42 ADVENTURES IX RADIOISOTOPE RESEARCH 



fit tempted to electrolyse hydrochloric acid solutions of pure platinum 

 chloride at a high current density, i. e. at 100 mA/cm^ and above. With 

 this heavy loading it was no longer possible to obtain an adherent deposit. 

 The deposited platinum powder fell into the solution, harl a black-grey 

 appearance and was extraordinarily fine grained. 



Summary 



The platinum black obtained from platinum chloride solutions containing lead 

 in accordance with the directions of Lummer and Kurlbaum, contains con- 

 siderable amounts (1.5 per cent) of lead. Variation of the lead content of the 

 platinum black with the lead content of the solution subjected to electrolysis 

 was observed. 



The electrolysis of a solution containing 1.9 per cent of lead acetate yields 

 a platinum black which contains 7 per cent of lead. By assuming the validity 

 of Vegard's additive law for the lattice dimensions it is found that the greater 

 part of the lead present in platinum black does not occur in solid solution. 



Of the various samples of platinum black the one prepared in accordance 

 with the directions of Lummer and Kuklbaum was the least electrolytically 

 polarizable and thus the most perfect. A determination of the paiticle size of 

 the platinum black samples by means of the half-width of the X-ray interferences 

 resulted in the fact that the best platinum black sample had the largest particle 

 size. 



Platinum, black was also prepared from platinum chloride solutions which 

 contained other added metals instead of lead. 



