rLATIXlM BLACK 



39 



obtained l)y llie electrolysis of solutions containing lead acetate, labelled 

 with thorium B, was determined before anrl af1(>r heating. The results 

 of this experiment are seen in Table 3. 



Table 3. — Effect of Heating for 16 hr in a Vacuum on the Lkad- 



CONTENT OF PlATINUM BlACK ORIGINALLY CONTAINING 1.5% LeAD 



The loss of lead is not entirely due to evaporation but partly also to 

 diffusion of the lead contained in the platinum black into the platinum 

 foil on which the coating was deposited. 



Differentiation between the loss by evaporation and diffusion, is 

 possible by making use of the y-radiation instead of the a-radiation 

 for making the comparison ; whereas the amount of lead removed by 

 diffusion weakens the a-radiation to the same extent as does the lead 

 disappearing through evaporation, this is is not the case when the 

 y-activity is measured. For example, the decrease in y-radiation after 

 16 hr heating at 685 to 700°(" amounted to only 42 per cent and thus 

 considerably less than the decrease in a-radiation. 



THE QUALITY OF THE VARIOUS SAMPLES OF PLATINUM BLACK 



An attempt was next made to measure the easily traced adsorption 

 of thorium B and thorium C from solutions of these radio-elements with 

 a vieW' to assessing the quality of the platinum black. Yet great difficulty 

 was encountered in obtaining reproducible results. This method was 

 therefore relinquished. It was then thought that a simple measure of the 

 quality of the various platinum black coatings could be obtained by 

 preparing hydrogen electrodes from the various platinum black samples 

 and comparing their potentials. It was shown, however, that the potential 

 of all the hydrogen electrodes prepared in this way m' as always the same 

 within the experimental error of about 1 mV. We then changed over to 

 determining how strongly the various samples of platinum black could 

 be polarized with the same cathodic loading and to making use of the 

 difference in polarizability as a measure of the quality of platinum black. 

 The polarization was performed in N sulphuric acid solution with a 

 current density of 20 mA/cm^ at room temperature for a period of 45 

 min ; the area of one side of the electrode amounted to 1 cm^. The polariz- 



