Phosphoric acid · Lablypedia

The compound is an acid. Removal of all three H⁺ ions gives the phosphate ion PO3−4. Removal of one or two protons gives dihydrogen phosphate ion H₂PO−4, and the hydrogen phosphate ion HPO2−4, respectively. Phosphoric acid forms esters, called organophosphates.

The name "orthophosphoric acid" can be used to distinguish this specific acid from other "phosphoric acids", such as pyrophosphoric acid. Nevertheless, the term "phosphoric acid" often means this specific compound; and that is the current IUPAC nomenclature.

Production

Phosphoric acid is produced industrially by one of two routes, wet processes and dry.

Wet process

In the wet process, phosphate-containing minerals such as calcium hydroxyapatite or fluorapatite are treated with sulfuric acid.

Ca₅(PO₄)₃OH + 5 H₂SO₄ → 3 H₃PO₄ + 5 CaSO₄ + H₂O

Ca₅(PO₄)₃F + 5 H₂SO₄ → 3 H₃PO₄ + 5 CaSO₄ + HF

By-products include calcium sulfate (CaSO₄) and hydrogen fluoride (HF). The HF gas may be recovered by streaming it into a wet (water) scrubber producing hydrofluoric acid. CaSO₄ is better known as gypsum, which is commonly used in the construction industry, however the CaSO₄ produced from phosphoric acid production can contain trace levels of radioactive elements such as radium. This makes it unsuitable for commercial use, and it is called phosphogypsum to distinguish it. It is typically stored indefinitely.

In both cases the phosphoric acid solution usually contains 23–33% P₂O₅ (32–46% H₃PO₄). It may be concentrated to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P₂O₅ (75–85% H₃PO₄). Further removal of water yields superphosphoric acid with a P₂O₅ concentration above 70% (corresponding to nearly 100% H₃PO₄). The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.

Dry process

To produce food-grade phosphoric acid, phosphate ore is first reduced with coke in an electric arc furnace, to give elemental phosphorus. This process is also known as the thermal process or the electric furnace process. Silica is also added, resulting in the production of calcium silicate slag. Elemental phosphorus is distilled out of the furnace and burned with air to produce high-purity phosphorus pentoxide, which is dissolved in water to make phosphoric acid. The thermal process produces phosphoric acid with a very high concentration of P₂O₅ (about 85%) and a low level of impurities.

However, this process is more expensive and energy-intensive than the wet process, which produces phosphoric acid with a lower concentration of P₂O₅ (about 26–52%) and a higher level of impurities. The wet process is the most common method of producing phosphoric acid for fertilizer use. Even in China, where the thermal process is still used quite widely due to relatively cheap coal as opposed to the sulfuric acid, over 7/8 of phosphoric acid is produced with wet process.

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Purification

Phosphoric acid produced from phosphate rock or thermal processes often requires purification. A common purification method is liquid–liquid extraction, which involves the separation of phosphoric acid from water and other impurities using organic solvents, such as tributyl phosphate (TBP), methyl isobutyl ketone (MIBK), or n-octanol. Nanofiltration involves the use of a premodified nanofiltration membrane, which is functionalized by a deposit of a high molecular weight polycationic polymer of polyethyleneimines. Nanofiltration has been shown to significantly reduce the concentrations of various impurities, including cadmium, aluminum, iron, and rare earth elements. The laboratory and industrial pilot scale results showed that this process allows the production of food-grade phosphoric acid.

Fractional crystallization can achieve higher purities typically used for semiconductor applications. Usually a static crystallizer is used. A static crystallizer uses vertical plates, which are suspended in the molten feed and which are alternatingly cooled and heated by a heat transfer medium. The process begins with the slow cooling of the heat transfer medium below the freezing point of the stagnant melt. This cooling causes a layer of crystals to grow on the plates. Impurities are rejected from the growing crystals and are concentrated in the remaining melt. After the desired fraction has been crystallized, the remaining melt is drained from the crystallizer. The purer crystalline layer remains adhered to the plates. In a subsequent step, the plates are heated again to liquify the crystals and the purified phosphoric acid drained into the product vessel. The crystallizer is filled with feed again and the next cooling cycle is started.

Properties

Acidic properties

In aqueous solution phosphoric acid behaves as a triprotic acid.

H₃PO₄ ⇌ H₂PO−4 + H⁺, pK_a1 = 2.14

H₂PO−4 ⇌ HPO2−4 + H⁺, pK_a2 = 7.20

HPO2−4 ⇌ PO3−4 + H⁺, pK_a3 = 12.37

The difference between successive pK_a values is sufficiently large so that salts of either monohydrogen phosphate, HPO2−4 or dihydrogen phosphate, H₂PO−4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK_a values.

Aqueous solutions

Aqueous solutions up to 62.5% H₃PO₄ are eutectic, exhibiting freezing-point depression as low as −85 °C. When the concentration of acid rises above 62.5% the freezing-point increases, reaching 21 °C by 85% H₃PO₄ (w/w; the monohydrate). Beyond this the phase diagram becomes complicated, with significant local maxima and minima. For this reason phosphoric acid is rarely sold above 85%, as beyond this adding or removing small amounts of moisture risks the entire mass freezing solid, which would be a major problem on a large scale. There exists a local maximum at 91.6% which corresponds to the hemihydrate 2H₃PO₄•H₂O, freezing at 29.32 °C. There is a second smaller eutectic depression at a concentration of 94.75% with a freezing point of 23.5 °C. At higher concentrations the freezing point rapidly increases. Concentrated phosphoric acid tends to supercool before crystallization occurs, and may be relatively resistant to crystallisation even when stored below the freezing point.

Self condensation

Phosphoric acid is commercially available as aqueous solutions of various concentrations, not usually exceeding 85%. If concentrated further it undergoes slow self-condensation, forming an equilibrium with pyrophosphoric acid:

2 H₃PO₄ ⇌ H₂O + H₄P₂O₇

Even at 90% concentration the amount of pyrophosphoric acid present is negligible, but beyond 95% it starts to increase, reaching 15% at what would have otherwise been 100% orthophosphoric acid.

As the concentration is increased higher acids are formed, culminating in the formation of polyphosphoric acids. It is not possible to fully dehydrate phosphoric acid to phosphorus pentoxide, instead the polyphosphoric acid becomes increasingly polymeric and viscous. Due to the self-condensation, pure orthophosphoric acid can only be obtained by a careful fractional freezing/melting process.

Uses

The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production.

The remaining 10% is primarily used in soaps and detergents:

Food-grade phosphoric acid (additive E338) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. The phosphoric acid also serves as a preservative. Soft drinks containing phosphoric acid, which would include Coca-Cola, are sometimes called phosphate sodas or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion. Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.

Specific applications of phosphoric acid include:

in anti-rust (iron and steel oxidation) treatment by phosphate conversion coating or passivation, including the Parkerization process
as an external standard for phosphorus-31 nuclear magnetic resonance
in phosphoric acid fuel cells
in activated carbon production
in compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide
in microfabrication to etch silicon nitride selectively with respect to silicon dioxide
in microfabrication to etch aluminium
as a pH adjuster in cosmetics and skin-care products
as a sanitizing agent in the dairy, food, and brewing industries
in chemical polishing (etching) of metals like aluminium

Safety

Phosphoric acid is not a strong acid. However, at moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage.