Alternative Processes for Phosphoric Acid Manufacture

Phosphoric acid is produced in the reaction between phosphorus vapour and steam (4.21). Although high yields can be obtained at 650-800°C using catalysts, the process cannot compete commercially 

Pure concentrated orthophosphoric acid is a clear, colourless syrupy liquid which is somewhat corrosive but less so than the other common mineral acids (Section 12.1). It is miscible with water and ethanol and many organic solvents. 

Orthophosphoric acid has only one strongly ionising hydrogen atom and dissociates according to the scheme 

Since the protons are always combined with water molecules in aqueous media as hyroxonium ions, the scheme can be represented below. 

In aqueous solutions of the add, all species are present, but in differing amounts. Since the first dissociation constant, A , is much larger than the second, very little of the H2PO4 produced in the first equation goes on to dissociate according to the second equation. It dissociates even less according to the third equation since the third constant, A3, is very small. 

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