Percarbonic Acid

Per-acids are derivatives of hydrogen peroxide, and, like that substance, contain a chain of two oxygen atoms. This chain may, however, occupy an internal or an external position with reference to the rest of the molecule; the difference is illustrated by perdi- and permono-sulphuric acids, in the former of which the oxygen chain is internal, in the latter necessarily external, one only of the two hydrogen atoms of H₂O₂ being replaced by an acidic group, thus:

Perdisulphuric acid:

Permonosulphuric acid:

It may be remarked that the former is prepared electrolytically and results from the union of two -OSO₂OH groups at the anode, whilst the latter is obtained by chemical means.

Two percarbonic acids may similarly be supposed to exist:

Perdicarbonic acid, H₂C₂O₆:
Permonocarbonic acid, H₂CO₄:

The first formula may be applied to the percarbonate obtained by electrolysis; a percarbonate of different constitution results from the union of a superoxide and carbon dioxide, to which the second formula may be applied. It may be noted also that an isomer with the former acid is possible, viz. .

Potassium percarbonate, K₂C₂O₆, was first prepared by Constam and von Hansen, in 1897-8, by the electrolysis of a concentrated solution of potassium carbonate at -10° C. to -15° C., and by subsequent treatment was obtained nearly pure. The following reaction appears to have taken place:

2KOCOOK → 2K +

and the product was a bluish-white, amorphous powder, stable at atmospheric temperature in the dry state, but decomposed by cold water with the evolution of oxygen, which comes off in a continuous stream at 45° C. It is important to note that sodium percarbonate could not be prepared by electrolysis because the carbonate does not yield a sufficiently strong solution with water.

Alternative methods, however, of preparing sodium percarbonate are by the interaction of sodium carbonate and hydrogen peroxide, or of sodium peroxide and carbon dioxide. Tanatar employed the former method, and isolated two salts having the composition 2Na₂CO₄.3H₂O and 2Na₂CO₄.H₂O₂.2H₂O, but was in doubt whether these were true percarbonates or compounds of sodium carbonate with hydrogen peroxide. By the interaction of solid hydrated sodium peroxide and solid carbon dioxide Bauer obtained a percarbonate of the composition Na₂CO₄, according to the reaction:

Na₂O₂.8H₂O + CO₂ = Na₂CO₄ + 8H₂O.

By means of hydrogen peroxide Kasanezky has prepared the potassium salt 2K₂CO₅.5H₂O or 2[(KOO)₂CO].5H₂O and the ammonium salt NH₄00COONH₄.2H₂O; whilst Wolffenstein and Peltner, by the interaction of sodium peroxide and carbon dioxide, obtained not only the salt 2Na₂CO₄.3H₂O, previously prepared by Tanatar, but "sodium dioxide dicarbonate" Na₂C₂O₆; and in other ways "sodium trioxide dicarbonate" NaHCO₄ or Na₂C₂O₇, another salt isomeric with this, and "sodium trioxide carbonate" Na₂CO₅. Subsequently Peltner obtained the following rubidium salts from the carbonate and hydrogen peroxide: Rb₂CO₄.2H₂O₂.H₂O, Rb₂CO₄.H₂O₂.2H₂O, and 2Rb₂CO₄.5H₂O; and shortly afterwards Riesenfeld and Reinhold, after preparing pure potassium percarbonate, K₂C₂O₆, by electrolysis, attacked the problem which was pressing in view of the supposed existence of all the above salts, as to whether a true percarbonate could be distinguished from a compound of a carbonate with hydrogen peroxide.

These authors found that the electrolytic percarbonate immediately liberates iodine from a cold solution of potassium iodide without loss of oxygen according to the equation:

C₂O₆'' + 2I' = 2CO₃'' + I₂.

This reaction is not due to the previous liberation of hydrogen peroxide, since this substance reacts but slowly with potassium iodide. Tanatar's percarbonate, however, prepared from carbonate and hydrogen peroxide, liberates oxygen when mixed with neutral potassium iodide solution, and was therefore supposed to be a compound of carbonate and hydrogen peroxide. At the same time doubt was thrown upon the nature of Wolffenstein and Peltner's percarbonates. Thereupon ensued a controversy between Riesenfeld on the one hand and Tanatar and Wolffenstein on the other; at the end of which Riesenfeld and Mau maintained that a true percarbonate can be distinguished from carbonates containing hydrogen peroxide of crystallisation by the quantitative liberation of iodine from neutral potassium iodide solution, and subsequently classified carbonates containing peroxidic oxygen in the following way: (1) carbonates containing hydrogen peroxide of crystallisation, e.g. 2Na₂CO₃.3H₂O₂; (2) monoperoxy-carbonates, e.g. Na₂CO₄; (3) percarbonates (monoperoxy-dicarbonates), e.g. Na₂C₂O₆. The latter salt, prepared from sodium peroxide and carbon dioxide, since no sodium percarbonate can be prepared by electrolysis, differs in its behaviour towards potassium iodide, and therefore in constitution, from electrolytic potassium percarbonate; consequently Riesenfeld and Mau prepared a potassium percarbonate from potassium peroxide and carbon dioxide, and found that it possessed the composition K₂C₂O₆, and resembled Na₂C₂O₆ in chemical reactivity. Thus two isomeric potassium percarbonates were found to exist - one prepared by electrolysis which was believed to have the constitution KOCOOOCOOK, and the other prepared from potassium peroxide and carbon dioxide to which the constitution KO-O-CO-O-CO-OK was attributed, and which resembled the only existing sodium percarbonate of the same empirical composition.

To the compound Na₂CO₄ the constitution NaOOCOONa was attributed, and consequently electrolytic K₂C₂O₆ and Na₂CO₄ stand in the same relation to one another as perdi- and permono-sulphates.

Thus:

Electrolytic percarbonate:

Monoperoxycarbonate:

Perdisulphate:

Permonosulphate:

Barium percarbonate is formed when carbon dioxide gas is passed through barium peroxide suspended in water; for hydrogen peroxide only appears in quantity in the solution after a considerable time. The gas is first absorbed to form the percarbonate, and when no barium peroxide remains the liquid becomes acid, and the percarbonate is hydrolysed with the liberation of hydrogen peroxide.

Percarbonic acid itself is very unstable, but if electrolytic potassium percarbonate is decomposed by phosphoric acid in presence of ether, the liberated percarbonic acid dissolves in the ether, and from this ethereal solution the potassium salt may be regenerated by adding potassium hydroxide.

Percarbonic acid may be estimated by decomposing its salts with dilute sulphuric acid, and titrating the liberated hydrogen peroxide with permanganate.