126 ON THE OCCLUSION OF HYDROGEN AND OXYGEN BY PALLADIUM. 



Whilst the heat of occlusion of hydrogen in palladium is less than the corresponding 

 number for platinum, the heat of absorption of oxygen is slightly greater. It will be 

 remembered that the heat of absorption of oxygen in platinum black, viz., -f 176 K. 

 per gram atom, is almost identical with the number given by JULIUS THOMSEN for 

 the heat of formation of platinous hydroxide, Pt(OH) 2 , viz., + 179 K., thereby 

 furnishing presumptive evidence that the absorption of oxygen by platinum black is 

 a true oxidation phenomenon, the water necessary for the formation of the hydroxide 

 being always present in platinum black. 



It therefore remains to be seen whether there is any sort of agreement between 

 the numbers representing the heat of absorption of oxygen by palladium black and 

 the heat of formation of an oxide or hydroxide of palladium. 



JOANNIS (' Comptes Rend.,' vol. 95, p. 295) determined the heat of formation of 

 palladous hydroxide, Pd(OH)., (oxide?), by precipitating the substance from a solu- 

 tion of (1) potassium palladobromide, (2) potassium palladochloride. by means of 

 caustic potash, and obtained the values + 193'4 K. and + 208'8 K., the mean of the 

 two being about + 200 K. 



JULIUS THOMSEN (' Thermochem. Untersuch.,' vol. 3, p. 436) also determined the 

 same heat of formation by precipitating a solution of sodium palladochloride by 

 caustic soda, and gives as its value + 227 K., thus : 



Pd + + H 2 O = Pd(OH) 3 + 227 K. 



There is thus a discrepancy of about 27 K. between the results obtained by the 

 two observers. 



The heat of formation of palladia hydroxide was also determined by THOMSEN in a 

 similar way, with the following result : 



Pd + 2O + 2H 2 O = Pd(OH), + 304 K. 



This number refers to 32 grams ot oxygen ; for 16 grams it therefore becomes 

 + 152 K. 



The value found by us for the heat of absorption of oxygen in palladium black lies 

 intermediate between the values given by THOMSEN for the heats of formation of 

 palladous and palladic hydroxide, and, considering the difference between the 

 numbers representing the heat of formation ot the lower compound as determined by 

 THOMSEN and JOANNIS, the heat of absorption of oxygen in presence of sufficient 

 moisture may be consistent with the formation of either the higher or lower 

 hydroxide, or possibly with a mixture of both. In any case, it is of the same order 

 of magnitude, and taken in conjunction with the behaviour of palladium black when 

 heated in an atmosphere of oxygen, is undoubtedly in harmony with the view that 

 the absorption of oxygen by palladium black is a true phenomenon of oxidation. 



