Mixter — Heat of Combination of Acidic Oxides. 131 



Chromium. 



The thermal constants of chromium are of considerable im- 

 portance. In Landolt and Bernstein's Physikalisch-Ohemische 

 Tabellen, p. 139, the statement is made : Die Bildungswaerme 

 der Chromverbindungen kann nicht angegeben werden, weil 

 keine Reaction untersucht wurde, an der metallische Chrom 

 betheiligt ist. Then, too, it is an interesting element to study, 

 forming basic and acidic oxides, both of which yield stable 

 salts. Since chromium and its sesquioxide do not burn in 

 oxygen, it is necessary to resort to indirect methods in deter- 

 mining the thermal constants of chromium compounds. This 

 has been done in solutions by Thomsen, Berthelot, and others. 

 The sodium peroxide method is a better one, and the reactions 

 are 



Cr + 3Na 2 <> 2 = Na 2 Cr0 4 + 2Na 2 

 Cr 2 O s + 3Na 2 2 =z 2Xa 2 Cr0 4 + Na 2 



.No perchromate is formed as shown by the following result: 

 A mixture of two grams of chromium trioxide and 6 grams of 

 sodium peroxide was placed in an ignition tube closed with a 

 calcium chloride tube to absorb escaping water. On heating 

 gently the mixture glowed. The loss in weight was 0*3S6 

 gram; calculated 032 gram. If perchromate had been formed, 

 less oxygen would have been given off. Moreover, it is im- 

 probable that sodium perchromate can exist in a molten mass 

 containing sodium oxide. 



To Dr. C. H. JMathewson I am indebted for a tine specimen 

 of crystalline chromium made at the Goldschmidt factory. The 

 metal was pulverized in a steel mortar and the powder was 

 digested with hydrochloric acid to remove the iron. Analysis 

 proved it to be free from aluminium and silicon and to con- 

 tain 0*7 per cent of iron. The last may have been from the 

 mortar. Metallic chromium as a very line powder will un- 

 doubtedly burn readily with sodium peroxide, but heat was 

 necessary to effect the reaction with that used, and carbon was 

 therefore added to the mixtures. After a combustion the 

 product was dissolved in water, the solution made acid with 

 nitric acid and the metal remaining was separated and weighed. 



Experiments. 



1 2 3 



Metal taken 2-000 2'5000 2*500 grams 



" unburned 0061 0-0043 0*021 " 



" burned 1*939 2*4957 2-479 " 



Chromium 1*925 2-4782- 2*4615 " 



Iron 0*014 00175 0*0175 " 



Carbon 0*2456 0*3767 0*3800 " 



Sodium peroxide 20* 20* 20*5 " 



Water equivalent of system 3428' 3514* 3487* " 



Temperature interval.' 2*601° 3*510° 3*520° 



