492 Mixter — Formation of the Oxides of Molyhdenum, etc. 



Selenium. 



Determinations were made of the heat effect when gray 

 metallic selenium is burned with sodium peroxide, using 5 to 

 10 grams of the former for a test. The mixture fused imper- 

 fectly. The water solution of the fusions after acidifying 

 with hydrochloric acid gave no precipitate of selenium when 

 sulphurous acid was added, showing that only selenic acid 

 was present. This was confirmed by the fact that a nitric acid 

 solution of the fusions did not decolorize permanganate. The 

 results for 1 gram of selenium were 1216 c , 1208 c , and 1208 c ; 

 average 1211 c . For 79*2 grams it is 95,900 c . 



Two combustions were made of a mixture of crystalline 

 selenium dioxide, sodium peroxide, and sulphur. 13*940 and 

 7*652 grams of selenium dioxide were taken respectively. The 

 results for 1 gram of Se0 2 were 644 c and 588 c . Two other 

 determinations were made, taking 6*118 and 7*109 grams of 

 selenium dioxide and an excess of sodium peroxide. These 

 two fusions were good and oxygen was not liberated, proving 

 conclusively that all of the Se0 2 was oxidized to Se0 3 , and 

 that the reaction was lSTa 2 2 -fSe0 2 =Na 2 Se0 4 and not Na 2 - 

 Se0 3 -f O. The heat effect for 1 gram of Se0 2 was 617 c and 

 637 c . The fusion giving 588 c contained some sodium selenite 

 and the result should be discarded. The average of the 

 remaining three is 632 c and of the two highest results it is 

 640 c . This last number multiplied by 111*2 gives 71*200 c for 

 the heat effect of Ka 2 2 , Se0 2 . 



The results of the combustions of sodium peroxide are as 

 follows : 



3Na 2 2 + Se = Na 2 Se0 4 + 2Na 2 + _. 95-9 c 



3Na o + 30 = 3NaO + --- 58*2 C 



Na 2 + Se + 30 = Na 2 Se0 4 + 154«l c 



Na 2 2 + Se0 2 = Na 2 Se0 4 + n*2 c 



Na 2 + O = Na 2 2 + - 19*4 C 



90'6 C 



The heat of formation of crystalline selenium dioxide derived 

 from these results is 154*1 — 90*6=63*5°. Thomsen's figures are 

 57*l c derived from the heat effect when the dioxide is reduced 

 in hydrochloric acid solution by sodium hydrosulphide and also 

 from the heat of formation of SeCl 4 and its hydrolysis. His 

 results by the two methods are practically identical. Thomsen 

 used amorphous selenium and the writer the grey metallic 

 modification, and as the change of the amorphous form into the 

 crystalline evolves heat the writer's results would have been a 

 little higher had he used amorphous selenium. The reason 



