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Phosphoric acid

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Phosphoric acid
Systematic name Phosphoric Acid[1]
Other names Orthophosphoric acid[2]
Molecular formula H3PO4
Molar mass 98.0 g/mol [2]
Appearance Clear, viscous liquid, white solid [3][2]
CAS number CAS number::7664-38-2
Density and phase Density::1.88 g/ml solid,
Solubility in water Miscible in water and ethyl[4][5] alcohol
Melting point 42.4°C [2]
Boiling point Boiling point::260°C decomposes
Acidity (pKa) 2.15, 7.09, 12.32[6]
Viscosity 43.5cP at 25°C at 85% concentration[7]
Molecular shape Tetrahedral[8]
MSDS Material safety data sheet
Main hazards Causes burns[9]
NFPA 704

NFPA 704 svg.png

Flash point Non-flammable
R/S statement R:34
S: 1/2, 26, 45
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Disclaimer and references

Phosphoric acid is an inorganic acid that contains phosphorus. It was discovered by Wilhelm Scheele when nitric acid with phosphorus in 1769. Since then phosphoric acid has been used all over the world and has a variety of different uses, and is ranked in the top ten chemicals used worldwide. In the United States, most phosphoric acid is used as fertilizer. [2]


As a solid, phosphoric acid is a white crystalline; in a liquid, phosphoric acid is a colorless substance. It is often dissolved in water and this is the most common form of phosphoric acid. [10]Phosphoric acid is comprised of hydrogen, phosphorus, and oxygen. Phosphoric is soluble in both water and alcohol. [11]Phosphoric acid is stronger than acetic acid, but weaker than stronger acids such as hydrochloric acid.[3]Phosphoric acid is a stable compound and is not flammable. Phosphoric acid should avoid incompatible materials such as combustible materials. [9] It can form three different kinds of salts and is also corrosive. Phosphoric acid reacts with metals; the product formed from the reaction is flammable hydrogen gas. [12] Phosphoric acid has a tetrahedral molecular shape. [13]


Phosphoric acid is an inorganic compound not found naturally on the earth. Instead, there are several methods in which phosphoric acid can be created. Phosphate rock occurs naturally in the earth, and if treated with sulfuric acid, phosphoric acid is created. This reaction produces a low quality phosphoric acid, and this type can be used in fertilizers. Another method that can create phosphoric acid is when phosphate rock is burned in an electric furnace. This produces gaseous phosphoric oxide. Phosphoric acid can be created through this method if the gas is then dissolved in water. This second reaction produces a better quality product which can be used in a variety of different ways including food. [5]


Phosphoric acid is used in soda for several different reasons. Soda makers use phosphoric acid to add to the flavor of the soda. Phosphoric acid is not only added to soda for flavor. Because of the high levels of sugar in soda, bacteria growth could be inhibited. However, with the addition of phosphoric acid to the soda, bacteria growth is reduced and slowed. [14] Phosphoric acid also has many other important uses. Phosphoric acid plays a key role in rust removal. The phosphoric acid changes the rust, which is ferric acid, to ferric phosphate. The rust that would previously not come off can be scraped once treated with phosphoric acid. The ferric phosphate can be removed from the item that needed to be treated. However, ferric phosphate can be beneficial if it not removed because it provides protection from future rust. Phosphoric acid is also used in several different ways in the field of dentistry. It is a component in dental cement as well as teeth whitening products. [15]One of phosphoric acid’s largest uses in fertilizer. Some of the fertilizers made from phosphoric acids are diammonium phosphate, monoammonium phosphate, granulated triple superphosphate, and superphosphoric acid. [5] In the United States, the majority of phosphoric acid is used to make fertilizer. Phosphoric acid really began to be produced on a large scale after World War II. [2]

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cell

In phosphoric acid fuel cells, phosphoric acid is used as an electrolyte. Inside the fuel cell, the positively charged ions travel from the anode to the cathode. Phosphoric acid fuel cells have several advantages over other fuel cells. Phosphoric acid fuel cells can handle a carbon monoxide level of about 1.5 percent. Carbon monoxide can hurt the fuel cell if too much is created and present in the fuel cell. Also, another advantage of the hydrogen fuel cell is that it can work above the boiling point of water. In the beginning stages of fuel cells, phosphoric acid was not selected as a main component in fuel cells. Scientists did not target phosphoric acid immediately because it does not conduct electricity. However, in 1961, two chemists, G.V Elmore and H.A. Tanner, released a paper describing some of their experiments with phosphoric acid in fuel cells. With power shortages in the 1970s, researchers began to experiment with ways that phosphoric acid fuel cells could be useful to generate electricity. Some of the experiments resulted in golf carts and metro buses being run on phosphoric acid fuel cells. However, phosphoric acid fuel cells are not very popular in the car industry because they take a long time to warm up before they can be used. Phosphoric acid fuel cells are good for supplying buildings with power because they are stationary and do not have to be turned off and on like cars. [16]

Health Dangers

There are many health dangers if exposed to large concentrations of phosphoric acid. One should not be exposed to more than 1 mg/m3 for safety reasons, and short term exposure is slightly higher than that. Small quantities in food will not do any harm, but exposure to large concentrations of phosphoric acid should be avoided. Phosphoric acid can be harmful if it comes in contact with skin, eyes, inhaled, or ingested. If it comes in contact with the eyes irritation may occur and even permanent eye damage is possible. Phosphoric acid can be very dangerous if ingested. Phosphoric acid may cause pain in the mouth and throat and may cause difficulty in swallowing and breathing. There are also many other effects if phosphoric is ingested. Phosphoric acid can also cause environmental damage to aquatic life. A large amount of phosphoric acid could affect the pH of the ecosystem and lower it to dangerous levels. [12]Phosphoric acid should be kept away from metals, bases, and combustible materials. Protective clothing should be worn when working with phosphoric acid because it can cause burns.[9]


  1. 1.0 1.1 Human Metabolome Database Metabolomics Toolbox. Web. May 27, 2013 (Accessed). Author Unknown.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Myers, Richard L. Phosphoric Acid Gale Virtual Reference Library.Web. 2007.
  3. 3.0 3.1 Prof. Shakhashiri. Phosphoric Acid. Scifun. Web. February 6, 2008.
  4. Material Safety Data Sheet ChemWatch. Web. August 16, 2006. Author Unknown.
  5. 5.0 5.1 5.2 Schlager, Neil, Weisblatt, and David E. Newton.Phosphoric Acid. Chemical Compounds. Web. 2006.
  6. Phosphoric Acid, ACS Reagent Sigma-Aldrich. Web. May 27, 2013 (Accessed) Author Unknown.
  7. Purified Phosphoric Acid. PotashCorp. Web. May 27, 2013 (Accessed) Author Unknown.
  8. Phosphoric Acid Chemistry. Web. January 6, 2008. Author Unknown.
  9. 9.0 9.1 9.2 Material Safety Data Sheet Phosphoric Acid. Web. November 30, 2005. Author Unknown.
  10. Ed. Lerner, Lee and BrendaWilmoth Lerner. Phosphoric Acid. The Gale Encyclopedia of Science. Web. 2008.
  11. The Columbia Encyclopedia Web. November 2004. Author Unknown.
  12. 12.0 12.1 Phosphoric acid: Overview. National Pollutant Inventory. Web. May 18, 2012 (Accessed) Author Unknown.
  13. Phosphoric Acid Chemistry. Web. January 6, 2008. Author Unknown.
  14. Senese, Fred. Why is phosphoric acid in soda pop?. General Chemistry Online?. Web. February 15, 2010.
  15. Khan, Sumaiya. Phosphoric Acid Uses. Buzzle. Web. October 3, 2011.
  16. Smithsonian Institution. Phosphoric Acid Fuel Cells Fuel Cells. Web. 2004.