Phosphoric acid

This article is about orthophosphoric acid. For other acids commonly called "phosphoric acid", see Phosphoric acids and phosphates.

Phosphoric acid
IUPAC name trihydroxidooxidophosphorus phosphoric acid
Other names Orthophosphoric acid trihydroxylphosphine oxide
Identifiers
CAS number	7664-38-2 Y,  16271-20-8 (hemihydrate)
PubChem	1004
ChemSpider	979 Y
UNII	E4GA8884NN Y
EC number	231-633-2
UN number	1805
KEGG	D05467 Y
ChEBI	CHEBI:26078 Y
ChEMBL	CHEMBL1187 Y
RTECS number	TB6300000
Jmol-3D images	Image 1
SMILES OP(=O)(O)O
InChI InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) Y Key: NBIIXXVUZAFLBC-UHFFFAOYSA-N Y InChI=1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) Key: NBIIXXVUZAFLBC-UHFFFAOYAI
Properties
Molecular formula	H3PO4
Molar mass	97.995 g/mol
Appearance	white solid or colourless, viscous liquid (>42 °C)
Density	1.885 g/mL (liquid) 1.685 g/mL (85% solution) 2.030 g/mL (crystal at 25 °C)
Melting point	42.35 °C (anhydrous) 29.32 °C (hemihydrate)
Boiling point	158 °C (decomposition)
Solubility in water	5.48 g/mL
Acidity (pKa)	2.148, 7.198, 12.377
Viscosity	2.4–9.4 cP (85% aq. soln.) 147 cP (100%)
Thermochemistry
Std enthalpy of formation ΔfHo298	-1288 kJ·mol−1
Standard molar entropy So298	158 J·mol−1·K−1
Hazards
MSDS	ICSC 1008
EU Index	015-011-00-6
EU classification	Corrosive (C)
R-phrases	R34
S-phrases	(S1/2) S26 S45
NFPA 704	0 3 0
Flash point	Non-flammable
Related compounds
Related phosphorus oxoacids	Hypophosphorous acid Phosphorous acid Pyrophosphoric acid Triphosphoric acid Perphosphoric acid Permonophosphoric acid
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a mineral (inorganic) acid having the chemical formula H₃PO₄. 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. The term phosphoric acid can also refer to a chemical or reagent consisting of phosphoric acids, such as pyrophosphoric acid or triphosphoric acid, but usually orthophosphoric acid.

In addition to being a chemical reagent, phosphoric acid has a wide variety of uses, including as a rust inhibitor, food additive, dental and orthop(a)edic etchant, electrolyte, flux, dispersing agent, industrial etchant, fertilizer feedstock, and component of home cleaning products.

Chemistry

Pure anhydrous phosphoric acid is a white solid that melts at 42.35 °C to form a colorless, viscous liquid.

Most people and even chemists refer to orthophosphoric acid as phosphoric acid, which is the IUPAC name for this compound. The prefix ortho is used to distinguish the acid from other phosphoric acids, called polyphosphoric acids. Orthophosphoric acid is a non-toxic, inorganic, rather weak triprotic acid, which, when pure, is a solid at room temperature and pressure. The chemical structure of orthophosphoric acid is shown above in the data table. Orthophosphoric acid is a very polar molecule; therefore it is highly soluble in water. The oxidation state of phosphorus (P) in ortho- and other phosphoric acids is +5; the oxidation state of all the oxygen atoms (O) is −2 and all the hydrogen atoms (H) is +1. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H⁺ each time, which typically combines with a water molecule, H₂O, as shown in these reactions:

H₃PO_4(s)   + H₂O_(l) H₃O⁺_(aq) + H₂PO₄⁻_(aq)       K_a1= 7.25×10⁻³

H₂PO₄⁻_(aq)+ H₂O_(l) H₃O⁺_(aq) + HPO₄²⁻_(aq)       K_a2= 6.31×10⁻⁸

HPO₄²⁻_(aq)+ H₂O_(l) H₃O⁺_(aq) +  PO₄³⁻_(aq)        K_a3= 3.98×10⁻¹³

The anion after the first dissociation, H₂PO₄⁻, is the dihydrogen phosphate anion. The anion after the second dissociation, HPO₄²⁻, is the hydrogen phosphate anion. The anion after the third dissociation, PO₄³⁻, 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 H₂SO₄), H₃PO₄ reacts to form mystery products, perhaps corrosive, acidic salts of the hypothetical tetrahydroxylphosphonium ion, which is isoelectronic with orthosilicic acid. The suspected reaction with HSbF₆, for example, is supposed to go:

H₃PO₄ + HSbF₆ → [P(OH)₄⁺][SbF₆]⁻

Aqueous solution

For a given total acid concentration [A] = [H₃PO₄] + [H₂PO₄⁻] + [HPO₄²⁻] + [PO₄³⁻] ([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:

[A] (mol/L)	pH	[H3PO4]/[A] (%)	[H2PO4−]/[A] (%)	[HPO42−]/[A] (%)	[PO43−]/[A] (%)
1	1.08	91.7	8.29	6.20×10−6	1.60×10−17
10−1	1.62	76.1	23.9	6.20×10−5	5.55×10−16
10−2	2.25	43.1	56.9	6.20×10−4	2.33×10−14
10−3	3.05	10.6	89.3	6.20×10−3	1.48×10−12
10−4	4.01	1.30	98.6	6.19×10−2	1.34×10−10
10−5	5.00	0.133	99.3	0.612	1.30×10−8
10−6	5.97	1.34×10−2	94.5	5.50	1.11×10−6
10−7	6.74	1.80×10−3	74.5	25.5	3.02×10−5
10−10	7.00	8.24×10−4	61.7	38.3	8.18×10−5

For large 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 H₂PO₄⁻. For [A] below 10⁻³, the solution is mainly composed of H₂PO₄⁻ with [HPO₄²⁻] becoming non negligible for very dilute solutions. [PO₄³⁻] is always negligible. Note that the above analysis does not take into account ion activity coefficients; as such, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.

Preparation

Phosphoric acid can be prepared by three routes – the thermal process, the wet process and the dry kiln process.

Thermal

This very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide and dissolving the product in dilute phosphoric acid. This 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.

Wet

See also: Nitrophosphate process

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 CaSO₄·2H₂O + HX

where X may include OH, F, Cl, and Br

The initial phosphoric acid solution may contain 23–33% P₂O₅, but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54% P₂O₅. Further evaporation of water yields superphosphoric acid with a P₂O₅ concentration above 70%.

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.

Uses

As a reagent

Pure 75–85% aqueous solutions (the most common) are clear, colourless, odourless, non-volatile, rather viscous, syrupy liquids, but still pourable. Phosphoric acid is very commonly used as an aqueous solution of 85% phosphoric acid or H₃PO₄. Because it is a concentrated acid, an 85% solution can be corrosive, although nontoxic when diluted. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids in a temperature-dependent equilibrium, but, for the sake of labeling and simplicity, the 85% represents H₃PO₄ as if it were all orthophosphoric acid. Other percentages are possible too, even above 100%, where the phosphoric acids and water would be in an unspecified equilibrium, but the overall elemental mole content would be considered specified. When aqueous solutions of phosphoric acid and/or phosphate are dilute, they are in or will reach an equilibrium after a while where practically all the phosphoric/phosphate units are in the ortho- form.

Hydrogen halide production

Phosphoric acid reacts with halides to form the corresponding hydrogen halide gas (steamy fumes are observed on warming the reaction mixture). This is a common practice for the laboratory preparation of hydrogen halides.

NaCl(s)	+ H3PO4(l) → NaH2PO4(s)	+ HCl(g)
NaBr(s)	+ H3PO4(l) → NaH2PO4(s)	+ HBr(g)
NaI(s)	+ H3PO4(l) → NaH2PO4(s)	+ HI(g)

Rust removal

Phosphoric acid may be used as a "rust converter", by direct application to rusted iron, steel tools, or surfaces. The phosphoric acid converts reddish-brown iron(III) oxide, Fe₂O₃ (rust) to black ferric phosphate, FePO₄.

"Rust converter" is sometimes a greenish liquid suitable for dipping (in the same sort of acid bath as is used for pickling metal), but it is more often formulated as a gel, commonly called "naval jelly". It is sometimes sold under other names, such as "rust remover" or "rust killer". As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces.

After treatment, the black ferric-phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes).

Food additive

Food-grade phosphoric acid (additive E338) is used to acidify foods and beverages such as various colas, but not without controversy regarding its health effects.  It provides a tangy or sour taste, and being a mass-produced chemical is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive seasonings that give comparable flavors, such as citric acid which is obtainable from citrus, but usually fermented by Aspergillus niger mold from scrap molasses, waste starch hydrolysates and phosphoric acid.

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 other applications, phosphoric acid is used:

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

Biological effects

In soft drinks

Phosphoric acid, used in many soft drinks (primarily cola), has been linked to lower bone density in epidemiological studies. For example, a study using dual-energy X-ray absorptiometry rather than a questionnaire about breakage, provides reasonable evidence to support the theory that drinking cola results in lower bone density. This study was published in the American Journal of Clinical Nutrition. A total of 1672 women and 1148 men were studied between 1996 and 2001. Dietary information was collected using a food frequency questionnaire that had specific questions about the number of servings of cola and other carbonated beverages and that also made a differentiation between regular, caffeine-free, and diet drinks. The paper cites significant statistical evidence to show that women who consume cola daily have lower bone density. Total phosphorus intake was not significantly higher in daily cola consumers than in nonconsumers; however, the calcium-to-phosphorus ratios were lower.

On the other hand, another study suggests that insufficient intake of phosphorus leads to lower bone density. The study does not examine the effect of phosphoric acid, which binds with magnesium and calcium in the digestive tract to form salts that are not absorbed, but rather studies general phosphorus intake.

A clinical study by Heaney and Rafferty using calcium-balance methods found no impact of carbonated soft drinks containing phosphoric acid on calcium excretion. The study compared the impact of water, milk, and various soft drinks (two with caffeine and two without; two with phosphoric acid and two with citric acid) on the calcium balance of 20- to 40-year-old women who customarily consumed ~3 or more cups (680 mL) of a carbonated soft drink per day. They found that, relative to water, only milk and the two caffeine-containing soft drinks increased urinary calcium, and that the calcium loss associated with the caffeinated soft drink consumption was about equal to that previously found for caffeine alone. Phosphoric acid without caffeine had no impact on urine calcium, nor did it augment the urinary calcium loss related to caffeine. Because studies have shown that the effect of caffeine is compensated for by reduced calcium losses later in the day, Heaney and Rafferty concluded that the net effect of carbonated beverages—including those with caffeine and phosphoric acid—is negligible, and that the skeletal effects of carbonated soft drink consumption are likely due primarily to milk displacement.

Other chemicals such as caffeine (also a significant component of popular common cola drinks) were also suspected as possible contributors to low bone density, due to the known effect of caffeine on calciuria. One other study, involving 30 women over the course of a week, suggests that phosphoric acid in colas has no such effect, and postulates that caffeine has only a temporary effect, which is later reversed. The authors of this study conclude that the skeletal effects of carbonated beverage consumption are likely due primarily to milk displacement (another possible confounding factor may be an association between high soft drink consumption and sedentary lifestyle)^{[citation needed]}.

See also

• Phosphate fertilizers, such as ammonium phosphate fertilizers

External links

• International chemical safety card 1008
• National pollutant inventory – Phosphoric acid fact sheet
• NIOSH Pocket guide to chemical hazards
• Excel spreadsheet containing phosphoric acid titration curve, distribution diagram and buffer pH calculation