What Is Orthophosphoric Acid?

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  • Written By: Douglas Bonderud
  • Edited By: Michelle Arevalo
  • Last Modified Date: 03 February 2020
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Orthophosphoric acid is an inorganic acid. It is also known as phosphoric acid. Technically, the term phosphoric acid refers to a broad variety of phosphorus-based acids, but it is often used to refer specifically to orthophosphoric acid by both scientists and laymen alike. The chemical structure is H3PO4, but these molecules can be re-combined to produce a number of of compounds. Any derivatives of this acid are also referred to as phosphoric acids.

The pure form of this acid is solid and white at room temperature. At 108.23°F (42.35°C), orthophosphoric acid will melt, resulting in a viscous, colorless liquid. At room temperature, this compound is typically anhydrous, which means it does not contain water. It has a polar molecular structure, however, which means that it is extremely soluble in water.

As a chemical reagent, this acid is often converted into aqueous, or water-containing, solutions. These can have a potentiometric hydrogen ion concentrations (pH) ranging from 1.08 to 7.00, depending on the amount of acid present. An 85% solution of the acid is corrosive, but can be made non-toxic by way of dilution.


One of the most common uses of this acid is as a food additive. In Europe, it is identified by the food additive code E338. It is primarily used to acidify foods and beverages, most notably cola soft drinks. Both Pepsi® and Coca-Cola® use orthophosphoric acid to give their drinks a slightly sour taste. This acid is mass-produced at a low cost, making it the second-most common choice for this process. Citric acid, which produces the same flavor, is typically first.

Orthophosphoric acid has been connected to a lack of bone density in several studies. One study, conducted from 1996 to 2001 and published in the American Journal of Clinical Nutrition, showed a statistically significant loss of bone density in women who consumed cola daily. Another study, funded by the Pepsi® Corporation, made the assertion that a lack of phosphorus in the body could lead to lower bone density.

Further investigation led to assertions that caffeine, not phosphoric acid, was responsible for the observed bone density loss. A 2001 study, using women who drank three cups of cola per day, found that any bone density loss was likely due to the displacement of milk in the body, rather than as a result of either phosphorus or caffeine consumption. This acid has also been linked to kidney stones and chronic kidney disease. Research again points to cola consumption as the culprit, but the exact mechanism of action and the extent to which orthophosphoric acid is involved in unknown.


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