Phosphoric acid

 

Phosphoric acid

Chemical formula	H3PO4
OTP appearance	white solid
Index of refraction	1.4320 (85% aq. soln, 25°C)
Molar Mass(g/mol)	97.995
Melting Point(°C)	42.35
Boiling Point(°C)	158 (decomp)
Solubility in water(g/L)	3922
Solubility in ethanol(g/L)	misc
NFPA 704	0 3 0
Thermal Decomposition	Phosphoric acid 158°C H2O + HPO3

Uses

Primary

• Industrial chemical (early accessible mineral acid)
• Fertilizer

Secondary

• Can be used to produce hydrochloric and nitric acids.

  H₃PO₄ + 3 NaCl

  {

  →

  110°C}

  Na₃PO₄ + 3 HCl(g) // Escaping HCl vapors keep reaction moving to the right

  H₃PO₄ + 3 NaNO₃

  {

  →

  120.5°C}

  Na₃PO₄ + 3 HNO₃(g) // Escaping HNO₃ vapors keep reaction moving to the right

Natural Occurrence

• Phosphoric does not occur naturally in quantity. If generated (by volcano, lightening, etc) it rapidly reacts with nearby materials and becomes phosphate.

Character

• 85% phosphoric acid has a density of approximately 1.65g/cc

Production

Synthesis

From phosphorus

1. Burn phosphorus, producing phosphorus pentoxide

   P₄ + 5 O₂ → 2 P₂O₅

2. Dissolve phosphorus pentoxide in water producing phosphoric acid

   3 H₂O + P₂O₅ → 2 H₃PO₄

From copper sulfate

Combine phosphorus with a copper sulfate solution leaving sulfuric and phosphoric acid along with metallic copper.

1. Combine white phosphorus with a solution of copper sulfate

   5 CuSO₄ + 2 P + 8 H₂O → 5 Cu + 5 H₂SO₄ + 2 H₃PO₄

Extraction

From apatite

Phosphoric acid is synthesized by treating apatite (mostly calcium triphosphate) with sulfuric acid, producing phosphoric acid, water, and gypsum.

Ca₅(PO₄)₃X + 5 H₂SO₄ + 10 H₂O → 3 H₃PO₄ + 5CaSO₄•2H₂O + HX where X ∈ {OH, F, Cl, Br}

Thus if fluorapatite is used, hydrofluoric acid will be produced in recoverable amounts.

From bone black

Phosphoric acid is synthesized by treating bone black with sulfuric acid, producing phosphoric acid, and calcium sulfate (dihydrate)

Ca₃(PO₄)₂ + 3 H₂SO₄ + 6 H₂O → 2 H₃PO₄ + 3CaSO₄•2H₂O

Purification

Much like sulfur trioxide can be introduced into dilute sulfuric acid to concentrate it, burnt phosphorus introduced into dilute phosphoric acid produces more concentrated phosphoric acid.

3 H₂O + P₂O₅ → 2 H₃PO₄ (Catalyzed by existing phosphoric acid)

See Also

• Mineral acid intraconversion
• Vogel, A. (1836) "Ueber die Reduction der Quecksilbersalze durch Kupfer.".
  J. Prakt. Chem. 8(1); pp107–109. 
  DOI:10.1002/prac.18360080118
• Straub, Walther (1903) "Phosphorus, reactions between copper and yellow"
  Journal of the Chemical Society 84(2) Gurney & Jackson