Also called orthophosphoric acid, phosphoric acid is a solid white crystal at room temperature. However, it’s more commonly available as an 85% aqueous solution, which is colourless, viscous, and non-volatile. It also has no distinctive odour.
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Chemical formula of phosphoric acid
Phosphoric acid is classed as a weak acid because it does not completely dissociate in an aqueous solution.
Its chemical composition and molecular structure mean it acts as a triprotic acid or, in other words, it has three hydrogen atoms that can be iodised.
The chemical formula for pure, anhydrous phosphoric acid in solid crystal form can be written as:
H3PO4 or PO(OH)3
It can also be written in a way that emphasises the three hydroxyl groups (see below). Its molecular structure is tetrahedral with an asymmetric charge distribution on the phosphorus as the central atom.
The three hydroxyl groups, which form the ‘legs’ of the tetrahedral molecule, are all 157 pm (trillionth of a metre) or 1.57 ångströms from the centre. The double-bonded oxygen is much closer to the centre of the molecule at 152 pm or 1.52 ångströms.
When dissolved in water, phosphoric acid salt molecules sequentially dissociate into ions in the manner shown in the dynamic equilibrium chemical equations below. As you can see, the bond-dissociation energy involved in each step decreases as the number of initial hydrogen also falls, beginning with three, then two, and finally, one.
Is phosphoric acid harmful?
Phosphoric acid is not harmful at low concentrations. In fact, it’s one of the most common additives in carbonated beverages and certain food products.
It can, however, be very harmful at a concentration of at least 1,000 mg/cm3. At high concentrations, phosphoric acid is considered corrosive and can cause damage to the eyes and mucous membranes.
How is phosphoric acid manufactured?
Most people are familiar with phosphoric acid as one of the ingredients in carbonated drinks. Not only does it add the tangy ‘acidic’ flavour, but it also helps to prevent microbial growth.
However, the main application of phosphoric acid is in manufacturing fertilisers. About 90% of phosphoric acid produced each year is used by the fertiliser manufacturing industry. In 2020, for example, the global production of phosphoric acid was 86.63 million metric tons. This translates to 77.97 million metric tons being used as a fertiliser ingredient.
Other applications of phosphoric acid include:
- Soaps and detergents
- Food additives
- Water treatment process
- Toothpaste manufacturing
- Pharmaceutical products.
Industrial and food-grade phosphoric acid can either be produced through the wet process or the dry process. The former involves the treatment of sulphuric acid, while the latter uses coke to reduce phosphate ore in an electric furnace.
The wet process
The wet process uses sulphuric acid to treat phosphate ores that contain minerals such as calcium hydroxyapatite and fluorapatite, as exemplified in the balanced chemical equations below:
- [eq. 1] Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O
- [eq. 2] Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF
As you can see, the reactions produce calcium sulphate (or gypsum) and hydrogen fluoride gas as unwanted byproducts. The gypsum is removed as phosphogypsum. Meanwhile, the hydrogen fluoride is streamed into a wet scrubber where it reacts with water to form hydrofluoric acid.
The wet process doesn’t normally produce a highly-concentrated phosphoric acid solution, with typical concentrations ranging between 32% and 46%. It also contains about 23% – 33% phosphorus pentoxide (P2O5).
The solution can, however, be further concentrated into a merchant-grade phosphoric acid solution, which has a concentration of 75% – 85%. Impurities, such as arsenic and other toxic substances, can also be removed.
The dry process
Also known as the thermal process, the dry process produces food-grade phosphoric acid through the reduction reaction of phosphate ore using coke in an electric furnace.
Phosphorus is sprayed into a furnace and burns at around 1,500-2,700°C. In some processes, steam may also be added to create a thin film of condensed polyphosphoric acid. This film helps to protect the stainless steel burner tower, which is externally cooled with water.
The initial reaction can be summarised as:
P4(I) + 5O2(g) → 2P2O5(g)
The steam passes through the burner tower and then mixes with the gaseous phosphorus in the hydration tower:
P2O5(g) + 3H2O(I) → 2H3PO4(I)
The dry process usually produces a higher concentration of phosphoric acid crystal, with a purity level of around 85%.
What is the raw material of phosphoric acid?
The raw materials used in the mass production of phosphoric acid depend on the manufacturing method.
In the wet process, the main raw materials are ores that have phosphate-containing minerals like calcium hydroxyapatite, which are treated with sulphuric acid. The dry process also uses phosphate ores, but these are heated to produce vapour, before being combined with oxygen and treated with steam.
Where can you buy phosphoric acid?
You can buy phosphoric acid either in crystal form or as a solution from a chemical company or distributor. You may also purchase it online from suppliers in various pack sizes – you may need to be a business to purchase this chemical.
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