Pentathionates

The Pentathionates are salts that are unstable, and even those of the alkali metals are so difficult to prepare as to arouse a suggestion that the acid H₂S₅O₆ had no real existence and that the so-called pentathionic acid was a colloidal solution of sulphur in aqueous tetrathionic acid. The definite existence of the acid was finally proved by the isolation of solid pentathionate by the cautious addition of a solution of hydroxide of barium or potassium to excess of the acid solution, when the tetrathionate crystallised first, followed by a deposit of pentathionate. In order to avoid further the harmful action of alkali on the free acid, Debus used the acetates of the metals in place of the hydroxides and recrystallised the pentathionate from water containing a little sulphuric acid. As potassium pentathionate is less dense than potassium tetrathionate, whilst a mixture of bromoform and xylene of specific gravity 2.2 has an intermediate density, it has been found possible to separate the potassium salts by addition of this liquid mixture, when the tetrathionate sinks and the pentathionate rises to the surface.

The pentathionates are not stable, decomposing spontaneously alone or in aqueous solution with gradual formation of tetrathionate, trithionate and sulphur, the change being retarded by the addition of a little hydrochloric or sulphuric acid. When heated in the dry condition they yield sulphate, sulphur and sulphur dioxide:

2K₂S₆O₆ = 2K₂SO₄ + 2SO₂ + 6S.

Hot alkaline solutions cause the pentathionates to undergo rapid conversion into thiosulphate, whilst with cold solutions some free sulphur is deposited, a corresponding amount of sulphite being produced, together with the thiosulphate:

2K₂S₅O₆ + 6KOH = 5K₂S₂O₃ + 3H₂O.

Sodium carbonate causes deposition of sulphur and formation of tetrathionate:

K₂S₅O₆ → K₂S₄O₆ + S,

but the reaction is not quantitative as stated by Raschig, and also proceeds beyond this stage.

Sulphites give tetra- and tri-thionates and thiosulphate,

S₅O₆'' + SO₃ ⇔ S₄O₆'' + S₂O₃'',

the decomposition of the tetrathionate proceeding as already described. The reaction with bisulphites is similar, tetrathionate first being produced.

The action of hydrogen sulphide and of sulphur dioxide has already received mention under the description of the acid.

Potassium amalgam reduces an aqueous pentathionate solution to tetrathionate and thiosulphate. Metallic copper and silver are blackened by pentathionate solutions.