Condensed phosphates

Key point: Dehydration of phosphoric acid leads to the formation of chain or ring structures that may be based on many PO4 units. When phosphoric acid, H₃PAO₄, is heated above 200ºC, condensation occurs resulting in the formation of P O P bridges between two neighbouring PO₄³⁻ units. The extent of this condensation depends upon the temperature and duration of heating.

The simplest condensed phosphate is thus H₄P₂ O₇. The most commercially important condensed phosphate is the sodium salt of the triacid, Na₅P₃O₁₀ (21). It is widely used in detergents for laundry and dishwashers, and in other cleaning products and in water treatment (Box 15.5). Polyphosphates are also used in various ceramics and as food additives. Triphosphates such as adenosine triphosphate (ATP) are of vital importance in living organisms (Section 26.8).

A range of condensed phosphates occurs with chain lengths ranging from those based on two PO4 units to polyphosphates having chain lengths of several thousand units. Di-, tri-, tetra-, and pentapolyphosphates have been isolated but higher members of the series always contain mixtures. However, the average chain length can be determined by the usual methods used in polymer analysis or by titration. Just as the three successive acidity constants for phosphoric acid differ, so do the acidity constants for the two types of OH group of the polyphosphoric acids. The terminal OH groups, of which there are two per molecule, are weakly acidic. The remaining OH groups, of which there is one per P atom, are strongly acidic because they are situated

opposite the strongly electron-withdrawing =O groups. The ratio of weakly to strongly acidic protons gives an indication of average chain length. The long-chain polyphosphates are vis cous liquids or glasses.

If NaH₂PO₄ is heated and the water vapour allowed to escape, then the tricyclo anion P₃O₉^3- (22) is formed. If this reaction is carried out in a closed system, the product is ‘Maddrell’s salt’, a crystalline material that contains long chains of PO4 units. The tetracyclo anion (23) is formed when P₄O₁₀ is treated with cold aqueous solutions of NaOH or NaHCO₃.

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مواضيع ذات صلة

Oxoanions of phosphorus, arsenic, antimony, and bismuth

Oxides of phosphorus, arsenic, antimony, and bismuth

Nitrogen(V) oxides and oxoanions

Oxides and oxoanions of nitrogen

Oxohalides

Halides of the heavy elements

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides