About Phosphoric Acid
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.
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.
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
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FAQs on Phosphoric Acid
1. What are the most common uses of phosphoric acid in everyday products?
Phosphoric acid has several important applications across different industries. The most significant uses include:
- Fertilizers: A vast majority of phosphoric acid is used to produce phosphate fertilizers, which are essential for agriculture.
- Food and Beverages: It acts as a food additive to provide a tangy or sour taste, especially in soft drinks like colas. It also serves as an acidity regulator and preservative.
- Rust Removal: It is effective in removing rust by converting iron oxide (rust) into a water-soluble phosphate compound.
- Detergents and Soaps: It is used to produce phosphates that act as water softeners in detergents, making them more effective.
2. What is the chemical formula and structure of phosphoric acid?
The chemical formula for phosphoric acid is H₃PO₄. Its structure consists of a central phosphorus (P) atom. This central atom is bonded to four oxygen (O) atoms in a tetrahedral arrangement. One oxygen atom is linked by a double bond, while the other three are single-bonded and are attached to a hydrogen (H) atom, forming hydroxyl (-OH) groups.
3. What happens when phosphoric acid is mixed with water?
When phosphoric acid is mixed with water, it behaves as a weak acid. It does not completely break apart or ionise. Instead, it partially dissociates by donating one of its hydrogen ions (H⁺) to a water molecule (H₂O). This reaction forms a hydronium ion (H₃O⁺) and a dihydrogen phosphate ion (H₂PO₄⁻). Because it has three hydrogen ions, this process can happen in three successive steps.
4. Why is phosphoric acid used as an additive in soft drinks like cola?
In soft drinks, phosphoric acid serves two main purposes. Firstly, it provides a distinct sharp and tangy flavour that balances the sweetness. Secondly, it acts as a preservative by creating an acidic environment that slows down the growth of mould and bacteria, helping to extend the shelf life of the beverage.
5. Is phosphoric acid considered a strong or weak acid, and why?
Phosphoric acid is considered a weak acid. This is because it does not fully ionise when dissolved in water. Unlike a strong acid, which releases all its hydrogen ions, only a small fraction of H₃PO₄ molecules donate a proton at any given moment. This limited dissociation is what defines it as a weak acid.
6. How do different manufacturing methods for phosphoric acid affect its purity and use?
The manufacturing method directly impacts the purity and application of phosphoric acid. The two main processes are:
- Wet Process: This method uses phosphate rock and produces a less pure, technical-grade acid. This version is cost-effective and primarily used for making fertilizers.
- Thermal Process: This method involves burning phosphorus to create a much purer, food-grade acid. This high-purity acid is used in the food industry, pharmaceuticals, and for specialised chemical applications.
7. Phosphoric acid is described as a polyprotic acid. What does this property mean for its chemical behaviour?
Describing phosphoric acid as polyprotic means that a single molecule has more than one acidic proton (H⁺) that it can donate in a reaction. Specifically, H₃PO₄ is triprotic because it has three protons. It can lose them one by one in three distinct steps, allowing it to form three different types of phosphate ions (H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻) and a variety of phosphate salts.
8. How does phosphoric acid work to remove rust from metal surfaces?
Phosphoric acid removes rust by chemically reacting with it. Rust is primarily iron(III) oxide. The acid converts the iron oxide into a black-coloured compound called ferric phosphate. This new compound forms a protective coating on the metal, which not only removes the original rust but also helps to prevent future corrosion. This layer can then be cleaned off or painted over.
