Phosphoric Acid

Phosphoric Acid, chemical formula H₃PO₄, 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 relative density 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 sulphuric acid, followed by filtration of the resultant liquid to remove calcium sulphate. It can also be prepared by burning phosphorus vapour 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 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. For example, NaH₂PO ₄, with one hydrogen atom replaced, is called primary sodium phosphate; Na ₃PO₄, with three hydrogen atoms replaced, is called tertiary sodium 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, Ca₃(PO₄)₂, 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, CaHPO₄ · 2H₂O, is used as a food supplement. Primary calcium phosphate, Ca(H ₂PO₄)₂, is an ingredient of plant fertilizers.

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

See Ecology; Environment; Water Pollution.