Phosphoric acid

Reactions

Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way.

Anhydrous phosphoric acid, a white low melting solid, is obtained by dehydration of 85% phosphoric acid by heating under a vacuum.

Orthophosphoric acid ionizes upon dissolving in water, mainly to give H₂PO−
4 and protons:

H₃PO₄(s) + H₂O(l) ⇌ H₃O⁺(aq) + H₂PO−
4(aq)       K_a1= 7.5×10⁻³ H₂PO−
4(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO2−
4(aq)       K_a2= 6.2×10⁻⁸ HPO2−
4(aq) + H₂O(l) ⇌ H₃O⁺(aq) +  PO3−
4(aq)        K_a3= 2.2×10⁻¹³

The anion after the first dissociation, H₂PO−
4, is the dihydrogen phosphate anion. The anion after the second dissociation, HPO2−
4, is the hydrogen phosphate anion. The anion after the third dissociation, PO3−
4, is the phosphate or orthophosphate anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant, called K_a1, K_a2, and K_a3 given at 25 °C. Associated with these three dissociation constants are corresponding pK_a1=2.12, pK_a2=7.21, and pK_a3=12.67 values at 25 °C. Even though all three hydrogen (H) atoms are equivalent on an orthophosphoric acid molecule, the successive K_a values differ since it is energetically less favorable to lose another H⁺ if one (or more) has already been lost and the molecule/ion is more negatively charged.

Because the triprotic dissociation of orthophosphoric acid, the fact that its conjugate bases (the phosphates mentioned above) cover a wide pH range, and, because phosphoric acid/phosphate solutions are, in general, non-toxic, mixtures of these types of phosphates are often used as buffering agents or to make buffer solutions, where the desired pH depends on the proportions of the phosphates in the mixtures. Similarly, the non-toxic, anion salts of triprotic organic citric acid are also often used to make buffers. Phosphates are found pervasively in biology, especially in the compounds derived from phosphorylated sugars, such as DNA, RNA, and adenosine triphosphate (ATP). There is a separate article on phosphate as an anion or its salts.

Upon heating orthophosphoric acid, condensation of the phosphoric units can be induced by driving off the water formed from condensation. When one molecule of water has been removed for each two molecules of phosphoric acid, the result is pyrophosphoric acid (H₄P₂O₇). When an average of one molecule of water per phosphoric unit has been driven off, the resulting substance is a glassy solid having an empirical formula of HPO₃ and is called metaphosphoric acid. Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or moisture-absorbing reagent. Further dehydrating is very difficult, and can be accomplished only by means of an extremely strong desiccant (and not by heating alone). It produces phosphoric anhydride (phosphorus pentoxide), which has an empirical formula P₂O₅, although an actual molecule has a chemical formula of P₄O₁₀. Phosphoric anhydride is a solid, which is very strongly moisture-absorbing and is used as a desiccant.

In the presence of superacids (acids stronger than H
2SO
4), H
3PO
4 reacts to form poorly characterized products, perhaps corrosive, acidic salts of the hypothetical tetrahydroxylphosphonium ion, which is isoelectronic with orthosilicic acid. The suspected reaction with HSbF
6, for example, is supposed to go:

H₃PO₄ + {HSbF₆} → [P(OH)+
4] [SbF₆]⁻

Aqueous solution

For a given total acid concentration [A] = [H₃PO₄] + [H₂PO−
4] + [HPO2−
4] + [PO3−
4] ([A] is the total number of moles of pure H₃PO₄ which have been used to prepare 1 liter of solution), the composition of an aqueous solution of phosphoric acid can be calculated using the equilibrium equations associated with the three reactions described above together with the [H⁺] [OH⁻] = 10⁻¹⁴ relation and the electrical neutrality equation. Possible concentrations of polyphosphoric molecules and ions is neglected. The system may be reduced to a fifth degree equation for [H⁺] which can be solved numerically, yielding:

For strong acid concentrations, the solution is mainly composed of H₃PO₄. For [A] = 10⁻², the pH is close to pK_a1, giving an equimolar mixture of H₃PO₄ and H₂PO−
4. For [A] below 10⁻³, the solution is mainly composed of H₂PO−
4 with [HPO2−
4] becoming non-negligible for very dilute solutions. [PO3−
4] is always negligible. Since this analysis does not take into account ion activity coefficients, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.

Preparation

Phosphoric acid is produced industrially by two general routes – the thermal process and the wet process, which includes two sub-methods. The wet process dominates in the commercial sector. The more expensive thermal process produces a purer product that is used for applications in the food industry.

Wet

Wet process phosphoric acid is prepared by adding sulfuric acid to tricalcium phosphate rock, typically found in nature as apatite. The reaction is:

Ca₅(PO₄)₃X + 5 H₂SO₄ + 10 H₂O → 3 H₃PO₄ + 5 CaSO₄·2 H₂O + HX where X may include OH, F, Cl, and Br

The initial phosphoric acid solution may contain 23–33% P₂O₅ (32–46% H₃PO₄), but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P₂O₅ (75–85% H₃PO₄). Further evaporation of water yields superphosphoric acid with a P₂O₅ concentration above 70% (corresponding to nearly 100% H₃PO₄; however, pyrophosphoric and polyphosphoric acids will start to form, making the liquid highly viscous).

Digestion of the phosphate ore using sulfuric acid yields the insoluble calcium sulfate (gypsum), which is filtered and removed as phosphogypsum. Wet-process acid can be further purified by removing fluorine to produce animal-grade phosphoric acid, or by solvent extraction and arsenic removal to produce food-grade phosphoric acid.

The nitrophosphate process is similar to the wet process except that it uses nitric acid in place of sulfuric acid. The advantage to this route is that the coproduct, calcium nitrate is also a plant fertilizer. This method is rarely employed.

Thermal

Very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide, which is subsequently dissolved in dilute phosphoric acid. This route produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting phosphorus from the rock in a furnace. The end result is food-grade, thermal phosphoric acid; however, for critical applications, additional processing to remove arsenic compounds may be needed.

Elemental phosphorus is produced by an electric furnace. At a high temperature, a mixture of phosphate ore, silica and carbonaceous material (coke, coal etc...) produces calcium silicate, phosphorus gas and carbon monoxide. The P and CO off-gases from this reaction are cooled under water to isolate solid phosphorus. Alternatively, the P and CO off-gases can be burned with air to produce phosphorus pentoxide and carbon dioxide.

Laboratory routes

A demonstrative process consists in the oxidation of red phosphorus by nitric acid.

¹/_n P_n + 5 HNO₃ → H₂O + H₃PO₄ + 5 NO₂

Uses

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

Food additive

Food-grade phosphoric acid (additive E338) is used to acidify foods and beverages such as various colas and jams. It provides a tangy or sour taste. Various salts of phosphoric acid, such as monocalcium phosphate, are used as leavening agents.

Rust removal

Phosphoric acid may be used to remove rust by direct application to rusted iron, steel tools, or other surfaces. The phosphoric acid changes the reddish-brown iron(III) oxide, Fe₂O₃ (rust) to ferric phosphate, FePO₄. An empirical formula for this reaction is:

2 H₃PO₄ + Fe₂O₃ → 2 FePO₄ + 3 H₂O

Liquid phosphoric acid may be used for dipping, but phosphoric acid for rust removal is more often formulated as a gel. As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces. Different phosphoric acid gel formulations are sold as "rust removers" or "rust killers". Multiple applications of phosphoric acid may be required to remove all rust. Rust may also be removed via phosphate conversion coating. This process can leave a black phosphate coating that provides moderate corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes).

In medicine

Phosphoric acid is used in dentistry and orthodontics as an etching solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed. Phosphoric acid is also an ingredient in over-the-counter anti-nausea medications that also contain high levels of sugar (glucose and fructose). This acid is also used in many teeth whiteners to eliminate plaque that may be on the teeth before application.

Other applications

Among many applications, phosphoric acid is used:

As a solution for anodizing

As an external standard for phosphorus-31 Nuclear magnetic resonance (NMR).

As a buffer agent in biology and chemistry; For example, a buffer for high-performance liquid chromatography.

As a chemical oxidizing agent for activated carbon production, as used in the Wentworth Process.

As the electrolyte in phosphoric acid fuel cells.

With distilled water (2–3 drops per gallon) as an electrolyte in oxyhydrogen generators.

As a catalyst in the hydration of alkenes to produce alcohols, predominantly ethanol.

As an electrolyte in copper electropolishing for burr removal and circuit board planarization.

As a flux by hobbyists (such as model railroaders) as an aid to soldering.

In compound semiconductor processing, phosphoric acid is a common wet etching agent: for example, in combination with hydrogen peroxide and water it is used to etch InGaAs selective to InP.

Heated in microfabrication to etch silicon nitride (Si₃N₄). It is highly selective in etching Si₃N₄ instead of SiO₂, silicon dioxide.

As a cleaner by construction trades to remove mineral deposits, cementitious smears, and hard water stains.

As a chelant in some household cleaners aimed at similar cleaning tasks.

In hydroponics pH solutions to lower the pH of nutrient solutions. While other types of acids can be used, phosphorus is a nutrient used by plants, especially during flowering, making phosphoric acid particularly desirable.

As a pH adjuster in cosmetics and skin-care products.

As a dispersing agent in detergents and leather treatment.

As an additive to stabilize acidic aqueous solutions within a wanted and specified pH range.

As a sanitizing agent in the dairy, food, and brewing industries.

Exposure

The exact dangers of phosphoric acid, H₃PO₄, depend on the concentration strength of the solution, with higher concentrations presenting greater hazards. Phosphoric acid, 85 wt.% is considered a corrosive chemical solution that can cause severe skin burns and permanent eye damage. Inhalation and ingestion can also have serious effects. See exposure symptoms below:

Eye exposure symptoms include: pain, redness, swelling, and blurred vision. Permanent eye damage including blindness can occur.

Skin exposure symptoms include: pain, redness, burns, blisters, and scarring of tissue. A serious skin exposure to high-strength phosphoric acid can even cause death.

Inhalation exposure symptoms include: cough, burning sensation, sore throat, and shortness of breath.

Ingestion exposure symptoms include: burning of lips, tongue, throat, and stomach, abdominal pain, shock or collapse.

Toxicology

Phosphoric acid is not a known carcinogen, mutagen, nor is it known to exhibit toxicity towards a growing embryo or the reproductive systems.

Incompatibilities

Strong bases (e.g. sodium hydroxide), strong oxidizers (e.g. perchloric acid), and certain metals (e.g. aluminum). Violent reactions and splattering of dangerous chemicals can occur upon mixing H₃PO₄ with a strong base. There is an increased risk of fire or causing an explosion from adding strong oxidizers to phosphoric acid. Phosphoric acid will attack many metals and drive the formation of highly flammable and potentially explosive hydrogen gas. If flammable combustion occurs, toxic fumes of phosphorous oxides will form.

In the Household

Examples of household products that contain or can contain H₃PO₄ are: cleaners for toilet bowls, grout, tile, automobile rims, some general purpose cleaners/degreasers; some hair/beauty products; and some toothpastes and mouth rinses may contain phosphoric acid. These examples, while the amount of H₃PO₄ may be dilute, still present potential health hazards and should always be handled safely. Additionally, never mix chemicals at home unless fully aware of the occurring chemical reaction, hazards, and safe-handling precautions necessary to remain safe. In example, the mixing of household bleach and phosphoric acid from a toilet bowl cleaner can cause the release of highly toxic chlorine gas.

Other Considerations

Phosphoric acid is not combustible, nor flammable, and will not burn in itself. Flammability of the chemical exists only from incompatible reactions with other compounds/materials. Decomposition of H₃PO₄ to toxic fumes readily occurs upon mixing with: alcohols, aldehydes, cyanides, ketones, phenols, esters, sulfides, and halogenated organics.

Handling

Personal protective equipment recommended for handling of H₃PO₄, 85 wt.%:

Eye/Face Protection: wear tight-fitting government approved safety goggles (NIOSH or EN 166).

Skin Protection: Nitrile rubber gloves are recommended for all handling conditions. Inspect gloves prior to usage for tears, holes, weak spots. Contaminated gloves should be disposed of according to all laws and regulations.

Body Protection: complete chemical-protective suit recommendation is dependent upon concentration and amount of phosphoric acid to be handled.

Respiratory Protection: use full-face air-purifying respirators with multi-purpose or type ABEK respirator cartridges as a backup to the engineering controls and when proper risk assessment recommends its usage.

To prevent exposure, the CDC recommends taking serious precautions to prevent the generation of mists, avoiding potentially hazardous exposure to eyes, skin, and nasal passageways. The CDC also recommends no eating, drinking, or smoking when handling phosphoric acid to avoid the possibility of ingestion.

Storage

Store H₃PO₄ in containers made of corrosion-resistant materials. Sigma-Aldrich recommends storing phosphoric acid in a stainless steel container with a resistant inner lining. Containers should always be stored upright to avoid leaks. Do not store in incompatible metals such as aluminum, the alloys of aluminum, and carbon steel. Store away from all incompatible chemicals and compounds. Keep away from food and feedstuffs. Ensure any solid material is kept dry. Do not reuse containers, even if they appear clean and empty.