Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is an inorganic mineral acid having the chemical formula H₃PO₄. Alternatively, orthophosphoric acid molecules can combine with themselves to form a variety of compounds referred to as phosphoric acids in a more general way. For a discussion of these, see Phosphoric acids and Phosphates. This article covers orthophosphoric acid.

The term "phosphoric acid" can also refer to a chemical or reagent consisting of phosphoric acids, usually mostly orthophosphoric acid. This article briefly discusses this chemical/reagent.

Orthophosphoric acid chemistry

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

Most people and even chemists simply refer to orthophosphoric acid as "phosphoric acid", which is the IUPAC name for this compound. The prefix ortho- usually is used when one wants to distinguish it 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 has 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 oxygens (O) is -2 and all the hydrogens (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:

H₃PO₄ + H₂O → H₃O⁺ + H₂PO₄^- K_a1= 7.5 x 10⁻³

H₂PO₄^- + H₂O → H₃O⁺ + HPO₄^2- K_a2= 6.2 x 10⁻⁸

HPO₄^2- + H₂O → H₃O⁺ + PO₄^3- K_a3= 2.14 x 10⁻¹³

The anion after the first dissociation, H₂PO₄^-, is the dihydrogen phosphate anion. The anion after the second dissociation, HPO₄^-2, is the hydrogen phosphate anion. The anion after the third dissociation, PO₄^-3, 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 and it conjugate bases, (the phosphates mentioned above) cover a wide pH range, and because phosphoric acid/phosphate solutions are generally non-toxic, mixtures of these types of phosphates are often used for buffering 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 and 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 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.phosphoric acid. The Columbia Encyclopedia, Sixth Edition. 2001-05 Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or moisture-absorbing reagent. Further dehydrating can produce produce phosphoric anhydride 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-aborbing and is used as a dessicant.

Phosphoric acid as a Chemical/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 not toxic 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.

Rust removal

Phosphoric acid may be used by direct application to rusted iron or steel tools or surfaces to convert iron(III) oxide (rust) to a water soluble phosphate compound. It is usually available as a greenish liquid, suitable for dipping (acid bath), but is more generally used as a component in a gel, commonly called Naval jelly. As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces. Care must be taken to avoid acid burns of the skin and especially the eyes, but the residue is easily diluted with water. When sufficiently diluted it can even be nutritious to plant life, containing the essential nutrients phosphorus and iron. It is sometimes sold under other names, such as "rust remover" or "rust killer". It should not be directly introduced into surface water such as creeks or into drains, however. After treatment, the reddish-brown iron oxide will be converted to a black iron phosphate compound coating that may be scrubbed off. Multiple applications may be required to remove all rust. The resultant black compound can provide further corrosion resistance (such protection is somewhat provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes.) After application and removal of rust using phosphoric acid compounds, the metal should be oiled (if to be used bare, as in a tool) or appropriately painted, most durably by using a multiple coat process of primer, intermediate, and finish coats.

Processed food use

It is also used to acidify foods and beverages such as various colas, but not without controversy as to its health effects. It provides a tangy taste, and being a mass produced chemical, is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive natural seasonings that give comparable flavors, such as ginger for tangyness, or citric acid for sourness, obtainable from lemons and limes. (However most citric acid in the food industry is not extracted from citrus fruit, but fermented by Aspergillus niger mould from scrap molasses, waste starch hydrolysates and phosphoric acid.)

Biological effects on bone calcium

Traditionally, the bone deficiency disease osteoporosis (literally "porous bones") has been seen mostly in post-menopausal women, particularly those who did not build high bone density in youth, typically due to insufficient calcium intake. Some researchers have observed a positive correlation between soft-drink consumption and increased risk of osteoporosis in young women, which is now also seen in increasing prevalence in men of late middle age.

Phosphoric acid, used in many soft drinks (primarily so in cola drinks), was initially suspected. It has been claimed that an excess of phosphorus may lead to poor bone density. However medical research indicates that exactly the opposite is the case; as one might expect from the simple application of Le Châtelier's principle, it is low phosphate intake which increases the risk of osteoporosis, while high phosphate intake reduces it.Elmståhl S, Gullberg B, Janzon L, et al. Increased incidence of fractures in middle-aged and elderly men with low intakes of phosphorus and zinc. Osteoporos Int 1998;8:333–40.

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. However other studies have shown that not only does phosphoric acid in colas have no such effect, but the caffeine has only a temporary effect which is later reversed. The authors of this study suggest that any correlation between osteoporosis and soft drink consumption is probably due to displacement of dairy products from the diet.Robert P Heaney and Karen Rafferty, Carbonated beverages and urinary calcium excretion, American Journal of Clinical Nutrition, Vol. 74, No. 3, 343-347, September 2001. (Another possible confounding factor may be an association between high soft drink consumption and sedentary lifestyle.)

Medical use

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 which also contain high levels of sugar (glucose and fructose). It should not be used by diabetics without consultation with a doctor.

Preparation of phosphoric acid

There are two distinct kinds of phosphoric acid: Thermal phosphoric acid and Wet phosphoric acid.

Thermal phosphoric acid This very pure phosphoric acid is obtained by burning elemental phosphorus and reacting the product with water. This is the cleanest way of producing 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.

Wet phosphoric acid Green phosphoric acid is prepared by adding sulfuric acid to calcium phosphate rock. While phosphoric acid has the potential to release three hydrogen ions, in aqueous solution the third requires a high pH because PO₄³⁻ is almost as strong a base as hydroxide ion.

Through modern filtering techniques the wet process acid can be cleaned up significantly but still isn't as pure as thermal phosphoric acid.

Other applications

Used as the electrolyte in Phosphoric-acid fuel cells. Used as an external standard for phosphorus-31 NMR.

Phosphoric acid is also used as a cleaner by construction trades to remove mineral deposits, cementitous smears, and hard water stains.

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

Phosphoric acid is used as a flux by hobbyists (such as model railroaders) as an aid to soldering.

Phosphoric acid is also used in hydroponics PH solutions to lower the PH of nutrient solutions. While other types of acids can be used, phosphor is a nutrient used by plants, especially during flowering, making phosphoric acid particularly desirable. General Hydroponics' ph Down solution contains phosphoric acid.

External links

http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc10/icsc1008.htm International Chemical Safety Card 1008]
National Pollutant Inventory - Phosphoric acid fact sheet
NIOSH Pocket Guide to Chemical Hazards

References

Phosphates | Acids

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