Pyrosulphuric acid

Ammonia is evolved here in the condensation process of melamine to melam whereas the respective ammonium salts are evolved in the case of the hydrobromide and nitrate. Reactions 9 and 9b show that ammonia should be neutralised by pyrosulphuric acid groups. In TG we have found that diammonium pyrosulphate eliminates ammonia above 350 C, with however complete decomposition ... 

It is pointed out that although aqueous sulphurous acid appears to contain most of its sulphur dioxide as such,1 yet solutions of the alkali bisulphites appear to contain some pyrosulphite (metabisulphite).2 It is therefore not unreasonable to assume that some free pyrosulphurous acid is present in aqueous sulphurous acid solutions. Since hydro-sulphurous acid is unstable under the prevailing conditions, it does not accumulate, but rapidly undergoes further change, which makes proof of its formation difficult. It is assumed 3 that some of the colourless acid formed in reaction (1) undergoes rearrangement to the isomeric coloured form ... 

Although the substance of the composition H2S207 is capable of definite existence, the commercial products deviate from the composition expressed by the formula and contain variable quantities of sulphuric acid with pyrosulphurie acid or of pyrosulphuric acid with sulphur trioxide. For the purpose of including all varieties of impure pyrosulphurie acid, the somewhat vague terms oleum, fuming sulphuric acid and Nordhausen sulphuric acid are advantageous. 

Physical Properties.—Pure pyrosulphuric acid is a colourless crystalline solid 2 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, (see also p. 167). 

Chemical Properties.—Pyrosulphuric acid, whether pure or mixed with excess of sulphuric acid1 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. 

The fuming sulphuric acid of commerce contains less sulphur trioxide than corresponds with the formula H3S207, and therefore consists of a mixture of pyrosulphuric acid and sulphuric acid. 

Constitution.—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 0H.S02.0.S02.0H, where the S02-group possesses the same character as in sulphuric acid. 

Estimation.—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 trioxidc during the earlier manipulations.1... 

At the ordinary temperature the concentration of the acid must not be much less than 252V for separation of sulphur to occur. It has been suggested, therefore, that it is really pyrosulphuric acid, H2S207, which reacts with the hydrogen sulphide, this acid being present in small equilibrium quantity in the concentrated acid solution. The first stage of the reaction would thus be ... 

In very acid solution, Bassett and Durrant believe that reaction (c) takes place, with formation of a new acid, di -thio - pyrosulphuric acid, HaS405 ... 

The pressure increases slowly at first but after a time a considerable increase suddenly occurs. The anhydride is also formed when the two gases react, only very slowly at ordinary temperatures, but more rapidly at higher temperatures. The reaction is never complete but goes somewhat further in the presence of platinised asbestos.8 The anhydride may also be prepared by the action of liquid nitrogen tetroxide on pyrosulphuric acid or pyrosulphuryl chloride. 

Sulphuric acid dissolves the element to form a red solution, the colour being due possibly to sulphur-tellurium sesquioxide, STe03, which has not been isolated.9 On warming, tellurium dioxide and sulphur dioxide are produced. If the sulphuric acid is hot and concentrated, a red solution is obtained from which tellurium may be separated by the addition of water. This solution is similar to that obtained with pyrosulphuric acid, in which Auerbach has shown that the tellurium is present in monatomic form (see p. 355). If the red solution is boiled for some time, white crystals of pyrotelluryl sulphate, 2Te02.S03, separate. These crystals are soluble in hydrochloric acid and decompose on heating with water.10 A red solution is also obtained with cold anhydrous selenic acid and presumably contains the corresponding selenium-tellurium sesquioxide, SeTeOa.u... 

Pyrosulphites, 131, 132. Pyrosulphurio acid, 144-147. Pyrosulphurous acid, 124. Pyrosulphuryl chloride, 95-97, 301. Pyrotellurates, 387. 

SYNS DISULPHURIC ACID DITHIONIC ACID FUMING SULFURIC ACID OLEUM PYROSULPHURIC ACID SULFURIC ACID, fiming > or =30% free sulfur trioxide (DOT) SULFURIC ACID, fuming <30% free sulfur trioxide (DOT) SULFURIC ACID MIXTURE with SULFUR TRIOXIDE... 

PYROSULFURYL CHLORIDE see PPR500 PYROSULPHURIC ACID see SOI520 PYROTARTARIC ACID NITRILE see TLR500 PYROTROPBLAU see CMO250 PYROXYLIC SPIRIT see MGB150... 

Mixtures of nitric and sulphuric acids contain nitronium sulphates which have been described by Ingold and associates, Woolf and Emeleus(Vo). I,p. 19). Reval-licr and co-workers [10] have found by Raman spectroscopy and vapour pressure measurements, that compounds made by acting with SOj on nitric acid are salts of nitronium ion (NO ) and sulphate anions. Vitse [11] established the structure of the compound N2 0s,4S03 as nitronium ion salt by X-ray crystallography. The salts of pyrosulphuric acid (Vol. 1, p. 12) can be present only in a mixture of nitric acid with oleum or SO3. They are described in the paragraph on nitronium salts (p. 27). The presence of NO in various solutions was discussed in Vol. I, pp. 14- 49. 

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