Phosphoric Acid
Structural Formula Vector Image
Title: Phosphoric Acid
CAS Registry Number: 7664-38-2
Additional Names: Orthophosphoric acid
Molecular Formula: H3O4P
Molecular Weight: 98.00
Percent Composition: H 3.09%, O 65.30%, P 31.61%
Line Formula: H3PO4
Literature References: Obtained commercially from phosphate rock deposits in Florida, Tennessee, and the Western United States. Phosphate rock is essentially tricalcium phosphate and one of the large scale processes is based on the equation: Ca3(PO4)2 + 3H2SO4 + 6H2O ® 2H3PO4 + 3(CaSO4.2H2O). Description of various processes: W. H. Waggaman, Phosphoric Acid, Phosphates and Phosphatic Fertilizers (Reinhold, New York, 1952); Phosphoric Acid Vol. 1, parts I, II, A. V. Slack, Ed. (Dekker, New York, 1968) 1159 pp; Faith, Keyes & Clark's Industrial Chemicals, F. A. Lowenheim, M. K. Moran, Eds. (Wiley-Interscience, New York, 4th ed., 1975) pp 628-639. Prepn of ultrapure, cryst H3PO4: Simon, Schulze, Z. Anorg. Allg. Chem. 242, 322 (1939); Weber, King, Inorg. Synth. 1, 101 (1939). Reviews: J. R. Van Wazer, Phosphorus and Its Compounds Vol. 1, Chemistry (Interscience, New York, 1958) pp 479-491; R. B. Hudson et al., "Phosphoric Acids and Phosphates" in Kirk-Othmer Encyclopedia of Chemical Technology Vol. 17 (Wiley-Interscience, New York, 3rd ed., 1982) pp 426-472.
Properties: Unstable, orthorhombic crystals, mp 42.35°, or clear, syrupy liquid; easily supercooled into a glass. Pleasing acid taste when suitably diluted. An acid containing ~88% H3PO4 will frequently crystallize on prolonged cooling; forms hemihydrate, mp 29.32°. Becomes anhydr at 150°, gradually changes to pyrophosphoric acid at ~200°, and changes to metaphosphoric acid when heated above 300°. The hot concd acid attacks porcelain and granite ware. May be stored in suitable stainless steel containers. Tribasic acid: pK1: 2.15; pK2: 7.09; pK3: 12.32. Other reported values of dissociation constants and viscosity data reviewed by Van Wazer. The pH of a 0.1N aq soln is 1.5. Heat of formation (crystals): -306.2 kcal/mole. Heat of soln: +2.79 kcal/mole. Sol in water, alc; sol in 8 vols of a 3:1 ether: alcohol mixture. Properties of phosphoric acid solns. d25 1.8741 (100% soln); 1.6850 (85% soln); 1.3334 (50% soln); 1.0523 (10% soln). Density measurements: Christensen, Reed, Ind. Eng. Chem. 47, 1277 (1950); Egan, Luff, ibid. 1280. nD17.5 1.34203 (10% soln); 1.35032 (20% soln); 1.35846 (30% soln). Spec heat (21.3°): 0.4359 (88% soln).
Melting point: mp 42.35°; mp 29.32°
pKa: pK1: 2.15; pK2: 7.09; pK3: 12.32
Index of refraction: nD17.5 1.34203 (10% soln); 1.35032 (20% soln); 1.35846 (30% soln)
Density: d25 1.8741 (100% soln); 1.6850 (85% soln); 1.3334 (50% soln); 1.0523 (10% soln)
CAUTION: Potential symptoms of overexposure are irritation of upper respiratory system, eyes and skin; burns skin and eyes; dermatitis. See NIOSH Pocket Guide to Chemical Hazards (DHHS/NIOSH 97-140, 1997) p 254.
Use: In the manuf of superphosphates for fertilizers, other phosphate salts, polyphosphates, detergents. Acid catalyst in making ethylene, purifying hydrogen peroxide. As acidulant and flavor, synergistic antioxidant and sequestrant in food. Pharmaceutic aid (solvent). In dental cements; process engraving; rustproofing of metals before painting; coagulating rubber latex; as analytical reagent.
Therap-Cat-Vet: Has been used in lead poisoning.

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