378 BURGESS AND SALE: QUALITY OF PLATINUM WARE 



the oxide layer is approximately constant for various sized sam- 

 ples from small iron tubes to 100 pound rails, and increases rapidly 

 with temperature, rising to about 100° at an outside temperature 

 of 1100°C. 



PHYSICAL CHEMISTRY.— A study of the quality of platinum 

 ware.^ George K. Burgess and P. D. Sale, Bureau of 

 Standards. 



There has been devised a simple, thermoelectric method 

 suitable for the determination of the purity of platinum ware.- 

 This method of analysis does not mar the article tested and 

 gives data for the classification of platinum in terms of its equiv- 

 alent iridium (or rhodium) content. 



There were examined by this thermoelectric method 164 

 pieces of platinum ware of which 26 per cent contained less 

 than 0.5 per cent iridium and 67 per cent less than 2 per cent of 

 iridium. Of 84 crucibles 36 per cent contained less than 0.5 

 per cent iridium and 87 per cent less than 2 per cent iridium. 



A method has been developed for determination of the exact 

 loss on heating of platinum crucibles, by means of a suitable 

 electric furnace containing no heated metal parts. 



Fourteen crucibles of various makes and grades were examined 

 for loss in weight on heating and after acid treatment following 

 each heating. Their magnetic susceptibilities were also deter- 

 mined. The susceptibility of pure platinum is zero and the 

 range of susceptibility of seven '' platinum" crucibles was found 

 to be 1 to 125. The value usually given for the magnetic sus- 

 ceptibility of pure platinum is about 20 in the same units. 



A summary of the results on losses in weight of a series of 

 crucibles is shown' in figure 1 in which each letter refers to a 

 crucible. As abscissae, are plotted the losses in mg. per 100 cm.^ 

 for 6 hours at 1200°C. and as ordinates the EMF developed 

 against pure platinum at 1100°C.; there is also indicated on the 



^ To appear in full as a Scientific Paper of the Bureau of Standards. Read 

 before the American Chemical Society, April 2. 1915. 

 " This Journal, 4: 282. 1914. 



