Originalh published in Z. phys. Chem. A 171, 41 (1934) 



2. PLATINUM BLACK 



G. Hevesy and T. Somya 

 From the Institute of Physical Chemistry, University of Freibuig 



For the preparation of platinum black, which is used in hydrogen 

 electrodes and for other purposes, the electrol3^sis of hydrochloric acid 

 solutions of platinum containing lead acetate is employed. The question 

 then arose as to whether the presence of lead in the solution is essential 

 to the preparation of good platinum black and. if so, as to the part 

 played by the lead. To obtain an answer to the first question, we have 

 electrolysed both hydrochloric acid solutions containing only platinum 

 chloride and others containing also small quantities of lead. It was 

 shown that platinum black cannot be obtained successfully b}^ the 

 electrolysis of a solution containing only platinum. On the contrary, 

 a grey or light brown deposit is always obtained. On the other hand, 

 the preparation of platinum black is accomplished from solutions which 

 contain a corresponding amount of other heavy metals in place of 

 lead. After this observation we proceeded to study whether lead is 

 carried into the deposit when a solution containing lead is electrolysed 

 and, if so, in what amount and form. 



DETERMINATION OF THE LEAD CONTENT OF PLATINUM BLACK 



Since the detection of small quantities of lead in platinum is very 

 tedious we have made use of a radioactive tracer method. A known 

 amount of lead acetate labelled with thorium-B was added to the 

 platinum chloride solution and the lead content of the platinum black 

 deposited on platinum electrodes having a surface area of 15.07 cm^. 

 at a current density of 10 mA/cm'-. was determined by measuring the 

 intensity of the a-radiation emitted by the deposit. The amount of lead 

 was calculated from this intensity measurement as follows : From the 

 same radioactive lead acetate solution, of which a known volume w^as 

 added to the platinum chloride solution, lead peroxide was precipitated 

 (see below) after adding nitric acid and a further amount of inactive 

 lead acetate and the a-radiation of this precipitate was compared with 



