Pyrosulphuric Acid, H₂S₂O₇

Although the substance of the composition H₂S₂O₇ is capable of definite existence, the commercial products deviate from the composition expressed by the formula and contain variable quantities of sulphuric acid with pyrosulphuric acid or of pyrosulphuric acid with sulphur trioxide. For the purpose of including all varieties of impure pyrosulphuric acid, the somewhat vague terms "oleum," "fuming sulphuric acid" and "Nordhausen sulphuric acid " are advantageous.

1. At one time fuming sulphuric acid was largely prepared by the dry distillation of basic ferric sulphate which had been obtained by submitting ferrous sulphate to atmospheric oxidation and subsequently dehydrating the oxidation product fairly completely:
   
   12FeSO₄ + 3O₂ = 4Fe₂(SO₄)₃ + 2Fe₂O₃,
   Fe₂(SO₄)₃ + 2H₂O = Fe₂O₃ + H₂S₂O₇ + H₂SO₄.
   
   The acid obtained will naturally vary in composition according to the extent of the previous dehydration.
   
   In the fifteenth and sixteenth centuries the alchemists obtained sulphuric acid in this manner, frequently aiding the process by the addition of silica, and the process was worked on a commercial scale until the end of the nineteenth century. From the fact that much of the fuming acid was distributed from Nordhausen in Prussia, the name "Nordhausen sulphuric acid" originated, but little if any of the acid was manufactured in this town.
2. The present method of manufacture is to treat ordinary sulphuric acid with the necessary amount of sulphur trioxide, by which procedure any desired degree of concentration may be attained. The trioxide may be introduced in the liquid condition, but commercially it is simpler to pass the trioxide vapours from the contact chambers directly into sulphuric acid of 98 to 99 per cent, concentration. The latter is usually contained in bubbling vessels or circulated in towers. The vapours meet the most concentrated acid in the first vessel, and then pass through slightly weaker acid, and so on until the trioxide is completely absorbed. The acid in the last absorption vessel should contain 98 to 99 per cent.H₂SO₄; if stronger than this the acid has the same vapour pressure as the trioxide and the latter is not absorbed; if weaker, a mist is formed which is difficult to absorb.

Ordinary commercial "oleum" is 20 per cent, oleum, that is, it contains 80 per cent, of H₂SO₄ and 20 per cent, of "free" SO₃. 100 parts of such "oleum," i.e. 80 parts of H₂SO₄ + 20 parts of SO₃, can take up 18/80×20 = 4.5 parts of H₂O, yielding 80 + 24.5 = 104.5 parts of H₂SO₄. Sulphuric acid containing high percentages of additional sulphur trioxide is frequently manufactured by distilling acid containing 20 to 30 per cent, of the trioxide.

Pure pyrosulphuric acid is a colourless crystalline solid of melting-point 35° C.; the addition of a little water or of sulphur trioxide lowers the melting-point; the colour of the acid is frequently brown, due to the presence of traces of organic matter.

In the following table are given the melting-points for ordinary sulphuric acid containing gradually increasing proportions of additional sulphur trioxide; the maxima for pure sulphuric acid and pure pyrosulphuric acid are evident at the values 0 per cent, and near 40 per cent.

Melting points of H₂SO₄-SO₃ mixtures

Percentage SO2.	Melting-point, ° C
0	+ 10.5
5	+ 3.5
10	- 4.8
15	- 11.2
20	- 11.0
25	- 0.6
30	+ 15.2
35	+ 26.0
40	+ 33.8
45	+ 34.8
50	+ 28.5
55	+ 18.4
60	+ 0.7
65	+ 0.8
70	+ 9.0
75	+ 17.2
80	+ 22.0

The specific gravity of " fuming sulphuric acid " reaches a maximum in the neighbourhood of 50 to 60 per cent, of " free " trioxide, the composition of maximum density acid being dependent to some extent on the temperature. In the next table are given figures for the specific gravities of concentrated sulphuric acid and the fuming acid at 15° C., which show clearly the irregularity in the variation of this physical characteristic with gradual alteration in composition.

Investigation has also been made of the viscosity, vapour pressure, boiling-point, specific heat, heat of solution in water, electrical resistance

Specific gravities of concentrated and fuming sulphuric acids

H2SO4 (per cent.).	Total SO3 (per cent.).	Free SO3 (per cent.).	Sp. Gr. at 15° C.	Sp. Gr. at 45° C.
95.98	78.35	. . .	1.8418	. . .
96.68	78.92	. . .	1.8429	. . .
96.99	79.18	. . .	1.8431	. . .
97.66	79.72	. . .	1.8434 max.	. . .
98.65	80.53	. . .	1.8403	. . .
99.40	81.14	. . .	1.8388 min.	. . .
99.76	81.44	. . .	1.8418	. . .
100.00	81.63	0	1.8500	1.822
. . .	83.46	10.0	1.888	1.858
. . .	85.30	20.0	1.920	1.887
. . .	87.14	30.0	1.957	1.920
. . .	88.97	40.0	1.979	1.945
. . .	90.81	50.0	2.009	1.964 max.
. . .	92.65	60.0	2.020 max.	1.959
. . .	94.48	70.0	2.018	1.942
. . .	96.32	80.0	2.008	1.890
. . .	98.16	90.0	1.990	1.864
. . .	100.00	100.0	1.984	1.814

and capillarity of fuming sulphuric acid of varying richness in sulphur trioxide.

Pyrosulphuric acid, whether pure or mixed with excess of sulphuric acid or sulphur trioxide, readily undergoes decomposition into sulphuric acid and the trioxide; indeed, the fumes evolved at the ordinary temperature are due to vaporisation of the trioxide from the molten acid or its solution in sulphuric acid. On heating the acid, sulphur trioxide is expelled until the mixture of sulphuric acid and water of minimum vapour pressure (98.3 per cent, sulphuric acid) remains. It is owing to this behaviour that pyrosulphuric acid and "fuming sulphuric acids" in general have no definite and constant boiling temperature.

In consequence of this readiness of "fuming sulphuric acid" to undergo fission into its two constituents, the chemical properties of the acid are largely a blend of those of sulphuric acid and its anhydride.

Platinum and lead are attacked more seriously by fuming sulphuric acid than by ordinary sulphuric acid, but of considerable technical value is the fact that although a weaker acid corrodes wrought iron, this metal becomes "passive" in acid containing more than 27 per cent, of "free" anhydride.

The fuming sulphuric acid of commerce contains less sulphur trioxide than corresponds with the formula H₂S₂O₇, and therefore consists of a mixture of pyrosulphuric acid and sulphuric acid.

The constitution of pyrosulphuric acid is to be regarded as that of an anhydride of sulphuric acid produced by the elimination from two molecules of the latter of a single molecule of water; the structural formula is therefore OH.SO₂.O.SO₂.OH, where the SO₂-group possesses the same character as in sulphuric acid.

The estimation of pyrosulphuric acid is generally effected by the addition of water to a definite quantity and determination of the amount of sulphuric acid formed. Various special forms of apparatus have been suggested to obviate loss of the trioxide during the earlier manipulations.

The amount of free trioxide in " fuming sulphuric acid " can be estimated by titration with water in a stoppered cylinder. The water is added until no further fuming occurs when air is blown through a glass tube on to the liquid surface. Each molecule of water added indicates the presence of one molecule of sulphur trioxide, thus:

H₂O + SO₃ = H₂SO₄.

A thermometric method of evaluating the "fuming acid" consists in adding excess of standard 80 per cent, sulphuric acid and titrating back with the "fuming acid"; the temperature of the solution rises with each addition of the "fuming acid" until the water content of the standard is exhausted.