1889.] On Selenic Acid and other Selenium Compounds. 



29 



which commenced to boil at 235° was heated until the temperature 

 rose to 325° : 2*1542 grams of the distillate neutralised 3 c.c. of semi- 

 normal soda solution, equivalent to 5"04 per cent, of selenic acid. 

 The residue was then further heated until the greater part of the 

 acid was decomposed and the strength of the distillate taken : 

 1'6176 grams neutralised 16*5 c.c. of seminormal soda solution, equiva- 

 lent to 36*90 per cent, of acid calculated as selenic, but much of the 

 acidity was due to selenious acid, a large amount of which was pre- 

 sent in the distillate. It is thus seen that the acid which distils 

 under ordinary pressures is always highly dilute. 



When anhydrous selenic acid is strongly heated under ordinary 

 pressures a portion of it is decomposed into selenium dioxide, oxygen, 

 and water, the latter serving to dilute the remaining acid. It will 

 then no longer solidify on addition of a crystal of the anhydrous 

 acid. After further heating it becomes sufficiently dilute to distil 

 over in the manner already described. It is instructive to compare 

 with this the action which takes place in the case of anhydrous 

 sulphuric acid. Mariguac found (' Annales de Chimie,' vol. 39, 1853, 

 p. 184), that the latter when heated gives off sulphuric anhydride 

 until the remaining acid is reduced to the same strength as that 

 yielded by a more dilute acid on prolonged ebullition — about 98"66 per 

 cent. In the case of selenic acid the anhydride is evidently unable to 

 exist free at elevated temperatures, but breaks up into selenium 

 dioxide and oxygen. 



It is worthy of note that a remarkable coincidence exists between 

 the temperature at which selenic acid is decomposed and that at 

 which selenium dioxide sublimes. The latter is not stated very 

 definitely in any of the chemical text-books ; we have found, however, 

 that at 280° rapid sublimation takes place, while at 250° volatilisation 

 proceeds slowly. When selenic acid is kept for some time at the 

 latter temperature it is always found to contain traces of selenium 

 dioxide, while at 280° decomposition proceeds with rapidity. It is 

 probable that a connexion exists to a considerable extent between the 

 phenomena, and that the explanation of the coincidence may be that 

 the comparatively weak affinities which bind the molecule of selenic 

 acid are unable to prevent its disruption when once a temperature is 

 reached at which the tendency of its constituents to separate is 

 aided by the expansion attending a change to the gaseous condition. 

 We have seen that both phenomena are affected by the temperature ; 

 they are also dependent to some extent upon the pressure. An- 

 hydrous selenic acid commences to dissociate in a vacuum at about 

 200°. We found that when selenious acid was heated under similar 

 circumstances water was quickly evolved, and after prolonged heating 

 of the residue a distinct white ring of selenium dioxide was deposited 

 on the cold portion of the neck of the flask. 



