POTASSIUM METASULPHITE. 485 



formation of these various potassium sulphites, and has found, 

 not without surprise, that the so-called anhydrous bisulphite, 

 far from belonging to the same type as the other sulphites, 

 constitutes in reility, by its chemical reactions and thermal 

 properties, a distinct and characteristic type of a new saline 

 series, viz. the mztasulphites, as distinct from the sulphites 

 properly so called, as the metaphosphates and pyrophosphates, 

 for example, are from the normal phosphates. 



Pure potassium metasulphite is obtained by saturating with 

 sulphurous acid gas a concentrated solution of potassium 

 carbonate, either warm or even cold, and by drying at 120 the 

 salt which separates by crystallisation. The anhydrous salt 

 already described under the name of anhydrous bisulphite by 

 Muspratt and Mirignac corresponds to the formula S2O5K2. 1 

 This salt is distinguished by its heat of formation, its stability, 

 its tendency to form hydrates, and even solutions distinct from 

 those of the normal bisulphite, and, finally, by its decomposi- 

 tion by heat. In leed, the normal bisulphite prepared in dilute 

 solutions by the saturation of the neutral sulphite by sulphurous 

 acid soon change 5 state in the liquid itself. It is dehydrated, 

 and becomes metasulphite, liberating +2*6 Cal., a fact which 

 accounts for the preponderance of the metasulphite and its 

 definite formation in solutions. 



The dissolved potash, moreover, reduces the metasulphite to 

 the state of neutral sulphite. 



Without dwelling any further here upon the characteristics of 

 the metasulphites, we shall describe the action which heat has 

 upon this one as entering into the scope of the present work. 



4. Decomposition of metasulphite by heat. The action of heat 

 forms one of the most striking characteristics of potassium 

 metasulphite. In fact, dry metasulphite does not lose sulphurous 

 acid even at 150. 



However, if it be brought to a dull red heat, it liberates 

 sulphurous acid, but without regenerating a corresponding 

 amount of neutral sulphite, and even changing in a well-defined 

 and entire manner into potassium sulphate and sublimed 

 sulphur when the reaction is carefully carried out 



2S 2 5 K 2 = 2S0 4 K 2 + S0 2 + S. 



This equation has been verified by accurate measurements. 

 These are characteristic. Sulphurous acid is actually liberated. 

 The volume of this gas indicated by the above formula should 

 be the half of that corresponding to the normal reaction of a 

 bisulphite, such as 



S 2 5 K 2 = S0 3 K 2 + S0 2 . 



1 " Comptes Rendus des stances de 1'Acade'mie des Sciences," torn. xcvi. 

 p. 142, and especially p. 208. 



