Phosphoric acid, H3PO4, and its derivatives

Phosphoric acid is made from phosphate rock (equation 15.132) or by hydration of P4O10 (equation 15.124). 

When H3PO4 is heated at 510 K, it is dehydrated to diphosphoric acid (equation 15.133). Comparison of the structures of these acids (Table 15.7) shows that water is eliminated with concomitant P—O—P bridge formation. Further heating yields triphosphoric acid (equation 15.134). 

Such species containing P-O-P bridges are commonly called condensed phosphates and equation 15.135 shows the general condensation process. 

Box 15.10 Phosphate fertilizers essential to crops but are they damaging our lakes  

Fertilizers are a major source of phosphates entering rivers and lakes. However, domestic and industrial waste water (e.g. from detergent manufacturing) also contains [P04] and condensed phosphates, and the levels that must be removed before the waste can be discharged are controlled by legislation. In most cases, phosphates are removed by methods based on precipitation (this is the reverse of the situation for nitrate removal see Box 15.9). Fe, Al and Ca are most commonly used to give precipitates that can be separated by filtration. 

Davison, D.G. George and N.J.A. Edwards (1995) Nature, vol. 377, p. 504 - Controlled reversal of lake acidification by treatment with phosphate fertilizer . 

Gachter and B. Muller (2003) Limnology and Oceanography, vol. 48, p. 929 - Why the phosphorus retention of lakes does not necessarily depend on the oxygen supply to their sediment surface . 

The controlled hydrolysis of P4O10 is sometimes useful as a means of preparing condensed phosphoric acids. In principle, the ermdensation of phosphate ions (e.g. reaction 15.140) should be favoured at low pH, but in practice such reactions are usually slow. 

In free condensed acids such as H5P3O10 (Table 15.7), different phosphoms environments can be distinguished by P NMR spectroscopy or chemical methods  

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More than 90 percent of commercial phosphorus production is in the form of calcium salts of phosphoric acid, H3PO4, used as fertilizers. Other significant uses of phosphorus compounds are in the manufacture of matches (phosphorus sulfides), food products and beverages (purified phosphoric acid and its salts), detergents (sodium polyphosphates), plasticizers for polymers (esters of phosphoric acid), and pesticides (derivatives of phosphoric acid). Related to the phosphorus pesticides are nerve gases, poisonons com-potmds that rapidly attack the central nervous system, initially developed during World War II. see also Deoxyribonucleic Acid (DNA) Fertilizer Pesticides. 

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