PL.A.TINUM BL.A.CK 



41 



Table 5. — Dependence of the Particle Size of Platinu.m Black on 



ITS Lead Context 



It is evident from this table that the platinum black sample whicli 

 has been found to have the best quality is distinguished by having the 

 largest particle size. 



PREPARATION OF PLATINUM BLACK FROM SOLUTIONS CONTAINING 



GOLD 



It is evident from Table 6 that platinum black of good quality was 

 prepared also from platinum chloride solutions which contained gold 

 instead of lead and was deposited at a current of 30 mA/cm^. 



Table 6. — Cathodic Polarizability of Platinum Deposits Prepared 



FROM Solutions Having Various Gold Contents. Polarization 



C URRENT Density 20 rnA/cm^. Electrolyte 1 N HoSO^ 



We have also prepared platinum black from platinum chloride solutions 

 which contained thallium, cadmium or zinc in place of lead. Whereas 

 thallium can substantially replace lead as far as the appearance of the 

 deposit is concerned, the behaviour with cadmium is different inasmuch 

 as a solution which contained about 0.02 per cent cadmium chloride 

 yielded a fine black deposit while the electrolysis of solutions which 

 contained only about 0.01 per cent cadmium chloride yielded a grey 

 deposit instead of platinum black. Electrolysis of platinum chloride 

 solutions containing zinc yielded a grey deposit in all cases. We then 



