase and then transported in rail tank cars, where it cools and solidifies. When it arrives at its destination, heating coils in the tanks warm it back to the liquid stage, and it is then stored for use. Because sodium has a high specific heat rating, a major use is as a liquid coolant for nuclear reactors. Even though sodium (both solid and liquid) is extremely reactive with water, it has proven safe as a coolant for nuclear reactors in submarines.

Soda niter or sodium nitrate (NaNO 3 ) is the most abundant of the nitrate minerals. It is used for fertilizer, explosives, and preservatives. The natural deposits are located in northern Chile, which was the original source for many years. More recently, nitrogen fixation, which extracts nitrogen from air, has been used for producing sodium nitrate. This synthetic process has greatly increased the availability of this useful sodium salt by eliminating the need for the natural source. It is used to preserve and cure meats and is used in photography, in pharma- ceuticals, and as a color fixative in fabrics.

Of course, the most common use is everyday table salt, sodium chloride (NaCl). Salt is vital for health; the body must have a small amount to survive. In the past, wars were fought over salt mines and salt deposits by nations that did not have any natural sources. Excessive sodium chloride in the diet can also be harmful to one’s health.

Following are some of the more useful of the hundreds of existing sodium compounds. Examples฀of฀Compounds

Sodium carbonate (Na 2 CO 3 ) is the eleventh most used industrial chemical in the United States. It is commonly used as a bleaching agent and is manufactured in a two-step process. First, ammonia is combined with carbon dioxide to form sodium chloride and water, which

reacts to form sodium bicarbonate and ammonium chloride (NH 3 + CO 2 + NaCl + H 2 O→ NaHCO 3 + NH 4 Cl). Sodium bicarbonate, commonly known as baking soda, is used as a leavening agent in baking, as an antacid to relieve stomach acid, and as a component for fire extinguishers. The second step is known as the Solvay process, wherein the sodium bicarbon-

ate is heated and converted into sodium carbonate (NaHCO 3 ∆→ Na 2 CO 3 +H 2 O + CO 2 ). There are two forms of sodium sulfate (Na 2 SO 4 ). One is a called salt cake and contains no water and, thus, is called anhydrous sodium sulfate. When one molecule of the dry form combines with 10 molecules of water (decahydrate), it is known as Glauber’s salt (Na 2 SO 4 • 10H 2 O) . Sodium sulfate is produced by several processes, one of which is 2NaCl + H 2 SO 4 → Na 2 SO 4 + 2HCl. A large number of sodium salts can exist in both the anhydrous (dry) form and the hydrous (attached to water) form.

Guide to the Elements | 53 There are several other compounds of sodium and sulfur, including the following:

Sodium sulfite (Na 2 SO 3 ) is an antioxidant, used as a preservative except with meats. It is also used for water treatment and in photography and textile bleaching. Sodium sulfide (Na 2 S) is used in the dye industry, in the oxidation process of gold, lead, and cooper metal ores, as a sheep dip, and to process paper. Sodium thiosulfate (Na 2 S 2 O 3 ) is known as photographers’ “hypo.” It dissolves the unex- posed silver salts from photographic negatives and prints during the process of “fixing” the image so that the film or print will no longer be light-sensitive.

Sodium permanganate (NaMnO 4 ) is a purple crystal that is soluble in water and is used as an oxidizing agent, disinfectant, and bactericide and as an antidote for morphine poisoning. Sodium silicate (Na 2 O) is known as “water glass” and is used in water treatment and in making soaps, detergents, adhesives, drilling fluids, bleaches. Sodium hydroxide (NaOH) is one of the most useful industrial sodium compounds. It is also known as lye or caustic soda and is one of the strongest base alkalis (high pH value) on the household market. It is used as a drain and oven cleaner, and it saponifies fats in the manufacture of soap. It must be used with care because it is also capable of producing serious skin burns.

Rock salt (halite, native NaCl) is an unpurified, coarse common salt that is spread on high- ways to melt snow and ice.

Hazards Sodium as the elemental metal is very dangerous because of its extreme electropositive

nature, particularly when it comes in contact with moist air, water, snow, or ice or other oxidizing agents. It readily gives up electrons to electronegative atoms (nonmetals). In these reactions, it releases hydrogen gas with enough heat to explosively ignite the hydrogen

Sodium perchlorate (NaClO 4 ) is extremely dangerous. It is easily detonated and thus used to set off explosives. It is also used as a jet fuel. Sodium peroxide (Na 2 O 2 ) is explosive when in contact with water. It is a strong oxidizing agent that is very irritating. Numerous sodium compounds are hazardous as carcinogens (cancer-causing) and as toxins (poisons) in plants and animals. On the other hand, we benefit greatly from the many compounds containing the element sodium. We could not live without it.