Phosphoric Acid

OVERVIEW

Phosphoric acid (fos-FOR-ik AS-id) melts at a temperature just above room temperature (about 20°C; 68°F), so would be expected to occur as a solid under those conditions. As a solid, the acid is a white crystalline material with a strong tendency to absorb moisture from the air. In fact, phosphoric acid may also occur as a supercooled liquid at room temperature. A supercooled liquid is one that remains in a liquid state at temperatures below its freezing point, at which temperature it would be expected to be a solid. As a liquid, phosphoric acid is a colorless, odorless, syrupy liquid whose character is sometimes described as sparkling.

Phosphoric acid was discovered independently as a component of bone ash in 1770 by two Swedish chemists, Johann Gottlieb Gahn (1745–1818) and Karl Wilhelm Scheele (1742–1786). Four years later, Scheele discovered that the acid could be made by adding nitric acid to phosphorus.

KEY FACTS

OTHER NAMES:

Orthophosphoric acid

FORMULA:

H₃PO₄

ELEMENTS:

Hydrogen, phosphorus, oxygen

COMPOUND TYPE:

Inorganic acid

STATE:

Solid. See Overview

MOLECULAR WEIGHT:

98.00 g/mol

MELTING POINT:

42.4°C (108°F)

BOILING POINT:

407°C (765°F)

SOLUBILITY:

Very soluble in water and ethyl alcohol

Phosphoric acid is the ninth highest volume chemical produced in the United States. In 2004, the U.S. chemical industry made about 5.2 million kilograms (11.5 million pounds) of phosphoric acid. About 90 percent of that amount went to the manufacture of fertilizers.

HOW IT IS MADE

The most economical method for making phosphoric acid is by treating phosphate rock with sulfuric acid (H₂SO₄). Phosphate rock is naturally-occurring rock with large amounts of calcium phosphate (Ca₃(PO₄)₂). The product of this reaction is generally not very pure, but sufficiently pure for use in the production of fertilizers. Higher quality phosphoric acid can be made by burning phosphorus or phosphate rock in an electric furnace, converting either to gaseous phosphoric oxide (diphosphorus pentoxide; P₂O₅). The oxide is then dissolved in water to form the acid.

COMMON USES AND POTENTIAL HAZARDS

By far the most important use of phosphoric acid is in the production of fertilizers. At least four major types of fertilizers are made from phosphoric acid: diammonium phosphate ((NH₄)₂HPO₄; DAP), monoammonium phosphate (NH₄H₂PO₄; MAP), granulated triple superphosphate (GTSP), and superphosphoric acid, the only liquid among the group. An additional 5 percent of the phosphoric acid produced is used as an animal feed supplement.

Interesting Facts

• Phosphoric acid is commercially available in a number of technical grades, ranging from agricultural (relatively low purity) to technical (from 50 to 100 percent purity) to FCC (Food Chemicals Codes) quality of at least 75 percent phosphoric acid.

The remaining 5 percent of phosphoric acid produced is used in a very wide range of commercial, industrial, and household products, including:

• For pickling cleaning and treating metallic surfaces, especially in the steel industry;
• In the synthesis of inorganic chemical compounds;
• As a catalyst in the manufacture of ethanol (ethyl alcohol), ethylene, and other organic compounds;
• As a food additive in a number of products, such as colas, beers, jams, and cheeses, where it adds a touch of tartness to the product;
• In the dentistry profession, where it is used to etch and clean teeth;
• In a number of consumer products, such as soaps, detergents, and toothpastes;
• As a refining and clarifying agent in the production of sugar;
• In the dyeing of cotton;

• As a binder for cement;
• In the manufacture of waxes and polishes; and
• In water and sewage treatment plants.

Phosphates (compounds made from phosphoric acid) were once used widely as "builders" in detergents. A builder is a compound that increases the cleaning efficiency of the detergent. The problem is that phosphates that escape into the natural environment can result in some undesirable changes in fresh water systems. Algae living in these systems use phosphate to grow and multiply, resulting in the conversion of fresh water lakes and ponds into swamps and bogs, and, eventually, into dry fields, a process known as eutrophication. Because of this effect, the use of phosphates in detergents has been banned in most parts of the United States.

Phosphoric acid is an extremely hazardous and toxic compound. In small amounts, it causes irritation of the skin, eyes, and respiratory system. If ingested, it can cause serious damage to the digestive system, resulting in nausea, vomiting, abdominal pain, difficulty in breathing, shock, and occasionally death by asphyxiation (suffocation). The most serious health hazards posed by phosphoric acid are of concern primarily to people who work with the product. The amount of phosphoric acid present in most domestic and household products is very small and poses little risk to users of those products.

Words to Know

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

PHOSPHATE

A compound that is manufactured from phosphoric acid

SUPERCOOLED

Refers to a substance that remains in a liquid state at temperatures below its freezing point.

FOR FURTHER INFORMATION

"Chemical of the Week: Phosphoric Acid." Science Is Fun. http://scifun.chem.wisc.edu/CHEMWEEK/H3PO4/H3PO4.html (accessed on October 24, 2005).

"Phosphate Primer." Florida Institute of Phosphate Research. http://www1.fipr.state.fl.us/PhosphatePrimer (accessed on October 24, 2005).

"Phosphoric Acid." DC Chemical Co., Ltd. http://www.dcchem.co.kr/english/product/p_basic/p_basic04.htm (accessed on October 24, 2005).

"Phosphoric Acid Fuel Cells." Smithsonian Institution. http://americanhistory.si.edu/fuelcells/phos/pafcmain.htm (accessed on October 24, 2005).

See AlsoNitric Acid; Sulfuric Acid

Phosphoric Acid

Phosphoric acid, H₃PO₄ (orthophosphoric acid), is a white crystalline substance which melts at 108°F (42°C). It is most commonly found in aqueous form (dissolved in water), where it forms a colorless, thick liquid. Phosphoric acid is widely used in the manufacturing of phosphate detergents and fertilizers. Because of increased algae growth in lakes with high levels of phosphate in them the use of phosphate detergents has been dramatically curtailed in many areas. Phosphoric acid is added to some foods (especially colas) to give a tart flavor to the final product. Since phosphoric acid can donate three protons (hydrogen ions) to other substances, it is known as a triprotic acid.

Phosphoric acid is a weak acid, with only a small percentage of the molecules in solution ionizing. Phosphoric acid is manufactured by the reaction of sulfuric acid upon phosphate rocks (commonly found in Florida), most notably calcium phosphate, as shown below:

Ca₃(PO₄)₂ +3H₂SO₄ +6H₂O → 3CaSO₄ • 2H₂O+ 2H₃PO₄

The other product of the reaction, calcium sulfate dihydrate is gypsum and is used in drywall in the construction industry.

In addition to using calcium phosphate as a starting material, fluorapatite Ca₅(PO₄)₃ may also be used. The two processes shown above are known as wet processes, which may give impure phosphoric acid as a product. Much higher levels of purity may be obtained by using the furnace process, in which phosphate containing minerals react with coke and silica at high temperatures. The resulting product is then dissolved in water to produce very pure phosphoric acid.

Alternatively, phosphoric acid may be produced by reacting tetraphosphorous decoxide with water:

P₄O₁₀ +6H₂O→ 4H₃PO₄

Phosphoric acid is used as an acidulant in the food industry (It is the second most common acidulant used, behind citric acid). As an acidulant it serves as a preservative and buffer, provides tartness, and modifies the viscosity (or resistance to flow) of liquids.

When pure phosphoric acid is heated, two molecules may condense (release water from a reaction between them) to form a polyphosphoric acid. Salts of polyphosphoric acids are used in the manufacturing of detergents to help bind calcium and magnesium ions from hard water.

Phosphoric acid

Phosphoric acid, H3PO4 (orthophosphoric acid), is a white crystalline substance which melts at 108°F (42°C). It is most commonly found in aqueous form (dissolved in water ), where it forms a colorless, thick liquid. Phosphoric acid is widely used in the manufacturing of phosphate detergents and fertilizers . Because of increased algae growth in lakes with high levels of phosphate in them the use of phosphate detergents has been dramatically curtailed in many areas. Phosphoric acid is added to some foods (especially colas) to give a tart flavor to the final product. Since phosphoric acid can donate three protons (hydrogen ions) to other substances, it is known as a triprotic acid.

Phosphoric acid is a weak acid, with only a small percentage of the molecules in solution ionizing. Phosphoric acid is manufactured by the reaction of sulfuric acid upon phosphate rocks (commonly found in Florida), most notably calcium phosphate, as shown below:

The other product of the reaction, calcium sulfate dihydrate is gypsum and is used in drywall in the construction industry.

In addition to using calcium phosphate as a starting material, fluorapatite Ca5(PO4)3 may also be used. The two processes shown above are known as wet processes, which may give impure phosphoric acid as a product. Much higher levels of purity may be obtained by using the furnace process, in which phosphate containing minerals react with coke and silica at high temperatures. The resulting product is then dissolved in water to produce very pure phosphoric acid.

Alternatively, phosphoric acid may be produced by reacting tetraphosphorous decoxide with water:

Phosphoric acid is used as an acidulant in the food industry (It is the second most common acidulant used, behind citric acid ). As an acidulant it serves as a preservative and buffer , provides tartness, and modifies the viscosity (or resistance to flow) of liquids.

When pure phosphoric acid is heated, two molecules may condense (release water from a reaction between them) to form a polyphosphoric acid. Salts of polyphosphoric acids are used in the manufacturing of detergents to help bind calcium and magnesium ions from hard water .

phosphoric acid Group of acids. Tetraoxophosphoric acid (H₃PO₄, formerly orthophosphoric acid) is a colourless liquid obtained by the action of sulphuric acid on phosphate rock (calcium phosphate); it is used in dental adhesives, flavoured syrups, fertilizers, soaps, detergents, and anticorrosive coatings for metals. Metaphosphoric acid (HPO₃) is obtained by heating tetraoxophosphoric acid; it is used as a dehydrating agent. Heptaoxodiphosphoric (H₄P₂O₇, formerly pyrophosphoric acid) is formed by moderately heating tetraoxophosphoric acid or by reacting phosphorus pentoxide (P₂O₅) with water; it is used as a catalyst and in metallurgy.

phosphoric acid May be one of three types: orthophosphoric acid (H₃PO₄), metaphosphoric acid (HPO₃), or pyrophosphoric acid (H₄P₂O₇). Orthophosphoric acid and its salts are used as acidity regulators and in acid‐fruit‐flavoured beverages.