Trioxophosphate(III) Acid: Phosphorous Acid, and Tetraoxophosphate(V) Acid: Phosphoric Acid, Tetraoxophosphate(V)

Both acids are formed based on the oxides of phosphorus. Trioxophosphate(III) acid is the most important acid based on phosphorus(III) oxide, and tetraoxophosphate(V) acid is the most important acid based on phosphorus(V) oxide.

Trioxophosphate(III) acid (phosphorus acid), H₃PO₃, is a slightly strong acid solution when dissolved in water. It is prepared by the action of cold water on phosphorus(III) oxide.

(cold)6H₂O(l) + P₄O₆(s) → 4H₃PO₃(aq)

Another method of preparation involves the hydrolysis of phosphorus(III) chloride, PCl₃, to form hydrochloric acid as a second product.

PCl₃(l) + 3H₂O(l) → H₃PO₃(aq) + 3HCl(aq)

It is a deliquescent solid in its pure form and a strong reducing agent that decomposes to phosphine with a second product: tetraoxophosphate(V) acid, when heated to 200°C.

4H₃PO₃(s) → PH₃(g) + 3H₃PO₄(s)

Phosphine is a strong reducing agent that exist as phosphorus(III) hydride and a less important diphosphane.

Although the structural formula of trioxophate(III) acid is visible as containing three hydroxyl groups, the acid actually has a basicity of two. The implication is that, it contains only two ionizable hydrogen atoms and two hydroxyl groups.

Tetraoxophosphate(V) acid (phosphoric acid), H₃PO₄, is the most hydrated of the three acid formed from phosphorus(V) oxide.

P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)

Its structural formula is one oxygen atom different from trioxophosphate(III) acid.

Tetraoxophasphate(V) acid is prepared by different means. The laboratory preparation involves the action of concentrated trioxonitrate(V) acid on red phosphorus.A unique method of preparation is the action of 60% tetraoxosulphate(VI) acid on rock phosphate: calcium tetraoxophosphate(V).

(rock phosphate) Ca₃(PO₄)₂(S) + 3H₂SO₄(aq) →3CaSO₄(s) + 2H₃PO₄(aq)

The most popular commercial method of preparation involves igniting white phosphorus in plentiful supply of air to form phosphorus(V) oxide. The phosphorus(V) oxide so formed, is then reduced in water to form the pure acid.

Unlike trioxophosphate(III) acid, tetraoxophospate(V) acid is a weak acid with a basicity of three. Here, it ionizes in water in three successive stages forming three oxonium ions, H₃O⁺.

• H₃PO₄(aq) + H₂O(l) ↔ H₃O⁺(aq) + H₂PO₄^-(aq)

• H₂PO₄^-(aq) + H₂O(l) ↔ H₃O⁺(aq) + HPO₄^2-(aq)

• HPO₄^2-(aq) + H₂O(l) ↔ H₃O⁺(aq) + PO₄^3-(aq)

This three stages of ionization allows it to go into neutralization reaction to form three different trioxophosphate(V) salts.

Another reaction of the acid is the that it is progessively dehydrated when heated to about 200°C in a reversible reaction.

(200°C) H₃PO₄(s) ↔ H₄P₂O₇(s) + H₂O(l) ↔ 2HPO₃(s) + 2H₂O(l)

Tetraoxophosphate(V) salts are mostly important in the application to manufacturing inorganic fertilizers. Here, ammonium tetraoxophosphate(V) and calcium tetraoxophosphate(V) are commonly used. Triple phosphate, 3Ca(H₂PO₄)₂,for example, is made from treating calcium tetraoxophasphate(V) with tetraoxophosphate(V) acid.

Ca₃(PO₄)₂(s) + 4H₃PO₄(aq) → 3Ca(H₂PO₄)₂(s)

Superphosphate, Ca(H₂PO₄)₂, is another phosphorus fertilizer that is a mixture of calcium dihydrogen tetraoxophosphate(V) and calcium tetraoxosulphate(VI) made from reacting calcium tetraoxophosphate(V) (bone ash or rock phosphate) with 65% tetraoxosulphate(VI) acid.

Ca₃(PO₄)₂(s) + 2H₂SO₄(aq) → Ca(H₂PO₄)₂(s) + 2CaSO₄(s)

The different stages of ionization of tetraoxophosphate(V) acid have actually resulted to three forms of tetraoxophosphate(V) salts. For example, sodium metal will form sodium hydrogen tetraoxophosphate(V), Na₂HPO₄, sodium dihydrogen tetraoxophosphate(V), NaH₂PO₄, and sodium tetraoxophosphate(V), Na₃PO₄.

The acid and its salts are used as water-softeners, detergents and catalysts. They are also used in refining petroleum, making matches, glazes for pottery and enamels.

Tetraoxophosphate(V) acid and its salts are important in food processing, drinks and in making baking powder. They are also useful in rustproofing steel materials before they are painted and in fireproofing textiles and timber.