The chemical formula \(\text{H}_3\text{PO}_4\) represents a significant inorganic compound used across various industries and biological systems. It is a major mineral acid of industrial importance due to its versatility. In its most commonly encountered form, it is a colorless, odorless, and non-volatile syrupy liquid, typically sold as an 85% aqueous solution. It serves as a foundational component in the production of goods, from agricultural fertilizers to consumer beverages.
Determining the Chemical Name
The systematic name for \(\text{H}_3\text{PO}_4\) is Phosphoric Acid, which follows the standard rules of inorganic chemical nomenclature for oxyacids. To determine the correct name, chemists look to the polyatomic anion derived from the acid, which is the phosphate ion, \(\text{PO}_4^{3-}\).
The naming rule dictates that an acid derived from an anion ending in the suffix “-ate” will have its name changed to end in “-ic acid.” Since the \(\text{PO}_4^{3-}\) ion is called phosphate, the resulting acid name becomes Phosphoric Acid. The full, systematic name is technically orthophosphoric acid, where the “ortho-” prefix is used to distinguish it from related condensed forms, such as pyrophosphoric acid.
Fundamental Chemical Properties
Phosphoric Acid is classified as a moderate or weak mineral acid, distinguishing it from strong acids like hydrochloric acid (\(\text{HCl}\)) or sulfuric acid (\(\text{H}_2\text{SO}_4\)). Its molecular structure contains three acidic hydrogen atoms, making it a triprotic acid. This means it can donate up to three protons in an aqueous solution, which is key to its utility in buffering systems.
The donation of each successive proton occurs at a different level of acidity, measured by its acid dissociation constant (\(\text{pKa}\)). The first dissociation (\(\text{pKa}_1\)) is approximately 2.12, allowing it to neutralize bases effectively. The second and third dissociations are significantly weaker, with \(\text{pKa}_2\) at about 7.21 and \(\text{pKa}_3\) at 12.32. These varied \(\text{pKa}\) values mean the acid can exist in three different ionic forms—dihydrogen phosphate, hydrogen phosphate, and phosphate—depending on the solution’s pH.
Common Uses and Consumer Exposure
The versatility of Phosphoric Acid stems from its mild acidity and ability to form various salts called phosphates. The largest volume produced globally is used in agriculture for the creation of phosphate fertilizers, which support plant growth and crop yields. These fertilizers, such as diammonium phosphate (\(\text{DAP}\)) and monoammonium phosphate (\(\text{MAP}\)), provide the necessary phosphorus nutrient for plants.
In consumer products, the acid is widely recognized as an acidulant and flavoring agent in soft drinks, especially cola beverages. Food-grade Phosphoric Acid provides the sharp, tangy “bite” that balances the high sweetness of the sugars or artificial sweeteners. It also serves as a preservative by lowering the beverage’s pH, which inhibits the growth of mold and bacteria.
Beyond food, technical-grade Phosphoric Acid is used in industrial applications, including metal treatment. Its ability to convert iron rust (iron oxide) into a stable iron phosphate compound makes it an effective rust remover and a surface preparation agent for painting or coating metals. The acid also finds use in dental cements and in the production of various cleaning agents.