Phosphoric acid

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Phosphoric acid
Chemical formula H3PO4
OTP appearance white solid 
Molar Mass(g/mol) 97.995 
Melting Point(°C) 42.35 
Boiling Point(°C) 158 (decomp) 
Index of refraction 1.4320 (85% aq. soln, 25°C) 
Solubility in water(g/L) 3922 
Solubility in ethanol(g/L) misc
NFPA 704
NFPA704.png
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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.

Hazards

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)

Testing

Storage

Disposal

See Also

  • 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

References

  1. Bird, R.M.; Diggs, S.H. (1914) "POSITIVE IONS FROM NON METALLIC ELEMENTS"
    Journal of the American Chemical Society XXXVI; pp1383.