BURGESS AND SALE: PURITY OF PLATINUM WARE 



283 



The most exact method for the determination of purity of 

 platinum appears to be by measurement of its temperature 

 coefficient of electrical resistance, which quantity has a mean 

 value of about 0.00391 per degree Centigrade for the interval 

 0° to 100°C. for the purest obtainable platinum, and decreases 

 with the addition of anything to the platinum. This measure- 

 ment can be made conveniently and exactly only with wires 

 and is therefore of little interest for the determination of the 

 pm-ity of platinum ware such as crucibles. 



The thermoelectromotive force of plati- 

 num against many of its alloys has also 

 been determined with considerable exact- 

 ness. - 



This property may evidently be made 

 use of, therefore, in devising a method for 

 the determination of platinum purity and 

 one that possesses, furthermore, the advan- 

 tages of accuracy, speed, convenience, and 

 preservation intact of the objects tested. 



The method as developed for use with 

 crucibles is shown in figm'e 1. To the rim 

 of the crucible C are arc-soldered two pure 

 platinum wires, at e and /, of small dia- 

 meter (0.1 or 0.2 mm.); these wires are 

 connected to an ordinary pyrometer gal- 

 vanometer or millivoltmeter G; the junction e is heated by a 

 small oxy-gas or other blast flame from d and the junction / 

 is kept cool by an air blast c; a sheet of asbestos A, cut as shown, 

 serves to prevent radiation from the heated portion of the cru- 

 cible reaching the cold junction/. 



Temperatures are measured by means of a 90 Pt — 10 Rh, Pt 

 thermocouple using the cold junction as above and a Pt - Rh wire 

 arc-soldered to the crucible near c, most conveniently adja- 

 cent to (0.5 to 1 mm. distance) but not touching the Pt wire 

 at e. 



^ ^ LEGEND >l> 



A- a.soestos a-icommulator a- ^33 tube 

 C-cruC(ble b-* po\es e-|t^errno€l€cInc 

 G - ^aJvdnoroeVc -itr ttte f-* lunctiona. 



Fig. 1. 



2 See in particular: W. Geibel, Zs. Anorg. Ch. 69: 38. 1910; 70: 240. 1911; 

 Burgess and LeChatelier, The Measurement of High Temperatures, 3d ed. 171, 

 1912. 



