Phosphoric Acid

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Phosphoric Acid, common acid of phosphorus that is the source of industrially important compounds called phosphates. At room temperature, phosphoric acid is a crystalline material with a specific gravity of 1.83. The solid melts at 42.35° C (108.23° F). Phosphoric acid is usually stored and sold as a solution. Phosphoric acid is made by treating calcium phosphate rock with sulfuric acid, followed by filtration of the resultant liquid to remove calcium sulfate. It can be prepared also by burning phosphorus vapor and treating the resulting oxide with steam. The acid is useful in the laboratory because of its resistance to oxidation, to reduction, and to evaporation. Among the many uses of phosphoric acid are as an ingredient in soft drinks and dental cements, as a catalyst, in rustproofing metals, and in making phosphates, which are used in water softeners, fertilizers, and detergents.

PHOSPHATES

Phosphates are products formed by the replacement of some or all of the hydrogen of a phosphoric acid by metals. Depending on the number of hydrogen atoms that are replaced, the resulting compound is described as a primary, secondary, or tertiary phosphate. Also known as trisodium phosphate, tertiary sodium phosphate is valuable as a detergent and water softener.. Primary and secondary phosphates contain hydrogen and are acid salts. Secondary and tertiary phosphates, with the exception of those of sodium, potassium and ammonium, are insoluble in water; the primary phosphates are more soluble.

Phosphates are important to metabolism in both plants and animals. Bones contain calcium phosphate, and the first step in the oxidation of glucose in the body is formation of a phosphate ester . To provide cattle with phosphate, dicalcium phosphate is used as a food supplement. Primary calcium phosphate is an ingredient of plant fertilizers.

Increasing attention has been focused on the environmentally harmful effects of phosphates in household detergents. Wastewater from laundering agents containing phosphates is known to be a water pollutant because phosphates are a primary nutrient of algae; when it grows in excess, algae can choke a lake or river and draw off needed oxygen from aquatic life.

See Ecology; Environment; Water Pollution.

Plutonium

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Plutonium, symbol Pu, radioactive metallic element that is used in nuclear reactors and nuclear weapons. The atomic number of plutonium is 94. The element is one of the transuranium elements in the actinide series of the periodic table.

Isotopes of plutonium were first prepared by the American chemist Glenn T. Seaborg and his associates at the University of California at Berkeley in 1940. Trace amounts of the element have since been found in uranium ores, but plutonium is prepared in relatively large quantities today in nuclear reactors.

Chemically, plutonium is reactive, its properties somewhat resembling those of the rare earth elements. The silvery metal, which becomes slightly yellow through oxidation caused by exposure to air, exists in six varying crystalline forms and has four different oxidation states. The metal gives off heat because of its radioactivity; 15 different isotopes of plutonium, ranging in mass number from 232 to 246, are known. Plutonium melts at about 641° C (about 1186° F), boils at about 3232° C (about 5850° F), and has a specific gravity of 19.84.

The most important isotope, plutonium-239, has a half-life of 24,360 years, and is produced by bombarding uranium-238 with slow neutrons. This forms neptunium-239, which in turn emits a beta particle and forms plutonium-239. Plutonium is the most economically important of the transuranium elements because plutonium-239 readily undergoes fission and can be both used and produced in quantity in nuclear reactors. It is also used in making nuclear weapons. It is an extremely hazardous poison due to its high radioactivity. Plutonium-238 has been used to power equipment on the moon by means of the heat it emits.

Polonium

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Polonium, symbol Po, rare, radioactive metallic element. Polonium is in group 16 (or VIa) of the periodic table. Its atomic number is 84.

The first element to be discovered by means of its radioactivity, polonium was found in pitchblende in 1898 by the French chemist Marie Curie, who named it for her native country, Poland. Polonium is one of the elements in the uranium-radium series of radioactive decay, the first member of which is uranium-238. Polonium occurs in radium-containing ores and is found in isotopic forms with mass numbers ranging from 192 to 218. Polonium 209 (also called radium-F), the only naturally occurring isotope, has a half-life of 138 days. Polonium melts at about 254° C (about 489° F), boils at about 962° C (about 1764° F), and has a specific gravity of 9.4.

Because most polonium isotopes disintegrate by emitting alpha particles, the element is a good source of pure alpha radiation. It is also used in nuclear research with elements such as beryllium that emit neutrons when bombarded by alpha particles. In printing and photography equipment, polonium is used in devices that ionize the air to eliminate accumulation of electrostatic charges.