Phosphoric Acid H3PO4 Structure Properties and Reactions

Phosphoric acid also called orthophosphoric acid is a weak acid with the chemical formulation H₃PO₄. Orthophosphoric acid is referred to as phosphoric acid, which is the IUPAC term for this compound. The prefix ortho- is used to differentiate the acid from linked phosphoric acids, known as polyphosphoric acids. Orthophosphoric acid is a non-toxic acid in nature, which, when pure, is a solid at room temperature and pressure. The conjugate base of phosphoric acid is the dihydrogen phosphate ion, H₂PO^-4, which in turn contain a conjugate base of hydrogen phosphate, HPO₂^-4, which also contain a conjugate base of phosphate, PO₃^-4. Phosphates are vital for life.

The most common form of phosphoric acid is an 85% liquid solution; these solutions are colorless, nonvolatile and odorless. The 85% solution is a thick liquid, but still transferable. Even though phosphoric acid does not meet the strict meaning of a strong acid, the 85% solution is acidic enough to be harsh.

Physical Properties: Pure phosphoric acid is a white crystal-like solid with a melting point of 42.35° C. When it is less dense, it is a colorless, viscous liquid, odorless with a density of 1.885 g/mL. It is non-toxic and non-volatile in nature. The most commonly used phosphoric acid concentration is 85% in H₂O water.

Chemical Properties: Phosphoric acid has three acidic and replaceable H atoms. Therefore, it reacts in a different way from other mineral acids. It can react with bases to produce three classes of salts by the substitution of one, two, or three hydrogen atoms, such as Na₂HPO₄, NaH₂PO₄, and Na₃PO₄, separately.

At high temperatures, phosphoric acid molecules can react together and combine to produce dimers, trimmers, and even long polymeric chains or series like metaphosphoric acids and polyphosphoric acids

2H₃PO₄ → H₄P₂O₇ (anhydride of phosphoric acid)

Manufacturing

Phosphoric acid is manufactured industrially in two general ways. 

Fluoroapatite is a substitute feedstock, in which case fluoride is removed as an insoluble compound Na2SiF6. The phosphoric acid solution typically contains 25–35% P₂O₅ (32–46% H3PO4). It can be concentrated to make commercial grade phosphoric acid, which has about 55–63% P₂O₅ (76–86% H₃PO₄). Further elimination of water produces super phosphoric acid with a P₂O₅ concentration of above 80% (equivalent to nearly 100% H₃PO₄). Calcium sulfate (gypsum) is formed as a by-product and is removed in the form of phosphogypsum.

The phosphoric acid from both procedures can be further purified by eliminating compounds of arsenic and other possibly toxic impurities.

Wet Process

Phosphoric acid is manufactured from fluorapatite, called phosphate rock, 3Ca₃(PO₄)₂. CaF₂, by the adding of concentrated (95%) sulfuric acid in a chain of well-stirred reactors. This results in calcium sulfate (gypsum) and phosphoric acid plus other insoluble impurities. water is added, and the gypsum is eliminated by filtration along with other insoluble substances (e.g. silica). Fluoride, as H₂SiF₆, is eliminated at a further stage by evaporation.

Although the reaction occurs in stages including calcium dihydrogen phosphate, the overall reaction can be written as:

On the other hand, there are side reactions; for instance, with calcium carbonate and calcium fluoride present in the rock:

Fluorosilicic acid is a vital by-product from this and from the production of hydrogen fluoride. It may be neutralized with sodium hydroxide to produce sodium hex fluorosilicate. The acid is also used to produce aluminum fluoride, used in turn in the production of aluminum.

The rock crystal structure of the calcium sulfate formation depends on the conditions of the reaction. At 345-355 K, the principal yield is dihydrate, CaSO₄.2H₂O. At 368-388 K, the hemihydrate is formed, CaSO₄.1/2H₂O.

Calcium sulfate is strained off and the acid is then concentrated to about 56% P₂O₅ using vacuum distillation.

The yield from the 'wet process' acid is contaminated but can be used, without additional purification, for fertilizer production. Instead it can be evaporated further to 70% P₂O₅, a solution known as super phosphoric acid which is used straight as a liquid fertilizer.

To produce industrial phosphates, the acid is filtered by solvent extraction, for instance, methyl isobutyl ketone (MIBK) in which the acid is somewhat soluble and concentrated to give 68% P₂O₅ content. This acid can be further purified using solvents to extract it from heavy metals and defluorinated (by vaporization) to create a product of food-grade quality.

Thermal Process

The raw materials for this procedure are air and phosphorous:

Originally, phosphorus is sprayed into the heater and is burnt in the air for about 1850-3050 K.

Most methods use moist air, and several involve the addition of vapor to the phosphorus flame to yield and preserve a film of compressed polyphosphoric acids which defend the stainless-steel burner tower. The products from the burner tower travel directly into a hydration tower (water is used) where the gassy phosphorus oxide is absorbed in reprocessed as phosphoric acid:

Phosphorus may be burnt in dry air. The phosphorus pentoxide is condensed as a white powder and distinctly hydrated to phosphoric acid. This technique allows heat to be recuperated and reused. Burning and direct hydration, as before defined, makes highly corrosive environments. The apparatus is made from stainless steel or is carbon brick-lined. To decrease corrosion, the walls of the burner and hydrator towers are cooled with water, but the reactor yields emerge at a temperature too low for useful heat retrieval. Yield acid has a concentration of 85%. tetraphosphoric acid, one of a group of polyphosphoric acids which can be selectively manufactured, is formed either by boiling off the water at high temperatures in a carbon container or by adding solid phosphorus pentoxide to nearly boiling phosphoric acid. The first technique usually gives a purer yield, due to the high arsenic concentration of phosphorus pentoxide.

Phosphates

The salts of phosphoric acid are compounds that are broadly used in agriculture, industry, and in domestic use.

1.  Ammonium Phosphates

diammonium hydrogen phosphate and mono ammonium dihydrogen phosphate and are much used as fertilizers and are prepared by mixing the correct quantity of phosphoric acid with anhydrous ammonia in a revolving drum. The selection of which ammonium phosphate to use relies on the amount of nitrogen and phosphorus required for the crop.

2.  Calcium Phosphates

Calcium phosphates are used widely as fertilizers. Calcium dihydrogen phosphate, Ca(H₂PO₄)₂, is manufactured by the reaction of sulfuric acid with phosphate rock:

This is called superphosphate. It contains about 20% P₂O₅. If phosphate crystal is reacted with phosphoric acid, other than sulfuric acid, a more intense form of calcium dihydrogen phosphate is made with a general higher P₂O₅ level (55%):

This Is called triple superphosphate. The developed level of phosphate is attained because the yield is no longer diluted with calcium sulfate.

(c)  Sodium Phosphates

Sodium phosphates are manufactured by treating phosphoric acid and a concentrated solution of sodium hydroxide in suitable (stoichiometric) quantities. The yield crystallizes out.

• Monosodium dihydrogen phosphate (MSP, NaH₂PO₄) is used in metal washing and surface formulations, as a foundation of phosphate in pharmaceutical production, and as a pH control agent in toothpaste, in glassy enamel coating (sanitary ware), and in the production of starch phosphates. One of the main uses is as a plumb solvency handling in drinking water. Also, phosphoric acid may be used to yield a thin insoluble coating of lead phosphate on lead pipes to stop the dissolution of the lead by the acids present in water.

• Disodium hydrogen phosphate (Na₂HPO₄) is also used as a softening agent in treated cheese, in enamels and ceramic glazes, in leather toasting, in dye production, and as a corrosion inhibitor in water treatment.

• Trisodium phosphate (Na₃PO₄) is used in heavy-duty cleaners, for instance in degreasing steel. It is an alkali and appropriate for calcium ions, keeping them in solution and preventing the development of scum.

• Disodium pyrophosphate (Na₂H₂P₂O₇) is used as a leavening agent in bread and cakes it helps the discharge of carbon dioxide from baking soda, as an iron oxide darkening or browning effect in the production of numerous foods and as a dispersant in oil-well boring mud.

• Food-grade phosphoric acid (preservative E338) is used to acidify foods and drinks like numerous colas and jams. It delivers a tangy or sour taste. Phosphoric acid in soft drinks contains the potential to cause dental erosion. Phosphoric acid also has the possibility to contribute to the development of kidney stones, particularly in those who have had kidney stones earlier.

Specific Applications of Phosphoric Acid include

• In anti-rust action by phosphate conversion coating

• As an outside typical for phosphorus-31 nuclear magnetic resonance NMR.

• In phosphoric acid energy cells.

• Inactivated carbon manufacture. 

• In compound semiconductor treating, etch Indium gallium arsenide selectively with detail to indium phosphide.

• In microfabrication to etch silicon nitride selectively with detail to silicon dioxide.

• As a pH adjuster in cosmetics and skin-care goods.

• As a sanitizing agent in dairy, food, and brewing productions.

Health Hazards/ Health Effects: Phosphoric acid is not well-thought-out toxic or hazardous. In little concentrations, it is safe on the skin and even for intake (it is used in cosmetics, food, and dental products). On the other hand, at very high concentrations, it is harsh and can produce skin burns.

Quick Summary

This was all about Phosphoric acid, its properties, uses, and its applications. For more such information, access free resources available on the Vedantu website useful for the state board, CBSE, ICSE, and competitive examinations. All NCERT Solutions for all subjects are available on the Vedantu website.