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Pyrophosphoric Acid, H4P2O7

A purer product was made by the double decomposition of a soluble lead salt with sodium pyrophosphate, Na4P2O7, whereby lead pyrophosphate, Pb2P2O7, was precipitated; this was then decomposed by H2S. The silver salt, Ag4P2O7, yielded anhydrous H4P2O7 when warmed in a current of dry hydrogen chloride.

Physical Properties

The concentrated acid, prepared by dehydration of orthophosphoric acid, is a highly viscous liquid and probably is polymerised. By the depression of the freezing-point the molecules in aqueous solution showed a complexity corresponding to (H4P2O7)4 and (H4P2O7)5. The molecular weight in glacial acetic acid corresponded to (H4P2O7)3, but diminished with time.2 The acid prepared as above—from Pb2P2O7—appeared to dissolve in water as simple molecules. The freezing-point curve of H4P2O7 showed two eutectics—one at 23° C. with a solution containing H4P2O7 + 1.25H2O and the other at -75° C. with H4P2O7 + 6.87H2O. The maximum freezing-points are (1) +61° C. or higher, the melting-point of the anhydrous acid; (2) + 26° C., the melting-point of the crystalline hydrate H4P2O7 + ½H2O. The acid which crystallised from concentrated solution was, however, found to contain anhydrous H3PO4.

The molar heat of fusion of the solid acid was about 2.3 Cals., therefore very similar to that of H3PO4. The heat of solution of H4P2O7 (solid) was about 7.85 Cals., that of H4P2O7.1.5H2O (solid) 4-5 Cals. The heats of transformation by means of liquid water were:—

H4P2O7 (solid) + H2O = 2H3PO4 (solid) +6.97 Cals.
H4P2O7 (liquid) + H2O = 2H3PO4 (liquid) + 9.09 Cals.
H4P2O7 aq. = 2H3PO4 aq. +4.25 Cals.

From these other thermal equations can be derived in the usual manner.

The heats of formation from the elements were close to those of 2H3PO4-H2O.

2P + 3½O2 + 2H2 = H4P2O7 + Q

H4P2O7 solidH4P2O7 liquidH4P2O7 solution
Q (Cals.)532.23529.94540.16


The electrical conductivities of aqueous solutions of pyrophosphoric acid are as follows:—

c, mols/litre0.001250.00250.0050.01250.0250.05
μ, molar conductivity602.0556.7503.3438.6384.9353.8


The degree of primary ionisation calculated from these values is high, being 96 per cent, at the lowest concentration. For dissociation constants, etc., see below.

All the evidence shows that pyrophosphoric acid is tetrabasic and that each hydrogen is that of a stronger acid, i.e. more dissociated at the same dilution, than orthophosphoric acid.

With regard to heats of neutralisation, 1 mol of the acid, to which n equivalents of strong alkali were added, gave the following successive amounts of heat, the sum of which is the total heat of neutralisation at each step:—

n12345
Q (Cals.)15.315.6513.17.81.8


The constants of the successive dissociations were deduced from the conductivities of the salts:

NaH3P2O7Na2H2P2O7Na3HP2O7Na4P2O7
K1K2K3K4
0.140.0110.06290.0836


or K3 =7.6×10-7, K4 = 4×10-9, or K4 = 4.9×10-9.

These constants determine the titration exponents pH and the best indicators for the successive hydrions. The acid can be titrated as dibasic, using methyl yellow, methyl orange or bromophenol blue, and as tetrabasic using phenolphthalein, thymolphthalein or thymol blue in the presence of a moderate excess of soluble barium salt. The values of pH in the partly neutralised acid were corrected for the salt error, and the constants K3 and which prevail in solutions of low concentration were thus deduced:—

K3 = 2.1×10-7, pK3 = 6.68; K4 = 4.06×10-10, pK4 = 9.4

The true constant found by introducing the correction for ionic strength was 0.45×10-10 at a concentration of 0.02 mol/litre, and increased with decreasing concentration to 4.6×10-10 at c = 0.00128. The uncorrected constant decreased from 4.9×10-9 at c = 0.02 to 1.5×10-9 at c = 0.00125.

Hydration to Orthophosphoric Acid

The process of depolymerisation 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. The velocity of the reaction has been followed by taking the difference in the relative titres to methyl orange and phenolphthalein.

H4P2O7 + H2O = 2H3PO4

A solution containing 15.6 grams of P2O5 per litre was about half- transformed in 121 days (H4P2O7/H3PO4 initially =87/4; finally 43.1/47.9). The reaction was greatly accelerated by hydrogen ions, which in practice are usually supplied by nitric acid. It was found to be unimolecular.

Constitution

The formulae must, on account of the properties of pyrophosphoric acid and its relation to orthophosphoric acid, show the phosphorus atoms as saturated. This can be done either by the electronic formulae or by those containing quinquevalent phosphorus, since as already shown the two kinds of formulae are interchangeable. The molecule may be (1) symmetrical or (2) unsymmetrical:—



The corresponding acid chloride is pyrophosphoryl chloride (q.v.). When this is treated with water it gives orthophosphoric acid, but when water vapour acts on a solution of the chloride in carbon disulphide some pyrophosphoric acid is produced.

Pyrophosphates may split up in an unsymmetrical manner. Thus by heating with PCl5 in a sealed tube:—

Na4P2O7 + 3PCl5 = NaPO3 + 4POCl3 + 3NaCl

and by fusion:—

NaAg3P2O7 = Ag3PO4 + NaPO3

The ethyl and methyl esters have been prepared by the usual methods. Thus (C2H5)4P2O7 by the action of C2H5I on Ag4P2O7 at 100° C. The product was a liquid soluble both in water and in alcohol. The elevation of the boiling-point of benzene by this ester corresponded to simple molecules. The decomposition of the ester on heating supports the asymmetrical constitution:—

(C2H5O)3P=O2=PO(OC2H5) → (C2H5O)3PO + O2P(OC2H5)
O2P(OC2H5) → Osub>2POH + C2H4

the products being triethyl phosphate, metaphosphoric acid and ethylene.

In most reactions pyrophosphoric acid is transformed into orthophosphoric acid. It was dehydrated by PCl5, thus

H4P2O7 + PCl5 = 2HPO3 + POCl3 + 2HCl

Thionyl chloride, SOCl2, and phosphorus trichloride also gave HPO3.

The acid gave compounds with albumin which, unlike those of HPO3, were soluble.

The pyrophosphates are more stable than the free acid and show some reactions. Among these may be mentioned the white precipitate of Ag2H2P2O7 which is insoluble in acetic acid.

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