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 .
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.
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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." Encyclopedia of Food and Culture. . Encyclopedia.com. (October 22, 2016). http://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/sodium
"Sodium." Encyclopedia of Food and Culture. . Retrieved October 22, 2016 from Encyclopedia.com: http://www.encyclopedia.com/food/encyclopedias-almanacs-transcripts-and-maps/sodium
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
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.
Group 1 (IA)
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.
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.
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.
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.
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.
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.
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
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.
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 (revised)." Chemical Elements: From Carbon to Krypton. . Encyclopedia.com. (October 22, 2016). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/sodium-revised
"Sodium (revised)." Chemical Elements: From Carbon to Krypton. . Retrieved October 22, 2016 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/sodium-revised
Known to most people in the form of table salt, sodium is one of the minerals that the body needs in relatively large quantities. Humankind's taste for sodium reaches far back into the distant past. Much like today, sodium was popular in antiquity as a food preservative and an ingredient in snacks. In some ancient societies, sodium was even used as a form of currency.
In modern times, most Americans and other Westerners consume far too much of the mineral, and it is easy to see why. One obvious culprit is table salt, which has a high sodium content. The mineral is also found in many of America's favorite foods (or the chemicals used to preserve those foods). Sodium can be found in potato chips and a variety of other snacks, processed foods, meat, fish, butter and margarine, soft drinks, dairy products, canned vegetables, and bread, just to name a few sources. A single slice of pizza can supply the body with all the sodium it needs for one day (about 500 mg), while a teaspoon of table salt contains four times that amount.
A certain intake of sodium is considered essential to life. The mineral is a vital component of all bodily fluids, including blood and sweat. Often working in combination with other minerals such as potassium , sodium helps to manage the distribution and pH balance of these fluids inside the body and plays an important role in blood pressure regulation. Sodium is referred to as an electrolyte because it possesses a mild electrical charge when dissolved in bodily fluids. Due to this charge, sufficient amounts of the mineral are necessary for the normal functioning of nerve transmissions and muscle contractions. Sodium also helps the body to retain water and prevent dehydration, and may have some activity as an antibacterial.
The important benefits associated with sodium become apparent in cases of sodium deficiency, which is relatively uncommon. Sodium deficiency is most likely to occur in cases of starvation, diarrhea , intense sweating, or other conditions that cause rapid loss of water from the body. People who suffer from low sodium levels may experience a wide range of bothersome or serious health problems, including digestive disorders, muscle twitching or weakness, memory loss, fatigue , and lack of concentration or appetite. Arthritis may also develop. These problems usually occur when fluids that belong in the bloodstream take a wrong turn and enter cells.
Most Americans consume anywhere from 3,000 mg to 20,000 mg of sodium a day. These amounts are much more than the body needs to function at an optimal level. Many nutrition experts are concerned about the rise in sodium intake in the general population in the last twenty years. Much of this increase is due to the popularity of fast foods and salty snacks, including the sale of high-sodium snack foods in school cafeterias or vending machines.
While sodium deficiencies are rare, supplements may be required in people with certain medical conditions such as Addison's disease, adrenal gland tumors, kidney disease, or low blood pressure. More sodium may also be needed by those who experience severe dehydration or by people who take diuretic drugs.
Though taking extra amounts of sodium is not known to improve health or cure disease, the mineral may have some therapeutic value when used externally. A number of medical studies in people suggest that soaking in water from the Dead Sea may be beneficial in the treatment of various diseases such as rheumatoid arthritis , psoriatic arthritis, and osteoarthritis of the knees. Located in Israel, the Dead Sea is many times saltier than ocean water and rich in other minerals such as magnesium , potassium, and calcium . In one small study, published in 1995 by researchers from the Soroka Medical Center in Israel, nine people with rheumatoid arthritis showed significant improvement in their condition after bathing in the Dead Sea for 12 days. The control group in the study, whose members did not bathe in the Dead Sea, failed to improve. The beneficial effects of the Dead Sea soaks lasted for up to three months after they had stopped bathing in the famous body of water. Despite intriguing findings such as these, no one knows for certain if sodium plays a major role in the therapeutic powers associated with the Dead Sea soaks.
Sodium has a reputation as a germ killer. Some people use a sodium solution as an antibacterial mouthwash to combat microorganisms that cause sore throat or inflamed gums. Plain saltwater soaks have also been recommended as a remedy for sweaty feet. Salt is believed to have a drying effect by soaking up excess perspiration. In ages past, saltwater soaks were used to relieve sore or aching muscles.
In the late 1990s the National Academy of Sciences established the recommended daily allowance (RDA) of sodium as between 1,100 and 3,300 milligrams.
To prepare a sodium mouthwash, mix 1 tsp of table salt with a glass of warm water. The solution should be swished around in the mouth for about a minute or so. Then spit the mixture out. Try not to swallow the solution, as it contains about 2,000 mg of sodium.
Sodium is available in tablet form, but supplements should only be taken under the supervision of a doctor. As mentioned earlier, most people already get far too much sodium in their diets .
A trip to the Dead Sea is not necessary in order to enjoy its potential benefits. Dead Sea bath salts are also available.
People who wish to take sodium supplements or increase their sodium intake should talk to a doctor first if they have high blood pressure (or a family history of the disease), congestive heart failure (or other forms of heart or blood vessel disease), hepatic cirrhosis, edema, epilepsy , kidney disease, or bleeding problems.
Studies investigating the role of sodium in the development of high blood pressure have produced mixed results. However, sodium is widely believed to contribute to the development of the disease in susceptible people. For this reason, most doctors and major health organizations around the world recommend a diet low in sodium. Eating a low-sodium diet may actually help to lower blood pressure, especially when that diet includes sufficient amounts of potassium.
A 20-year-long follow-up study to the National Health and Nutrition Examination Survey that was conducted between 1971–1975 reported in 2002 that high levels of sodium in the diet are an independent risk factor for congestive heart failure (CHF) in overweight adults. The authors of the study suggested that lowering the rate of sodium intake may play an important role in lowering the risk of CHF in overweight populations as well as individuals.
Another good reason for limiting one's intake of sodium is the link between high levels of dietary sodium and an increased risk of stomach cancer . This risk is increased if a person's diet is also low in fresh fruits and vegetables.
Apart from an increase in blood pressure, high levels of sodium may cause confusion, anxiety , edema, nausea, vomiting , restlessness, weakness, and loss of potassium and calcium.
People who are concerned about consuming too much sodium should try to keep their sodium intake below 2500 mg per day. This is the level recommended by the US Department of Health and Human Services and the US Department of Agriculture in their 2000 Dietary Guidelines for Americans. Ways to reduce sodium intake include the following:
- Reading the Nutrition Facts labels on processed food items. The amount of sodium in a specific processed food, such as cake mix or canned soup, can vary widely from brand to brand.
- Retraining the taste buds. A taste for salt is acquired. A gradual decrease in the use of salt to season foods gives the taste buds time to adjust.
- Using other spices and herbs to season food.
- Cooking from scratch rather than using processed foods.
- Substituting fresh fruits and vegetables for salty snack foods.
- Tasting food at the table before adding salt. Many people salt their food automatically before eating it, which often adds unnecessary sodium to the daily intake.
- Choosing foods that are labeled "low sodium" or "sodium free."
- Watching the sodium content of over-the-counter medications, and asking a pharmacist for information about the sodium content of prescription drugs.
Restricting sodium intake is not usually recommended for women who are pregnant or breast-feeding.
Dietary sodium is not associated with any bothersome or significant short-term side effects. In some people, however, salt tablets may cause upset stomach or affect kidney function.
Sodium may promote the loss of calcium and potassium from the body. In addition, sodium in the diet should be restricted for such medications as antihypertensives (drugs to control blood pressure) and anticoagulants (blood thinners) to be fully effective.
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Rebecca J. Frey, PhD
"Sodium." Gale Encyclopedia of Alternative Medicine. . Encyclopedia.com. (October 22, 2016). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/sodium
"Sodium." Gale Encyclopedia of Alternative Medicine. . Retrieved October 22, 2016 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/sodium
sodium, a metallic chemical element; symbol Na [Lat. natrium]; at. no. 11; at. wt. 22.98977; m.p. 97.81°C; b.p. 892.9°C; sp. gr. 0.971 at 20°C; valence +1. Sodium is a soft, silver-white metal. Extremely reactive chemically, it is one of the alkali metals in Group 1 of the periodic table. Like potassium, which it closely resembles, it oxidizes rapidly in air; it also reacts violently with water, liberating hydrogen (which may ignite) and forming the hydroxide. It must be stored out of contact with air and water and should be handled carefully. Sodium combines directly with the halogens. The metal is usually prepared by electrolysis of the fused chloride (the Downs process); formerly, the chief method of preparation was by electrolysis of the fused hydroxide (the Castner process). Metallic sodium has limited use. It is used in sodium arc lamps for street lighting; pure or alloyed with potassium, it has found use as a heat-transfer liquid, e.g., in certain nuclear reactors. It is used principally in the manufacture of tetraethyl lead (a gasoline antiknock compound) and of sodamide, NaNH2, sodium cyanide, NaCN, sodium peroxide, Na2O2, and sodium hydride, NaH. Sodium compounds are extensively used in industry and for many nonindustrial purposes. Among the most important compounds are chloride (common salt, NaCl), bicarbonate (baking soda, NaHCO3), carbonate (soda ash, or washing soda, Na2CO3), hydroxide (caustic soda, or lye, NaOH), nitrate (Chile saltpeter, NaNO3), thiosulfate (hypo, Na2S2O3·5H2O), phosphates, and borax (Na2B4O7·10H2O). Sodium hydroxide is used wherever a cheap alkali is needed, for example, in making soap. Substances containing sodium impart a characteristic yellow color to a flame. Because of its activity sodium is not found uncombined in nature. It occurs abundantly and widely distributed in its compounds, which are present in rocks and soil, in the oceans, in salt lakes, in mineral waters, and in deposits in various parts of the world. Sodium compounds are found in the tissues of plants and animals. Sodium is an essential element in the diet, but some people must limit the amount of sodium in their food for medical reasons. Discovery of sodium is usually credited to Sir Humphry Davy, who prepared the metal from its hydroxide in 1807; its compounds have been known since antiquity.
"sodium." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (October 22, 2016). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/sodium
"sodium." The Columbia Encyclopedia, 6th ed.. . Retrieved October 22, 2016 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/sodium
melting point: 97.8°C
boiling point: 883°C
density: 0.971 g/cm 3
most common ions: Na +
Sodium is a soft, silvery alkali metal and reacts vigorously with water to generate hydrogen gas. The word sodium is derived from "sodanum" (a Medieval Latin name for a headache remedy), and "natrium" (Latin for "soda") is the origin of the element's symbol. Humphry Davy isolated the element in 1807 via the electrolysis of caustic soda, NaOH. Currently, sodium metal is obtained from the electrolysis of a molten mixture of sodium chloride and
calcium chloride (in an electrochemical cell called the Downs cell). In nature it is never found in its elemental form, but sodium compounds are quite common. Sodium is the most abundant alkali metal and the seventh most abundant element in Earth's crust (22,700 ppm). Sodium burns yellow-orange in the flame test.
The demand for metallic sodium is declining. Its primary use had been as a substance used in the production of tetraethyl lead, an antiknocking gasoline additive; however, because of its damaging effects on the environment, tetraethyl lead is being phased out. Sodium is used to produce sodamide from reaction with ammonia and to reduce TiCl4, ZrCl4, and KCl to Ti, Zr, and K, respectively. An alloy of Na and K is used in nuclear reactors as a heat transfer agent.
Several sodium compounds are economically important. NaCl (ordinary salt) is a de-icing compound, a condiment, and a food preservative. NaOH finds use in the manufacture of soaps, detergents, and cleansers. Na2CO3 (washing soda) is used to make glass, soaps, fire extinguishers, and "scrubbers" that remove SO2 from gases generated in power plants before it escapes into the atmosphere. The paper industry uses Na2SO4 (salt cake) to make brown wrapping paper and corrugated boxes.
Appropriate sodium ion levels (along with potassium levels) are essential for proper cell function in biological systems.
see also Alkali Metals.
Nathan J. Barrows
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so·di·um / ˈsōdēəm/ • n. the chemical element of atomic number 11, a soft silver-white reactive metal of the alkali metal group. (Symbol: Na)
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