40 ADVENTURES IN RADIOISOTOPE RESEARCH 



ation potential was measured by using a normal hydrogen electrode. 

 A constant A^alue of the polarization potential was established after 

 about 30 min. The result of the measurements is evident in Table 4. 



Tablk 4. — Cathodic Polarizability of Platintm Deposits Obtained 

 FROM Solutions with Different Lead Contents. Polarization Current 



Density 20 mA/cm^ 



Lead content of the 

 electrode (%) 



Polarization potential 

 (mV) 



7.1 1.5 0.1.3 0.035 



84.4 77.5 81.0 97.9 103.7 



The least polarizable and, therefore, ihe one of highest quality is 

 platinum black with a lead content of 1.5 per cent, and it is interesting 

 to notice that this sample of platinum black is identical w-ith that 

 obtained by electrolysing a solution containing 1 part of platinum chloride 

 and 0.008 parts of lead acetate in 30 parts of water, and that Lummer 

 and KuRLBAUM a long time ago used the electrolysis of a solution of 

 Ihis composition for preparing platinum black. This set of directions is 

 also included in the Textbook of Practical Physics by Kohlrausch and 

 other similar works. 



Attempts were then made to heat the electrodes before they were 

 polarized. In all instances the heating spoiled the quality of the platinum 

 black. After heating the electrode containing 1.5 per cent lead for 16 hr 

 at about 610°C the polarization potential rose from 77.5 to 88.3 mV, 

 and after 16 hr heating at about 700° it became 183 mV. 



( -hanging the current density from 10 to 30mA/cm2 when preparing 

 the platinum black had no detectable effect on the quality. 



CONNEXION BETWEEN THE PARTICLE SIZE AND QUALITY OF 



PLATINUM BLACK 



The particle size of the platinum black was determined from the half 

 breadth of the X-ray lines in accordance with Brill's method^. The 

 Debye-Scherrer camera used for this purpose had a diameter of 5.73 cm. 

 The diameter of the platinum wire covered with platinum black was 

 0.34 mm. Lines of the (220) and (311) faces were used for the investigation. 

 The results of this study are shown by the data in Table 5. 



IK. JiRiLL, KuUuid Z. 55, 104 (1931); Z. Krist. 74, (147 (1930). 



