Pyrophosphoric acid hydration

Pyrophosphate (also known as diphosphate) or 1 207" . The conjugate base of pyrophosphoric acid (or, diphosphoric acid), H4P2O7 (having pi a values, at 25°C, of 0.85, 1.96, 6.60, and 9.41). Crystals of tetrasodium pyrophosphate (mp = 988°C) are very soluble in water. The pyrophosphate anion hydrolyzes to orthophosphate however, the rate of hydrolysis is much slower than that of pyrophosphoric acid. The hydrated form of tetrasodium pyrophosphate exhibits a shght degree of efflorescence. 

Orthophosphoric acid can thus be regarded as a hydrate, H Oy. O, of pyrophosphoric acid, and pyrophosphoric acid in turn as a hydrate of the meta-acid, in harmony with the view that pyrophosphoric acid is intermediate between ortho-and meta-phosphoric acids. 

The hydration of pyrophosphoric add.—T. Graham kept an aq. soln. of dil. pyrophosphoric acid for six months without change but, when heated, it was converted into the ortho-acid. The hydration of dil. soln. of pyrophosphoric acid is therefore slow at ordinary temp., but it is rapid when boiled. M. Berthelot and G. Andre 4 found that a soln. of pyrophosphoric acid contained after standing ... 

The familiar phosphoric acid, H3P04 (melting point, 43° C), is more specifically orthophosphoric acid, as distinct from pyrophosphoric acid, (OH)sP—O—P(OH)2, and from the less hydrated derivatives of... 

If even less water is used for hydration, a product known as polyphosphoric acid results. Ordinary phosphoric acid is a solution of the monomer, H3P04, in water, and is called orthophosphoric acid. If a molecule of water is removed between two orthophosphate molecules, the dimer, pyrophosphoric acid, H4P207, is formed. Similarly, the trimerand higher polymers can be made. Superphosphoric acid is a mixture of orthophosphoric acid and polyphosphoric acid and is now made from wet process acid as described later in this chapter. 

Hydration to Orthophosphoric Acid.—The process of depolymerisa-tion of concentrated solutions of pyrophosphoric acid which has already been noted very likely consists in the hydration of the molecules of condensed or poly-acid. The further hydration, with formation of orthophosphoric acid, proceeds only slowly at low temperatures and in dilute solutions. A dilute aqueous solution was kept for six months without change.7 The velocity of the reaction has been followed by... 

It cannot be assumed that, because pyrophosphoric acid is produced as an intermediate product in the dehydration of H3P04 that it will also be produced during the hydration of HP03. The amounts observed may be present in the original HP03 or be produced by the heat developed when this is placed in water. The pyro-acid has been detected in the last fractional precipitates of silver phosphates, etc. (see Estimation ).2 Since there is some evidence that metaphos-phoric add is hydrated more rapidly than pyrophosphoric acid6 the latter may accumulate up to a certain maximum concentration. 

Water interacts with the H3PO4 imbibed membranes either by reacting with the pyrophosphoric acid or by hydrating H3PO4 according to reactions (1) and (2) respectively. Both reactions can be considered as the main contributing processes for the proton conductivity. In this respect the significance of equilibrium reactions... 

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