What Is Phosphoric Acid?

by Kate Onissiphorou

Phosphoric acid is a colourless, odourless and crystalline liquid that contains the element phosphorus, and is found in many sugary carbonated beverages or soft drinks.

As well as contributing to the tangy flavour, it stops the reproduction of bacteria and mould that may contaminate the beverage during manufacturing, packaging, transportation, and storage.

Phosphoric acid can be a source of dietary phosphorus, which, as we discuss below, has several important functions in the body.

What is phosphoric acid used for?

In addition to drinks, phosphoric acid is also added to many processed foods as a flavouring and preservative. As an ion or component of other compounds, phosphorus plays an important role in the body. For example, it’s the main component of the energy-carrying molecules ATP and ADP. ReAgent's Phosphoric Acid

Phosphorus also works with calcium to help build healthy bones and teeth, and is essential in the production of DNA and RNA. Both of these genetic molecules have sugar-phosphate backbones.

Despite being essential to many biological functions, some studies suggest that too much phosphoric acid can lead to various medical problems. Potential complications can include a weakening of the bones (osteoporosis) and an increased risk of heart-related ailments.

What is phosphoric acid used for in food?

Most species of known bacteria grow at an ideal pH of between 6.5 and 7.0 on the pH scale

Adding non-toxic acids to beverages and food products can lower the pH to 2.5, thereby reducing the chance of bacteria growing in sugar-rich drinks and food.

 Phosphoric acid is a good additive in soft drinks and some processed foods because it’s inexpensive and non-toxic. It also adds a tangy sour flavour, similar to that of citric acid in fruits. 

What is the pH of phosphoric acid?

Just like other acids, the pH of a phosphoric acid solution is directly proportional to the concentration of the solute. It also varies depending on other factors such as temperature and the types of ions that dissociate when the acidic substance is dissolved. For example, at 0.1 N aqueous solution, phosphoric acid has a pH of 1.5.

However, this only tells you the concentration of the hydronium ions at a logarithmic scale; it does not indicate the type of ions and their relative dissociations. It’s important to note that phosphoric acid is a triprotic acid, meaning it has three acidic protons with corresponding pKa values:  The three dissociation reactions of phosphoric acid

As you can see, the pKa values of the three acidic protons become progressively higher as more protons are donated.

Formula of phosphoric acid

Pure phosphoric acid has the chemical formula H3PO4. The hydrogens form three different sets of ions per molecule when dissolved in water with corresponding conjugate ions. An illustration of the phosphoric acid formula

Phosphoric acid can be manufactured industrially through either the wet process or the dry (electric furnace) process.

In the wet process, phosphoric acid is produced via the reaction between sulphuric acid and a phosphate-containing mineral. For example, when calcium hydroxyapatite reacts with sulphuric acid, the balanced chemical equation is:

Ca5(PO4)3OH + 5 H2SO4 + 5 CaSO4↓ + H2O

The dry process, however, involves the use of an electric furnace. Firstly, phosphate ore is reduced using coke to produce elemental phosphorus. Silicon is also added to produce calcium silicate slag. The elemental phosphorus can then be distilled out of the furnace, before being exposed to air and burned to obtain highly pure phosphorus pentoxide. This is then dissolved in water to produce phosphoric acid. Further purification processes may be performed to remove toxic impurities from the solution.

Is phosphoric acid toxic to humans?

In soft drinks like cola, phosphoric acid only appears in a very low concentration of between 0.26% and 0.30%. Although it has some health effects, it is not toxic at this level. Phosphoric acid only becomes toxic at a concentration of 10% and may start to irritate the skin at 25% concentration.

Cola being poured into a glass filled with ice

Much like calcium, the main problem with phosphorus is that it can accumulate in the body. If there are excessive or imbalanced amounts in proportion to calcium, phosphorus can have long term toxic effects.

How do you treat a phosphoric acid burn?

Highly concentrated phosphoric acid can burn the skin and may cause serious injuries. It’s therefore important to take all necessary precautions when handling this chemical at high concentrations. This means using goggles and gloves, as well as a laboratory apron or gown. 

If your skin is exposed to highly concentrated phosphoric acid, immediately flush the affected area with lukewarm water for at least 30 minutes or until proper medical treatment is applied.


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