SODIUM

SODIUM. Sodium is normally present in food and in the body in its ionic (charged) form rather than as metallic sodium. Sodium is a positively charged ion or cation (Na⁺), and it forms salts with a variety of negatively charged ions (anions). Table salt or sodium chloride (NaCl) is an example of a sodium salt. In solution, NaCl dissociates into its ions, Na⁺ and Cl^-. Other sodium salts include those of both inorganic (e.g., nitrite or bicarbonate) and organic anions (e.g., citrate or glutamate) in aqueous solution, these salts also dissociate into Na⁺ and the respective anion.

Types and Amounts of Common Foods that Contain the Recommended Levels of Sodium

Only small amounts of salt or sodium occur naturally in foods, but sodium salts are added to foods during food processing or during preparation as well as at the table. Most sodium is added to foods as sodium chloride (ordinary table salt), but small amounts of other salts such as sodium bicarbonate (baking soda and baking powder), monosodium glutamate, sodium sulfide, sodium nitrate, and sodium citrate are also added. Studies in a British population found that 75 percent of sodium intake came from salts added during manufacturing and processing, 15 percent from table salt added during cooking and at the table, and only 10 percent from natural foods (Sanchez-Castillo et al., 1987). Most sources of drinking water are low in sodium. However, the use of home water softening systems may greatly increase the sodium content of water; the system should be installed so that water for cooking and drinking bypasses the water softening system.

The estimated minimum safe daily intake of sodium for an adult (0.5 grams) can be obtained from ¼ teaspoon of salt, ¼ of a large dill pickle, ⅕ can of condensed tomato soup, one frankfurter, or fifteen potato chips. The effect of salt added in processing is noted by the calculation that, whereas one would need to consume 333 cups of fresh green peas (with no salt added during cooking or at the table) in order to consume 0.5 grams of sodium, the estimated minimum safe daily intake of sodium is provided by only 1.4 cups of canned or 2.9 cups of frozen green peas.

Whereas the estimated minimum safe intake for an adult is 0.5 g/day of sodium (1.3 g/day of sodium chloride), average Americans consume between 2 and 5 g/day of sodium (between 5 and 13 g/day of sodium chloride) (National Research Council, 1989). Sodium chloride, or salt, intake varies widely among cultures and among individuals. In Japan, where consumption of salt-preserved fish and the use of salt for seasoning are customary, salt intake is high, ranging from 14 to 20 g/day (Kono et al., 1983). On the other hand, the unacculturated Yanomamo Indians, who inhabit the tropical rain forest of northern Brazil and southern Venezuela, do not use salt in their diet and have an estimated sodium chloride intake of less than 0.3 g/day (Oliver et al., 1975). In the United States, individuals who consume diets high in processed foods tend to have high sodium chloride intakes, whereas vegetarians consuming unprocessed food may ingest less than 1 g/day of salt. Individuals with salt intakes less than 0.5 g/day do not normally exhibit chronic deficiencies, but appear to be able to regulate sodium chloride retention adequately.

Recommended Intake of Sodium

The daily minimum requirement of sodium for an adult is the amount needed to replace the obligatory loss of sodium. The minimum obligatory loss of sodium by an adult in the absence of profuse sweating or gastrointestinal or renal disease has been estimated to be approximately 115 mg/day, which is due to loss of about 23 mg/day in the urine and feces and of 46 to 92 mg/day through the skin (National Research Council, 1989). Because of large variations in the degrees of physical activity and in environmental conditions, the estimated level of safe minimum intake for a 70-kg adult was set at 500 mg/day of sodium (equivalent to 1,300 mg/day of sodium chloride) by the National Research Council (1989). Although there is no established optimal range of intake of sodium chloride, it is recommended that daily salt intake should not exceed 6 grams because of the association of high intake with hypertension (National Research Council, 1989). The Dietary Guidelines for Americans, published in 2000, include a recommendation to choose and prepare foods with less salt.

Individuals who wish to lower their sodium or salt intakes should use less salt at the table and during cooking, avoid salty foods such as potato chips, soy sauce, pickled foods, and cured meat, and avoid processed foods such as canned pasta sauces, canned vegetables, canned soups, crackers, bologna, and sausages. Individuals should also become aware of and avoid "hidden" sources of sodium such as softened water, products made with baking soda, and foods containing additives in the form of sodium salts.

The need for sodium chloride is increased during pregnancy and lactation, with the estimated safe minimum intake being increased by 69 mg/day and 135 mg/day, respectively, for women during pregnancy and lactation. The estimated minimum requirement for sodium is 120 mg/day for infants between birth and 5 months of age and 200 mg/day for infants 6 to 11 months of age (National Research Council, 1989); these intakes are easily met by human milk or infant formulas. The estimated minimum requirements of sodium for children range from 225 mg/day at one year of age to 500 mg/day at 10 to 18 years of age.

General Overview of Role of Sodium in Normal Physiology

Total body sodium has been estimated at 100 grams (4.3 moles) for a 70-kg adult. In general, the cytoplasm of cells is relatively rich in potassium (K>) and poor in sodium (Na>) and chloride (Cl<) ions. The concentrations of sodium (and potassium and chloride) ions in cells and the circulating fluids are held remarkably constant, and small deviations from normal levels in humans are associated with malfunction or disease. Na⁺, K⁺, and Cl^- are referred to as electrolytes because of their role in the generation of gradients and electrical potential differences across cell membranes. Sodium and sodium gradients across cell membranes play several important roles in the body. First, sodium gradients are important in many transport processes. Sodium tends to enter cells down its electrochemical gradient (toward the intracellular compartment that has a lower Na⁺ concentration and a more negative charge compared to the extracellular fluid compartment). This provides a secondary driving force for absorption of Cl^- in the same direction as Na⁺ movement or for the secretion of K⁺ or hydrogen ions (H⁺) in the opposite direction in exchange for Na⁺. The sodium gradient is also used to drive the coupled transport of Na⁺ and glucose, galactose, and amino acids by certain carrier proteins in cell membranes; because as Na⁺ enters down its electrochemical gradient, uptake of glucose/galactose or amino acids can occur against their concentration gradient. Second, sodium ions, along with potassium ions, play important roles in generating resting membrane potentials and in generating action potentials in nerve and muscle cells. Nerve and muscle cell membranes contain gated channels through which Na⁺ or K⁺ can flow. In the resting state, these cell membranes are highly impermeable to Na⁺ and permeable to K⁺ (i.e., Na⁺ channels are closed and K⁺ channels are open). These gated channels open or close in response to chemical messengers or to the traveling current (applied voltage). Action potentials are generated in nerve and muscle due to opening of Na⁺ channels followed by their closing and the re-opening of K⁺ channels.

A third important function of sodium is its osmotic role as a major determinant of extracellular fluid volume. The volume of the extracellular fluid compartment is determined primarily by the total amount of osmotic particles present. Because Na⁺, along with Cl^-, is the major determinant of osmolarity of extracellular fluid, disturbances in Na⁺ balance will change the volume of the extracellular fluid compartment. Finally, because Na⁺ is a fixed cation, it also plays a role in acid-base balance in the body. An excess of fixed cations (versus fixed anions) requires an increase in the concentration of bicarbonate ions.

Consequences of Deficiency or Excessive Intake Levels

Sodium balance in the body is well controlled via regulation of Na⁺ excretion by the kidneys. The kidneys respond to a deficiency of Na⁺ in the diet by decreasing its excretion, and they respond to an excess of Na⁺ by increasing its excretion in the urine. Physiological regulatory mechanisms for conservation of Na⁺ seem to be better developed in humans than mechanisms for excretion of Na⁺, and pathological states characterized by inappropriate retention of Na⁺ are more common than those characterized by Na⁺ deficiency.

Retention of Na⁺ occurs when Na⁺ intake exceeds the renal excretory capacity. This can occur with rapid ingestion of large amounts of salt (for example, ingestion of seawater) or with too-rapid intravenous infusion of saline. Hypernatremia (abnormally high plasma concentration of Na⁺) and hypervolemia (abnormally increased volume of blood), resulting in acute hypertension, usually occur in these situations, and the Na⁺ regulatory mechanisms will cause natriuresis (urinary excretion of Na⁺) and water retention.

The body may be depleted of Na⁺ under extreme conditions of heavy and persistent sweating or when conditions such as trauma, chronic vomiting or diarrhea, or renal disease produce an inability to retain Na⁺. Sodium depletion produces hyponatremia (abnormally low plasma concentration of Na⁺) and hypovolemia (abnormally decreased volume of blood) which place the individual at risk of shock. Medical treatment includes replacement of Na⁺ and water to restore the circulatory volume. If the loss of Na⁺ is not due to renal disease, mechanisms to conserve Na⁺ and water are activated. Loss of Na⁺ can also be caused by the administration of diuretics, which inhibit Na⁺ and Cl^- reabsorption, or by untreated diabetes mellitus, which causes diuresis.

Regulatory Processes that Govern the Uptake and Excretion of Sodium

The kidneys are the main site of regulation of Na⁺ balance. The intestines play a relatively minor role. Under normal circumstances, about 99 percent of dietary Na⁺ and Cl^- are absorbed, and the remainder is excreted in the feces. Absorption of Na⁺ and Cl^- occurs along the entire length of the intestines; 90 to 95 percent is absorbed in the small intestine and the rest in the colon. Intestinal absorption of Na⁺ and Cl^- is subject to regulation by the nervous system, hormones, and paracrine agonists released from neurons in the enteric nervous system in the wall of the intestines. The most important of these factors is aldosterone, a steroid hormone produced and secreted by the zona glomerulosa cells of the adrenal cortex. Aldosterone stimulates absorption of Na⁺ and secretion of K⁺, mainly by the colon and, to a lesser extent, by the ileum.

The kidneys respond to a deficiency of Na⁺ in the diet by decreasing its excretion, and they respond to an excess by increasing its excretion in the urine. Urinary loss of Na⁺ is controlled by varying the rate of Na⁺ reabsorption from the filtrate by renal tubular cells. Individuals consuming diets that are low in Na⁺ efficiently reabsorb Na⁺ from the renal filtrate and have low rates of excretion of Na⁺. When there is an excess of Na⁺ from high dietary intake, little Na⁺ is reabsorbed by renal tubular cells, resulting in the excretion of the excess Na⁺ in the urine. As much as 13 g/day of Na⁺ can be excreted in the urine.

The most important regulator of renal excretion of Na⁺ and Cl^- is the renin-angiotensin-aldosterone system (Laragh, 1985). Sensors in the nephrons of the kidney respond to changes in Na⁺ load by influencing the synthesis and secretion of renin (Levens et al., 1981). A decrease in renal perfusion or Na⁺ load will increase the release of renin. In the circulation, renin acts to initiate the formation of active angiotensin II from angiotensinogen, a protein produced by the liver. Angiotensin II conserves body Na⁺ by stimulating Na⁺ reabsorption by the renal tubules and indirectly via stimulating secretion of aldosterone. Secretion of aldosterone by the adrenal cortex is stimulated by a low plasma Na⁺ concentration and by angiotensin II. Aldosterone stimulates cells of the renal tubules to reabsorb Na⁺.

Because of the close association of Na⁺ and Cl^- concentrations with effective circulating volume, Na⁺ (and Cl^-) retention results in proportionate water retention, and Na⁺ (and Cl^-) loss results in proportionate water loss. Expansion or contraction of the extracellular volume affects the activation of vascular pressure receptors, as well as the release of natriuretic peptides by certain tissues, and result in changes, mediated largely by antidiuretic hormone (ADH), in renal excretion of Na⁺, Cl^-, and water. A deficiency of sodium chloride and hypovolemia have also been shown to produce an increase in appetite for salt, which will increase sodium chloride intake.

Evidence that Sodium Intake May Be Related to Risk of Hypertension

Both epidemiological and experimental studies implicate habitual high dietary salt intake in the development of hypertension (Weinberger, 1996). Primary hypertension, or abnormally high blood pressure, is a significant risk factor for cardiovascular disease, stroke, and renal failure in industrialized societies. Diets that are high in fat, high in sodium, low in potassium, low in calcium, and low in magnesium may contribute to the development of hypertension (Reusser and McCarron, 1994).

Although epidemiological and experimental evidence suggest a positive correlation between habitual high-salt consumption and hypertension, controversy remains regarding the importance of sodium salts in the regulation of blood pressure and the mechanisms by which salt influences blood pressure. This is not surprising, because the response of blood pressure depends on an interplay of various factors, such as genetic susceptibility, body mass, cardiovascular factors, regulatory mechanisms mediated through the neural and hormonal systems, and renal function.

A large comprehensive study on the role of sodium in hypertension was carried out in fifty-two geographically separate centers in thirty-two countries by the INTERSALT Cooperative Research Group (Stamler, 1997). Four centers included in the study had median values for Na⁺ excretion that were under 1.3 g/day. Subjects in these four unacculturated centers had low blood pressure, rare or absent hypertension, and no age-related rise in blood pressure as occurred in populations in the other forty-eight centers in which mean values for Na⁺ excretion were between 2.4 and 5.6 grams Na⁺ per day. Although blood pressure and sodium intake appeared to be associated when all fifty-two centers were included, the correlation between systolic blood pressure and excretion of sodium was not significant when the four centers with the lowest median values of sodium excretion were excluded from the analysis.

Intervention studies of dietary salt restriction to lower blood pressure have produced mixed results. This may be explained by the facts that not all hypertensive patients are salt-sensitive and that many cases of hypertension are due to other causes. Nevertheless, various clinical trials indicate some beneficial effects of dietary restriction of sodium on blood pressure (Cutler et al., 1997; Reusser and McCarron, 1994) with response being greater in older patients, patients with the highest degree of restriction, and in nonoverweight, mildly hypertensive patients.

Researchers are currently attempting to identify the genetic basis of salt-sensitive hypertension and to identify polymorphisms associated with salt-sensitive hypertensive individuals. More than thirty different gene variations could be responsible for essential hypertension, and hypertension is considered to have a complex genetic basis. Further insight into the basis of hypertension may help to determine individuals for whom lowering salt intake would be beneficial and to facilitate the prescription of appropriate drugs.

See also Dietary Guidelines ; Fast Food ; Fish, Salted ; Health and Disease ; Meat, Salted ; Preserving ; Salt .

BIBLIOGRAPHY

Church, Charles F., and Helen N.Church. Food Values of Portions Commonly Used : Bowes and Church. Philadelphia: J. B. Lippincott, 1970.

Cutler, Jeffrey A., Dean Follmann, and P. Scott Allender. "Randomized Trials of Sodium Reduction: An Overview." American Journal of Clinical Nutrition 65 (1997, Supp.): 643S–651S.

Kono, Suminori, Masato Ikeda, and Michiharu Ogata. "Salt and Geographical Mortality of Gastric Cancer and Stroke in Japan." Journal of Epidemiology and Community Health 37 (1983): 43–46.

Laragh, John H. "Atrial Natriuretic Hormone, the Renin-Aldosterone Axis, and Blood Pressure—Electrolyte Homeostasis." New England Journal of Medicine 313 (1985): 1330–1340.

Levens, Nigel R., Michael J. Peach, and Robert M. Carey. "Role of Intrarenal Renin-Angiotensin System in the Control of Renal Function." Circulation Research 48 (1981):157–167.

National Research Council. Recommended Dietary Allowances. 10th ed. Washington, D.C.: National Academy Press, 1989, pp. 247–261.

Oliver, Walter J., Erik L. Cohen, and James V. Neel. "Blood Pressure, Sodium Intake and Sodium-Related Hormones in the Yanomamo Indians, a 'No-Salt' Culture." Circulation 52 (1975): 146–151.

Reusser, Molly E., and David A. McCarron. "Micronutrient Effects on Blood Pressure Regulation." Nutrition Reviews 52 (1994): 367–375.

Sanchez-Castillo, C. P., S. Warrender, T. P. Whitehead, and W. P. James. "An Assessment of the Sources of Dietary Salt in a British Population." Clinical Science 72 (1987): 95–102.

Sheng, Hwai-Ping. "Sodium, Chloride, and Potassium." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk, pp. 686–710. Philadelphia: W. B. Saunders Co., 2000.

Stamler, Jeremiah. "The INTERSALT Study: Background, Methods, Findings, and Implications." American Journal of Clinical Nutrition 65 (1997, Supp.): 626S–642S.

United States Department of Agriculture. Nutrition and Your Health: Dietary Guidelines for Americans. 5th ed.. Washington, D.C.: U. S. Government Printing Office, 2000.

Weinberger, Myron H. "Salt Sensitivity of Blood Pressure in Humans." Hypertension 27 (1996): 481–490.

Martha H. Stipanuk

Brief Outline of the History of Salt

Common salt is the chemical compound NaCl. Salt makes up nearly 80 percent of the dissolved material in seawater and is also widely distributed in solid deposits. It is found in many evaporative deposits, where it crystallizes out of evaporating brine lakes, and in ancient bedrock, where large extinct salt lakes and seas evaporated millions of years ago. Salt was in general use long before history began to be recorded. Salt has been used widely for the curing, seasoning, and preserving of foods.

Sodium

Discovery and Naming

General properties

Where it comes from

How the metal is obtained

How it is used

Compounds of sodium

Chemical properties

Resources

Sodium—an element that is the sixth most abundant found on Earth—is the second member of the alkali family, the group of elements that make up Group 1 of the periodic table. Sodium has an atomic number of 11, an atomic mass of 22.98977, and a chemical symbol of Na. Its chemical symbol reflects its Latin name of natrium. The element was first isolated by English chemist Sir Humphry Davy (1778– 1829) in 1807. Only one stable isotope of sodium exists in nature, sodium-23. However, at least six radioactive isotopes have been prepared synthetically. They include sodium-20, sodium-21, sodium-22, sodium-24, sodium-25, and sodium-26.

Discovery and Naming

Probably the best known compound among ancient civilizations was sodium carbonate (Na₂CO₃), commonly known as soda. Soda is one of the most common ores of sodium found in nature and it was used very early in human history to make glass. The Egyptians called soda natron, from which the Romans later derived the term natrium. It is from these names that sodium’s modern chemical symbol, Na, is derived.

Most compounds of sodium are very stable, so it is not surprising that the element itself was not discovered until fairly recent times. Davy first prepared a sample of pure sodium metal by electrolyzing molten sodium chloride. In essence, the system developed by Davy is still used in some cases to prepare pure sodium. Davy went on to apply his method for preparing sodium to the extraction of potassium, calcium, and other active metals.

General properties

Sodium is a soft metal that can be cut easily with a table knife. Its density is so low that it will float when placed into water. At the same time, the metal is so active that it reacts violently with the water, producing sodium hydroxide and hydrogen gas as products. Sufficient heat is produced in the reaction to cause the metal to heat and to ignite the hydrogen produced in the reaction.

Freshly cut sodium metal has a bright, shiny surface that quickly becomes a dull gray as it reacts with oxygen in the air around it. Over time, the metal becomes covered with a white crust of sodium oxide that prevents further reaction of the metal and oxygen.

Sodium forms a very large number of compounds in nature, and an even larger number have been prepared synthetically. These compounds include binary compounds of sodium with metals, non-metals, and metalloids, as well as ternary, and more complex compounds. Included among these are such well-known substances as sodium chloride (table salt), sodium bicarbonate (baking soda), sodium borate (borax), sodium carbonate (soda ash), monosodium glutamate (MSG), sodium hydroxide (caustic soda or lye), sodium nitrate (Chilean saltpeter), sodium silicate (water glass), and sodium tartrate (sal tartar).

Where it comes from

Sodium is the sixth most common element in Earth’s crust with an estimated abundance of 2.83%. It is the second most abundant element in seawater after chlorine. One point of interest is that, although the abundance of sodium and potassium is approximately equal in crustal rocks, the former is 30 times more abundant in seawater than is the latter. The explanation for this difference lies in the greater solubility of sodium compounds than of potassium compounds.

Sodium never occurs free in nature because it is so active. For all practical purposes, the only compound from which it is prepared commercially is sodium chloride. That compound is so abundant and so inexpensive that there is no economic motivation for selecting another sodium compound for its commercial production.

By far the largest producer of sodium chloride in the world is the United States, where about a quarter of the world’s supply is obtained. China, Germany, the United Kingdom, France, India, and members of the former Soviet Union are other major producers of salt. The greatest portion of salt obtained in the United States comes from brine, a term used for any naturally occurring solution of sodium chloride in water. The term includes, but is not restricted to, seawater, subterranean wells, and desert lakes such as the Great Salt Lake and the Dead Sea. The second largest source of sodium chloride in the United States is rock salt. Rock salt is generally obtained from underground mines created by the evaporation and then the burying of ancient seas.

How the metal is obtained

The isolation of sodium from its compounds long presented a problem for chemists because of the element’s reactivity. Electrolysis of a sodium chloride solution will not produce the element, for example, because any sodium produced in the reaction will immediately react with water.

The method finally developed byDavy inthe early nineteenth century has become the model on which modern methods for the production of sodium are based. In this method, a compound of sodium (usually sodium chloride) is first fused (melted) and then elec-trolyzed. In this process, liquid sodium metal collects at the cathode of the electrolytic cell and gaseous chlorine is released at the anode.

The apparatus most commonly used today for the preparation of sodium is the Downs cell, named for its inventor, J. Cloyd Downs. The Downs cell consists of a large steel tank lined with a refractory material containing an iron cathode near the bottom of the tank and a graphite anode near the top. A molten mixture of sodium chloride and calcium chloride is added to the tank. The presence of calcium chloride to the extent of about 60% lowers the melting point of the sodium chloride from 1,472° F (800° C) to about 1,076°F (580°C).

When an electrical current is passed through the mixture in the cell, sodium ions migrate to the cathode, where they pick up electrons and become sodium atoms. Chlorine ions migrate to the anode, where they lose electrons and become chlorine atoms. Since the molten sodium metal is less dense than the sodium chloride/calcium chloride mixture, it rises to the top of the cell and is drawn off. The chlorine gas escapes through a vent attached to the anode at the top of the cell. Sodium metal produced by this method is about 99.8% pure. The Downs cell is such an efficient and satisfactory method for preparing sodium that the vast majority of the metal’s production is accomplished by this means.

How it is used

Sodium metal has relatively few commercial uses. The most important is as a heat exchange medium in fast breeder nuclear reactors. A heat exchange medium is a material that transports heat from one place to another. In the case of a nuclear reactor, the heat exchange medium absorbs heat produced in the reactor core and transfers that heat to a cooling unit. In the cooling unit, the heat is released to the atmosphere, is used to boil water to power an electrical generating unit, or is transferred to a system containing circulating water for release to the environment.

Liquid sodium is a highly effective heat exchange medium for a number of reasons. First, it has a high heat capacity (that is, it can absorb a lot of heat per gram of metal) and a low neutron absorption cross-section (that is, it does not take up neutrons from the reactor core). At the same time, the metal has a low melting point and a low viscosity, allowing it to flow through the system with relatively little resistance.

For many years, the most important commercial application of sodium metal was in the manufacture of anti-knock additives such as tetraethyl and tetra-methyl lead. An alloy of sodium and lead was used to react with alkyl chlorides (such as ethyl chloride) to produce these compounds. In 1959, about 70% of all the sodium produced in the United States was used for this purpose. As compounds of lead such as tetraethyl and tetramethyl lead have been phased out of use for environmental reasons, however, this use of sodium has declined dramatically.

Another important use of sodium metal is in the manufacture of other metals, such as zirconium and titanium. Originally, magnesium metal was the reducing agent of choice in these reactions, but sodium has recently become increasingly popular in the preparation of both metals. When sodium is heated with a chloride of one of these metals, it replaces (reduces) the metal to yield the pure metal and sodium chloride.

About 10% of all sodium produced is used to make specialized compounds such as sodium hydride (NaH), sodium peroxide (Na₂O₂), and sodium alkoxides (NaOR). Small amounts of the metal are used as a catalyst in the manufacture of synthetic elastomers.

Compounds of sodium

Sodium chloride is the most widely used sodium compounds. Due to its availability and minimal amount of preparation, there is no need for it to be manufactured commercially. A large fraction of the sodium chloride used commercially goes to the production of other sodium compounds, such as sodium hydroxide, sodium carbonate, sodium sulfate, and sodium metal itself.

For many centuries, sodium chloride has also been used in the food industry, primarily as a preservative and to enhance the flavors of foods. In fact, many seemingly distinct methods of food preservation, such as curing, pickling, corning, and salting differ only in the way in which salt is used to preserve the food. Scientists are uncertain as to the mechanism by which salting preserves foods, but they believe that some combination of dehydration and high salinity create conditions unfavorable to the survival of pathogens.

Sodium hydroxide and sodium carbonate traditionally rank among the top 25 chemicals in terms of volume produced in the United States.

The number one use of sodium hydroxide is in the manufacture of a large number of other chemical products, the most important of which are cellulose products (including cellulose film) and rayon. Soap manufacture, petroleum refining, and pulp and paper production account for about one tenth of all sodium hydroxide use.

Two industries account for about one third each of all the sodium carbonate use in the United States. One of these is glass-making and the other is the production of soap, detergents, and other cleansing agents. Paper and pulp production, the manufacture of textiles, and petroleum production are other important users of sodium carbonate.

Sodium sulfate is produced in large amounts in the United States. For many years, the largest fraction of sodium sulfate (also known as salt cake) was used in the production of kraft paper and paper-board. In recent years, an increasing amount of the chemical has gone to the manufacture of glass and detergents.

Just behind sodium sulfate in U.S. production levels is sodium silicate, also known as water glass. Water glass is used as a catalyst, in the production of soaps and detergents, in the manufacture of adhesives, in the treatment of water, and in the bleaching and sizing of textiles.

Chemical properties

As described above, sodium reacts violently with water and with oxygen to form sodium hydroxide and sodium oxide, respectively. The element also reacts vigorously with fluorine and chlorine, at room temperature, but with bromine and iodine only in the vapor phase. At temperatures above 392°F (200°C), sodium combines with hydrogen to form sodium hydride, NaH, a compound that then decomposes, but does not melt, at about 752°F (400°C).

Sodium reacts with ammonia in two different ways, depending upon the conditions under which the reaction takes place. In liquid ammonia with a catalyst of iron, cobalt, or nickel, sodium reacts to form sodium amide (NaNH₂) and hydrogen gas. In the presence of hot coke (pure carbon), sodium reacts with ammonia to form sodium cyanide (NaCN) and hydrogen gas.

Sodium also reacts with a number of organic compounds. For example, when added to an alcohol, it reacts as it does with water, replacing a single hydrogen atom to form a compound known as an alkoxide. Sodium also reacts with alkenes and dienes to form addition products, one of which formed the basis of an early synthetic rubber known as buna (for bu tadiene and Na [for sodium]) rubber. In the presence of organic halides, sodium may replace the halogen to form an organic sodium derivative.

See also Sodium benzoate; Sodium hypochlorite.

KEY TERMS

Alkene —An organic compound whose molecules contain a carbon-carbon double bond.

Diene —An organic compound whose molecules contain two carbon-carbon double bonds.

Electrolysis —The process by which an electrical current is used to break down a compound into its component elements.

Heat exchange medium —A material that transports heat from one place to another.

Metalloid —An element with properties intermediary between those of a metal and a nonmetal.

Ternary compound —A compound that contains three elements.

Viscosity —The internal friction within a fluid that makes it resist flow.

Resources

BOOKS

Ede, Andrew. The Chemical Element: A Historical Perspective. Westport, CT: Greenwood Press, 2006.

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2003.

Merck. The Merck Index. Whitehouse Station, NJ: Merck; London: Harcourt, 2001.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 2002.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

Wiley-Interscience, eds. Kirk-Othmer Encyclopedia of Chemical Technology. Hoboken, NJ: Wiley-Interscience, 2004.

David E. Newton

SODIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Most people have never seen sodium metal. But it is almost impossible not to see many compounds of sodium every day. Ordinary table salt, baking soda, baking powder, household lye (such as Drano), soaps and detergents, aspirin and other drugs, and countless other consumer products are sodium products.

Sodium is a member of the alkali metals family. The alkali family consists of elements in Group 1 (IA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Other Group 1 (IA) elements are lithium, potassium, rubidium, cesium, and francium. The members of the alkali metals family are among the most active elements.

SYMBOL
Na

ATOMIC NUMBER
11

ATOMIC MASS
22.98977

FAMILY
Group 1 (IA)
Alkali metal

PRONUNCIATION
SO-dee-um

Compounds of sodium have been known, of course, throughout human history. But sodium metal was not prepared until 1807. The reason is that sodium attaches itself very strongly to other elements. Its compounds are very difficult to break apart. It was not until 1807 that English chemist Sir Humphry Davy (1778-1829) found a way to extract sodium from its compounds. (See sidebar on Davy in the calcium entry in Volume 1.) Sodium metal itself has relatively few uses. It reacts with other substances easily, sometimes explosively. However, many sodium compounds have many uses in industry, medicine, and everyday life.

Discovery and naming

Sodium carbonate, or soda (Na₂CO₃), was probably the sodium compound best known to ancient peoples. It is the most common ore of sodium found in nature.

This explains why glass was one of the first chemical products made by humans. Glass is made by heating sodium carbonate and calcium oxide (lime) together. When the mixture cools, it forms the hard, clear, transparent material called glass. Glass was being manufactured on a large scale in Egypt as early as 1370 b.c.

The Egyptians called soda natron. Much later, the Romans used a similar name for the compound, natrium. These names explain the chemical symbol used for sodium, Na.

The name sodium probably originated from an Arabic word suda, meaning "headache." Soda was sometimes used as a cure for headaches among early peoples. The word suda also carried over into Latin to become sodanum, which also means "headache remedy."

In the early 1800s, Davy found a way to extract a number of active elements from their compounds. Sodium was one of these elements. Davy's method involved melting a compound of the active element, then passing an electric current through the molten (melted) compound. Davy used sodium hydroxide (NaOH) to make sodium.

Physical properties

Sodium is a silvery-white metal with a waxy appearance. It is soft enough to be cut with a knife. The surface is bright and shiny when first cut, but quickly becomes dull as sodium reacts with oxygen in the air. A thin film of sodium oxide (Na₂O) forms that hides the metal itself.

Sodium's melting point is 97.82°C (208.1°F) and its boiling point is 881.4°C (1,618°F). Its density is slightly less than that of water, 0.968 grams per cubic centimeter. Sodium is a good conductor of electricity.

Chemical properties

Sodium is a very active element. It combines with oxygen at room temperature. When heated, it combines very rapidly, burning with a brilliant golden-yellow flame.

Sodium also reacts violently with water. (See accompanying sidebar.) It is so active that it is normally stored under a liquid with which it does not react. Kerosene or naphtha are liquids commonly used for this purpose.

Sodium also reacts with most other elements and with many compounds. It reacts with acids to produce hydrogen gas. It also dissolves in mercury to form a sodium amalgam. An amalgam is an alloy of mercury and at least one other metal.

Occurrence in nature

Sodium never occurs as a free element in nature. It is much too active. It always occurs as part of a compound. The most common source of sodium in the Earth is halite. Halite is nearly pure sodium chloride (NaCl). It is also called rock salt.

Halite can be found in underground deposits similar to coal mines. Those deposits were formed when ancient oceans evaporated (dried up), leaving sodium chloride behind. Earth movements eventually buried those deposits. Now they can be mined to remove the sodium chloride.

Sodium and water aren't friends

O il and vinegar don't mix. But sodium and water really don't mix! Sodium reacts violently with water. The effect is fascinating.

When sodium metal is first placed into water, it floats. But it immediately begins to react with water, releasing hydrogen gas:

A great deal of energy is released in this reaction. It is enough to set fire to the hydrogen gas. The sodium metal reacts with water. So much heat is released that the sodium melts. It turns into a tiny ball of liquid sodium. At the same time, the sodium releases hydrogen from water. The hydrogen gas catches fire and causes the ball of sodium to go sizzling across the surface of the water.

Sodium reacts violently with water.

Sodium chloride can also be obtained from seawater and brine. Brine is similar to seawater, but it contains more dissolved salt. Removing sodium chloride from seawater or brine is easy. All that is needed is to let the water evaporate. The sodium chloride is left behind. It only needs to be separated from other chemicals that were also dissolved in the water.

Isotopes

There is only one naturally occurring isotope of sodium, sodium-23. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Six radioactive isotopes of sodium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Two radioactive isotopes of sodium—sodium-22 and sodium-24—are used in medicine and other applications. They can be used as tracers to follow sodium in a person's body. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by means of detectors placed around the system.

Sodium-24 also has non-medical applications. For example, it is used to test for leaks in oil pipe lines. These pipe lines are usually buried underground. It may be difficult to tell when a pipe begins to leak. One way to locate a leak is to add some sodium-24 to the oil. If oil leaks out of the pipe, so does the sodium-24. The leaking oil may not be visible, but the leaking sodium-24 is easily detected. It is located by instruments that are designed to detect radiation.

Extraction

One way to obtain pure sodium metal is by passing an electric current through molten (melted) sodium chloride:

This method is similar to the one used by Humphry Davy in 1808.

But there is not much demand for sodium metal. Sodium compounds are much more common. A second and similar method is used to make a compound known as sodium hydroxide (NaOH). The sodium hydroxide is then used as a starting point for making other sodium compounds.

The method for making sodium hydroxide is called the chloralkali process. The name comes from the fact that both chlorine and an alkali metal (sodium) are produced at the same time. In this case, an electric current is passed through a solution of sodium chloride dissolved in water:

Three useful products are obtained from this reaction: chlorine gas (Cl₂), hydrogen gas (H₂), and sodium hydroxide (NaOH). The chlor-alkali process is one of the most important industrial processes used today.

Uses

Sodium metal has a relatively small, but important, number of uses. For example, it is sometimes used as a heat exchange medium in nuclear power plants. A heat exchange medium is a material that picks up heat in one place and carries it to another place. Water is a common heat exchange medium. Some home furnaces burn oil or gas to heat water that travels through pipes and radiators in the house. The water gives off its heat through the radiators.

Sodium does a similar job in nuclear power plants. Heat is produced by nuclear fission reactions at the core (center) of a nuclear reactor. In a nuclear fission reaction, large atoms break down to form smaller atoms. As they do so, large amounts of heat energy are given off.

Liquid sodium is sealed into pipes that surround the core of the reactor. As heat is generated, it is absorbed (taken up) by the sodium. The sodium is then forced through the pipes into a nearby room. In that room, the sodium pipes are wrapped around pipes filled with water. The heat in the sodium converts the water to steam. The steam is used to operate devices that generate electricity.

Another use of sodium metal is in producing other metals. For example, sodium can be combined with titanium tetrachloride (TiCl₄) to make titanium metal:

Sodium is also used to make artificial rubber. (Real rubber is made from the collected sap of rubber trees and is expensive.) The starting material for artificial rubber is usually a small molecule. The small molecule reacts with itself over and over again. It becomes a much larger molecule called a polymer. The polymer is the material that makes up the artificial rubber. Sodium metal is used as a catalyst in this reaction. A catalyst is a substance used to speed up or slow down a chemical reaction without undergoing any change itself.

The combination of an electric current and sodium vapor produces a yellowish glow in street lamps.

Sodium is frequently used in making light bulbs. Sodium is first converted to a vapor (gas) and injected into a glass bulb. An electric current is passed through a wire or filament in the gas-filled bulb. The electric current causes the sodium vapor to give off a yellowish glow. Many street lamps today are sodium vapor lamps. Their advantage is that they do not produce as much glare as do ordinary lights.

Compounds

Almost all sodium compounds dissolve in water. When it rains, sodium compounds dissolve and are carried into the ground. Eventually, the compounds flow into rivers and then into the oceans. The ocean is salty partly because sodium compounds have been dissolved for many centuries.

But that means that finding sodium compounds on land is somewhat unusual. They tend to be more common in desert areas because deserts experience low rainfall. So sodium compounds are less likely to be washed away. Huge beds of salt and sodium carbonate are sometimes found in desert areas.

Dozens of sodium compounds are used today in all fields. Some of the most important of these compounds are discussed below.

Sodium chloride (NaCl). The most familiar use of sodium chloride is as a flavor enhancer in food. It is best known as table salt. Large amounts of sodium chloride are also added to prepared foods, such as canned, bottled, frozen, and dried foods. One purpose of adding sodium chloride to these foods is to improve their flavors. But another purpose is to prevent them from decaying. Sodium chloride kills bacteria in foods. It has been used for hundreds of years as a food preservative. The "pickling" or "salting" of a food, for example, means the adding of salt to that food to keep it from spoiling.

This process is one reason people eat so much salt in their foods today. Most people eat a lot of prepared foods. Those prepared foods contain a lot of salt. People are often not aware of all the salt they take in when they eat such foods.

Sodium chloride is also the starting point for making other sodium compounds. In fact, this application is probably the number one use for sodium chloride.

Almost all sodium compounds dissolve in water. They tend to be more common in desert areas because deserts experience low rainfall.

Sodium carbonate (Na2CO3). Sodium carbonate is also known by other names, such as soda, soda ash, sal soda, and washing soda. It is also used as the starting point in making other sodium compounds. A growing use is in water purification and sewage treatment systems. The sodium carbonate is mixed with other chemicals that react to form a thick, gooey solid. The solid sinks to the bottom of a tank, carrying impurities present in water or waste water.

Sodium carbonate is also used to make a very large number of commercial products, such as glass, pulp and paper, soaps and detergents, and textiles.

Sodium bicarbonate (NaHCO3). When sodium bicarbonate is dissolved in water, it produces a fizzing reaction. That reaction can be used in many household situations. For example, the fizzy gas can help bread batter rise. The "rising" of the batter is caused by bubbles released when sodium bicarbonate (baking soda) is added to milk in the batter. Certain kinds of medications, such as Alka-Seltzer, also include sodium bicarbonate. The fizzing is one of the effects of taking Alka-Seltzer that helps settle the stomach. Sodium bicarbonate is also used in mouthwashes, cleaning solutions, wool and silk cleaning systems, fire extinguishers, and mold preventatives in the timber industry.

Examples of lesser known compounds are as follows:

sodium alginate (NaC₆H₇O₆): a thickening agent in ice cream and other prepared foods; manufacture of cement; coatings for paper products; water-based paints

sodium bifluoride (KHF₂): preservative for animal specimens; antiseptic (germ-killer); etching of glass; manufacture of tin plate

sodium diuranate, or "uranium yellow" (Na₂U₂O₇): used to produce yellowish-orange glazes for ceramics

sodium fluorosilicate (Na₂SiF₆): used to make "fluoride" toothpastes that protect against cavities; insecticides and rodenticides (rat-killers); moth repellent; wood and leather preservative; manufacture of laundry soaps and "pearl-like" enamels

sodium metaborate (NaBO₂): herbicide

sodium paraperiodate (Na₃H₂IO₆): helps tobacco to bum more completely and cleanly; helps paper products retain strength when wet

sodium stearate (NaOOCC₁₇H₃₅): keeps plastics from breaking down; waterproofing agent; additive in toothpastes and cosmetics

sodium zirconium glycolate (NaZrH₃(H₂COCOO)₃): deodorant; germicide (germ-killer); fire-retardant

Health effects

Sodium has a number of important functions in plants, humans, and animals. In humans, for example, sodium is involved in controlling the amount of fluid present in cells. An excess or lack of sodium can cause cells to gain or lose water. Either of these changes can prevent cells from carrying out their normal functions.

Dietary concerns

P eople sometimes talk about the amount of "sodium" in their diet. Or they may refer to the amount of "salt" in their diet. The two terms are similar, but not exactly alike. In the body, sodium occurs most often as sodium chloride. A common name for sodium chloride is salt.

The Committee on Dietary Allowance of the U.S. Food and Nutrition Board recommends that a person take in about 1,100 to 3,300 milligrams of sodium per day. The human body actually needs only about 500 milligrams of sodium. Studies show that the average American takes in about 2,300 to 6,900 milligrams of sodium per day.

This high level of sodium intake troubles many health experts. Too much sodium can affect the body's ability to digest fats, for example. The most serious problem, however, may be hypertension. Hypertension is another name for "high blood pressure." A person with high blood pressure may be at risk for stroke, heart attack, or other serious health problems.

Sodium is also involved in sending nerve messages to and from cells. These impulses control the way muscles move. Again, an excess or lack of sodium can result in abnormal nerve and muscle behavior. Sodium is also needed to control the digestion of foods in the stomach and intestines.

Sodium

The chemical element of atomic number 11. Symbol Na, atomic weight 22.9898, specific gravity 0.97, melting point 208°F (97.8°C), boiling point 1,621.4°F (883°C).

Sodium is the second element in group 1 of the periodic table . Its chemical symbol reflects its Latin name of natrium. The element was first isolated by the English chemist Sir Humphry Davy in 1807. Only one stable isotope of sodium exists in nature, sodium-23. However, at least six radioactive isotopes have been prepared synthetically. They include sodium-20, sodium-21, sodium-22, sodium-24, sodium-25, and sodium-26.

General properties

Sodium is a soft metal that can be cut easily with a table knife. Its density is so low that it will float when placed into water . At the same time, the metal is so active that it reacts violently with the water, producing sodium hydroxide and hydrogen gas as products. Sufficient heat is produced in the reaction to cause the metal to heat and to ignite the hydrogen produced in the reaction.

Freshly cut sodium metal has a bright, shiny surface that quickly becomes a dull gray as it reacts with oxygen in the air around it. Over time, the metal becomes covered with a white crust of sodium oxide that prevents further reaction of the metal and oxygen.

Sodium forms a very large number of compounds in nature, and an even larger number have been prepared synthetically. These compounds include binary compounds of sodium with metals, non-metals, and metalloids, as well as ternary, and more complex compounds. Included among these are such well-known substances as sodium chloride (table salt ), sodium bicarbonate (baking soda), sodium borate (borax), sodium carbonate (soda ash), monosodium glutamate (MSG) , sodium hydroxide (caustic soda or lye), sodium nitrate (Chilean saltpeter), sodium silicate (water glass ), and sodium tartrate (sal tartar).

Where it comes from

Sodium is the sixth most common element in the Earth's crust with an estimated abundance of 2.83%. It is the second most abundant element in sea water after chlorine . One point of interest is that, although the abundance of sodium and potassium is approximately equal in crustal rocks , the former is 30 times more abundant in sea water than is the latter. The explanation for this difference lies in the greater solubility of sodium compounds than of potassium compounds.

Sodium never occurs free in nature because it is so active. For all practical purposes, the only compound from which it is prepared commercially is sodium chloride. That compound is so abundant and so inexpensive that there is no economic motivation for selecting another sodium compound for its commercial production.

By far the largest producer of sodium chloride in the world is the United States, where about a quarter of the world's supply is obtained. China, Germany, the United Kingdom, France, India, and members of the former Soviet Union are other major producers of salt. The greatest portion of salt obtained in the United States comes from brine, a term used for any naturally occurring solution of sodium chloride in water. The term includes, but is not restricted to, sea water, subterranean wells, and desert lakes such as the Great Salt Lake and the Dead Sea. The second largest source of sodium chloride in the United States is rock salt. Rock salt is generally obtained from underground mines created by the evaporation and then the burying of ancient seas.

How the metal is obtained

The isolation of sodium from its compounds long presented a problem for chemists because of the element's reactivity. Electrolysis of a sodium chloride solution will not produce the element, for example, because any sodium produced in the reaction will immediately react with water.

The method finally developed by Sir Humphry Davy in the early nineteenth century has become the model on which modern methods for the production of sodium are based. In this method, a compound of sodium (usually sodium chloride) is first fused (melted) and then electrolyzed. In this process, liquid sodium metal collects at the cathode of the electrolytic cell and gaseous chlorine is released at the anode .

The apparatus most commonly used today for the preparation of sodium is the Downs cell, named for its inventor, J. Cloyd Downs. The Downs cell consists of a large steel tank lined with a refractory material containing an iron cathode near the bottom of the tank and a graphite anode near the top. A molten mixture of sodium chloride and calcium chloride is added to the tank. The presence of calcium chloride to the extent of about 60% lowers the melting point of the sodium chloride from 1,472°F (800°C) to about 1,076°F (580°C).

When an electrical current is passed through the mixture in the cell, sodium ions migrate to the cathode, where they pick up electrons and become sodium atoms . Chlorine ions migrate to the anode, where they lose electrons and become chlorine atoms. Since the molten sodium metal is less dense than the sodium chloride/calcium chloride mixture, it rises to the top of the cell and is drawn off. The chlorine gas escapes through a vent attached to the anode at the top of the cell. Sodium metal produced by this method is about 99.8% pure. The Downs cell is such an efficient and satisfactory method for preparing sodium that the vast majority of the metal's production is accomplished by this means.

How we use it

Sodium metal has relatively few commercial uses. The most important is as a heat exchange medium in fast breeder nuclear reactors. A heat exchange medium is a material that transports heat from one place to another. In the case of a nuclear reactor , the heat exchange medium absorbs heat produced in the reactor core and transfers that heat to a cooling unit. In the cooling unit, the heat is released to the atmosphere, is used to boil water to power an electrical generating unit, or is transferred to a system containing circulating water for release to the environment.

Liquid sodium is a highly effective heat exchange medium for a number of reasons. First, it has a high heat capacity (that is, it can absorb a lot of heat per gram of metal) and a low neutron absorption cross-section (that is, it does not take up neutrons from the reactor core). At the same time, the metal has a low melting point and a low viscosity , allowing it to flow through the system with relatively little resistance.

For many years, the most important commercial application of sodium metal was in the manufacture of antiknock additives such as tetraethyl and tetramethyl lead . An alloy of sodium and lead was used to react with alkyl chlorides (such as ethyl chloride) to produce these compounds. In 1959, about 70% of all the sodium produced in the United States was used for this purpose. As compounds of lead such as tetraethyl and tetramethyl lead have been phased out of use for environmental reasons, however, this use of sodium has declined dramatically.

Another important use of sodium metal is in the manufacture of other metals, such as zirconium and titanium . Originally, magnesium metal was the reducing agent of choice in these reactions, but sodium has recently become increasingly popular in the preparation of both metals. When sodium is heated with a chloride of one of these metals, it replaces (reduces) the metal to yield the pure metal and sodium chloride.

About 10% of all sodium produced is used to make specialized compounds such as sodium hydride (NaH), sodium peroxide (Na2O2), and sodium alkoxides (NaOR). Small amounts of the metal are used as a catalyst in the manufacture of synthetic elastomers.

Compounds of sodium

Sodium chloride is the most widely used sodium compounds. Due to its availability and minimal amount of preparation, there is no need for it to be manufactured commercially. A large fraction of the sodium chloride used commercially goes to the production of other sodium compounds, such as sodium hydroxide, sodium carbonate, sodium sulfate, and sodium metal itself.

For many centuries, sodium chloride has also been used in the food industry, primarily as a preservative and to enhance the flavors of foods. In fact, many seemingly distinct methods of food preservation , such as curing, pickling, corning, and salting differ only in the way in which salt is used to preserve the food. Scientists are uncertain as to the mechanism by which salting preserves foods, but they believe that some combination of dehydration and high salinity create conditions unfavorable to the survival of pathogens .

Sodium hydroxide and sodium carbonate traditionally rank among the top 25 chemicals in terms of volume produced in the United States. In 1988, for example, the first of these was the seventh most widely produced chemical, with a production of 24.0 billion lb (10.9 billion kg), and the latter ranked number eleven, with a production of 19.1 billion lb (8.65 billion kg).

The number one use of sodium hydroxide is in the manufacture of a large number of other chemical products, the most important of which are cellulose products (including cellulose film) and rayon. Soap manufacture, petroleum refining, and pulp and paper production account for about one tenth of all sodium hydroxide use.

Two industries account for about one third each of all the sodium carbonate use in the United States. One of these is glass-making and the other is the production of soap, detergents, and other cleansing agents. Paper and pulp production, the manufacture of textiles , and petroleum production are other important users of sodium carbonate.

Ranking number 45 on the list of the top 50 chemicals produced in the United States in 1988 was sodium sulfate. For many years, the largest fraction of sodium sulfate (also known as salt cake) was used in the production of kraft paper and paperboard. In recent years, an increasing amount of the chemical has gone to the manufacture of glass and detergents.

Just behind sodium sulfate on the list of top 50 chemicals in 1988 was sodium silicate, also known as water glass. Water glass is used as a catalyst, in the production of soaps and detergents, in the manufacture of adhesives, in the treatment of water, and in the bleaching and sizing of textiles.

Chemical properties

As described above, sodium reacts violently with water and with oxygen to form sodium hydroxide and sodium oxide, respectively. The element also reacts vigorously with fluorine and chlorine, at room temperature , but with bromine and iodine only in the vapor phase. At temperatures above 392°F (200°C), sodium combines with hydrogen to form sodium hydride, NaH, a compound that then decomposes, but does not melt, at about 752°F (400°C).

Sodium reacts with ammonia in two different ways, depending upon the conditions under which the reaction takes place. In liquid ammonia with a catalyst of iron, cobalt or nickel, sodium reacts to form sodium amide (NaNH2) and hydrogen gas. In the presence of hot coke (pure carbon ), sodium reacts with ammonia to form sodium cyanide (NaCN) and hydrogen gas.

Sodium also reacts with a number of organic compounds. For example, when added to an alcohol , it reacts as it does with water, replacing a single hydrogen atom to form a compound known as an alkoxide. Sodium also reacts with alkenes and dienes to form addition products, one of which formed the basis of an early synthetic rubber known as buna (for butadiene and Na [for sodium]) rubber. In the presence of organic halides, sodium may replace the halogen to form an organic sodium derivative.

See also Sodium benzoate; Sodium hypochlorite.

Resources

books

Emsley, John. Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2002.

Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinemann Press, 1997.

Hawley, Gessner G. The Condensed Chemical Dictionary. 9th ed. New York: Van Nostrand Reinhold Company, 1977.

Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

David E. Newton

KEY TERMS

Alkene

—An organic compound whose molecules contain a carbon-carbon double bond.

Diene

—An organic compound whose molecules contain two carbon-carbon double bonds.

Electrolysis

—The process by which an electrical current is used to break down a compound into its component elements.

Heat exchange medium

—A material that transports heat from one place to another.

Metalloid

—An element with properties intermediary between those of a metal and a nonmetal.

Ternary compound

—A compound that contains three elements.

Viscosity

—The internal friction within a fluid that makes it resist flow.

Sodium

Definition

Purpose

Description

Precautions

Interactions

Complications

Parental concerns

Resources

Definition

Sodium is a mineral that exists in the body as the ion Na+. Sodium is acquired through diet, mainly in the form of salt (sodium chloride, NaCl). Regulating the amount of Na+ in the body is absolutely critical to life and health.

Purpose

Sodium is possibly the most important mineral in the body. It plays a major role in controlling the distribution of fluids, maintaining blood pressure and blood volume, creating an electrical gradient that allows nerve transmission and muscle contraction to occur, maintaining the mechanisms that allow wastes to leave cells, and regulating the acidity (pH) of the blood. Many different organ working together, including the kidneys, endocrine glands, and brain, tightly control the level of Na+ in the body. Researchers estimate that between 20% and 40% of an adult’s resting energy use goes toward regulating sodium. Sodium affects every cell in the body, and a major failure of sodium regulatory mechanisms means death.

Description

In the body, sodium exists as electrolyte. Electrolytes are ions that form when salts dissolve in water or fluids. These ions have an electric charge. Positively charged ions are called cations. Negatively charged ions are called anions. Electrolytes are not evenly distributed within the body, and their uneven distribution allows many important metabolic reactions to occur. Sodium (Na+), potassium (K+), calcium (Ca 2+), magnesium (Mg 2+), chloride (Cl-), phosphate

Sodium

mg = milligram

(Illustration by GGS Information Services/Thomson Gale.)

(HPO₄ 2-), bicarbonate (HCO₃-), and sulfate (SO₄ 2-) are important electrolytes in humans.

Na+ is ten times more concentrated in fluid outside cells (i.e. extracellular fluid and blood) than it is in fluid inside cells. This difference in concentration is maintained through the expenditure of cellular energy, and it is critical to many metabolic functions, including maintaining the proportion of water that exists inside and outside of cells. (See the entry on electrolytes for a more detailed explanation of how this occurs). When Na+ is too high or too low, it is almost never because an individual has eaten too much or too little salt. Instead, it is because organs such as the kidneys or endocrine glands that regulate the conservation or removal of sodium from the body have broken down.

Sodium requirements

Researchers estimate that humans can remain healthy taking in only 500 mg of sodium daily. Salt is 40% sodium by weight, and 500 mg is slightly less than the amount of sodium found in 1/4 teaspoon of salt. Humans almost never take in too little salt; their health problems result from too much salt in the diet.

The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for many vitamins and minerals including sodium. The DRIs

KEY TERMS

Diuretic— a substance that removes water from the body by increasing urine production

Ion— an atom or molecule that has an electric charge. In the body ions are collectively referred to as electrolytes.

consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.

The IOM has not set RDAs for sodium, but instead it has set AI levels for all age groups based on observed and experimental information about the amount of sodium needed to replace what is lost by a moderately active individual each day. Sodium is lost in both urine and sweat. IAs for sodium are measured in milligrams (mg). UL levels have not been set. However, the IOM recommends that adults limit their sodium intake to less than 2,400 mg per day, and the American Heart Association recommends an adult daily intake of 1,500-2,300 mg.

The following list gives the recommended daily AL levels of sodium for each age group.

• children birth-6 months: AI 120 mg
• children 7-12 months: AI 370 mg
• children 1-3 years: AI 1,000 mg
• children 4-8 years: AI 1,200 mg
• children 9-13 years: AI 1,500 mg
• adolescents 14-18 years: IA 1,500 mg
• adults age 19-50: AI 1,500 mg
• adults ages50-70 1,300 mg
• adults 71 years or older: AI 1,200 mg
• pregnant women: IA 1,500 mg
• >breastfeeding women: AI 1,500 mg

Sources of sodium

Many people think that the main source of salt in their diet is what they add to food when they are cooking or at the table while eating. In reality, more than three-quarters of the sodium in the average American’s diet is added to food during processing. Another 12% is already naturally in the food. For example, 1 cup of low-fat milk contains 110 mg of sodium. About 6% of sodium in the diet is added as salt during cooking and another 5% from salting food while eating.

Although most sodium in diet comes from salt, other sources of sodium include preservatives and flavor enhancers added during processing. Sodium content is required to be listed on food labels of processed foods. Some common ‘‘hidden’’ sources of sodium include:

• baking soda
• baking powder
• disodium phosphate
• monosodium glutamate (MSG)
• sodium nitrate or sodium nitrite

Below are some common foods and their sodium content.

• table salt, 1 teaspoon:2,300 mg
• dill pickle, large: 1731 mg
• canned chicken noodle soup, 1 cup: 850-1,100 mg
• ham, 3 ounces: 1,000 mg
• sauerkraut, 1/2 cup: 780 mg
• pretzels, 1 ounce: 500 mg
• potato chips, 1 ounce: 165-185 mg
• soy sauce, 1 teaspoon: 304
• deli turkey breast, 1 ounce: 335 mg

Fresh fruits, vegetables, unsalted nuts, and rice, dried beans and peas are examples of foods that are low in sodium.

Sodium and health

Too high a concentration of sodium in the blood causes a condition called hypernatremia. Too much sodium in the diet almost never causes Hypernatremia. Causes include excessive water loss (e.g. severe diarrhea), restricted water intake, untreated diabetes (causes water loss), kidney disease, and hormonal imbalances. Symptoms include signs of dehydration such as extreme thirst, dark urine, sunken eyes, fatigue, irregular heart beat, muscle twitching, seizures, and coma.

Too low a concentration of sodium in the blood causes hyponatremia. Hyponatremia is not usually a problem in healthy individuals, although it has been known to occur in endurance athletes such as ultra-marathoners. It is common in seriously ill individuals and can result from vomiting or diarrhea (extreme loss of sodium), severe burns, taking certain drugs that cause the kidney to selectively excrete sodium, extreme overconsumption of water (water intoxication, a problem among the elderly with dementia), hormonal imbalances, kidney failure, and liver damage. Symptoms include nausea, vomiting, headache, tissue swelling (edema), confusion, mental disorientation, hallucinations, muscle trembling, seizures, and coma.

Hypernatremia and hyponatremia are at the extreme ends of sodium imbalance. However, high dietary intake of salt can cause less visible health damage in the form of high blood pressure (hypertension) . Hypertension silently damages the heart, blood vessels, and kidney and increases the risk of stroke, heart attack, and kidney damage. A low-salt diet significantly lowers blood pressure in 30-60% of people with high blood pressure and a quarter to half of people with normal blood pressure. Some individuals are more sensitive to sodium than others. Those people who are most likely to see a rise in blood pressure with increased sodium intake include people who are obese, have type 2 diabetes, are elderly, female, and African American.

The American Heart Association recommends reducing sodium in the diet to between 1,500 mg and 2,300 mg daily. Below are some suggestions for cutting down on salt.

• Eat more fresh fruits and vegetables.
• Look for processed foods that say ‘‘no salt added’’
• Limit or eliminate salty snacks such as chips and pretzels.
• Restrict the amount processed meats such as hot dogs, pepperoni, and deli meats.
• Avoid high salt canned soups; choose heart-healthy lower salt soups instead.
• Use spices instead of salt to give foods flavor.

Precautions

People who are salt-sensitive may need to keep their salt intake at levels below the suggested daily amounts to control their blood pressure.

Interactions

Certain drugs cause large amounts of sodium to be excreted by the kidneys and removed from the body in urine. Diuretics (‘‘water pills’’) are among the best known of these drugs. Other types of drugs that may cause low sodium levels, especially in ill individuals, include non-steroidal anti-inflammatory drugs (NSAIDs) such as Advil, Motrin, and Aleve, opiates such as codeine and morphine, selective serotonin-reuptake inhibitors (SSRIs) such as Prozac or Paxil, and tricyclic antidepres-sants such as Elavil and Tofranil.

Complications

Health concerns about sodium have been discussed above. Most problems related to high blood pressure are chronic, slow to develop disorders that do not cause serious complications until the second half of an individual’s lifetime. Kidney failure, heart attack, and stroke are all complications of high blood pressure and potentially of high sodium intake.

Parental concerns

Salt is an acquired taste. Parents can help their children control their salt intake and discourage the development of a craving for salt by substituting low-salt foods for high-salt foods.

Resources

BOOKS

American Heart Association. American Heart Association Low-Salt Cookbook: A Complete Guide to Reducing Sodium and Fat in Your Diet. 3rd ed. New York: Clarkson Potter Pubs., 2006.

Hawkins, W. Rex. Eat Right—Electrolyte: A Nutritional Guide to Minerals in Our Daily Diet. Amherst, NY: Prometheus Books, 2006.

James, Shelly V, The Complete Idiot’s Guide to Low-Sodium Meals. Indianapolis, IN: Alpha Books, 2006.

Pressman, Alan H. and Sheila Buff.The Complete Idiot’s Guide to Vitamins and Minerals. 3rd ed. Indianapolis, IN: Alpha Books, 2007.

ORGANIZATIONS

American Heart Association. 7272 Greenville Avenue, Dallas, TX 75231. Telephone: (800) 242-8721. Website: <http://www.americanheart.org>

International Food Information Council. 1100 Connecticut Avenue, NW Suite 430, Washington, DC 20036. Telephone: 02-296-6540. Fax: 202-296-6547. Website: <http://ific.org>

Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331-6512. Telephone: (541) 717-5075. Fax: (541) 737-5077. Website:<http://lpi.oregonstate.edu>

OTHER

American Heart Association. ‘‘Sodium.’’ undated, accessed April 27, 2007, <http://www.americanheart.org/presenter.jhtml?identifier=4708>

Higdon, Jane. ‘‘Sodium.’’ Linus Pauling Institute-Oregon State University, February 16, 2004.<http://lpi.ore-gonstate.edu/infocenter/minerals/sodium>

Mayo Clinic Staff. ‘‘Sodium: Are You getting Too Much?’’ MayoClinic.com, May 24, 2006. <http://www.mayoclinic.com/health/sodium/NU00284>

Medline Plus. ‘‘Dietary Sodium.’’ U. S. National Library of Medicine, April 23, 2007. <http://www.nlm.nih/gov/medlineplus/dietarysodium.html>

Murray, Robert. ‘‘The Risk and Reality of Hyponatremia.’’ Gatorade Sports Science Institute, 2006. <http://www.gssiweb.com>

Northwesternutrition ‘‘Nutrition Fact Sheet: Sodium.’’ Northwestern University, September 21, 2006. <http://www.feinberg.northwestern.edu/nutrition/factsheets/sodium.html>

United States Department of Health and Human Services and the United States Department of Agriculture.

‘‘Dietary Guidelines for Americans 2005.’’ January 12, 2005. <http://www.healthierus.gov/dietaryguidelines>

Tish Davidson, A.M.

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