8 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



METHOD OF MEASUREMENT. 



The solution-tension of the iron was measured as its electromotive force 

 in a normal solution of ferrous salt, in this case ferrous sulphate. The 

 sulphate ion is desirable here, since it is neutral as regards passivity phe- 

 nomena, having neither passivizing nor activizing effect. 9 The voltage of 

 the cell Fe, FeS0 4 , w/ioKCl, HgCl, Hg, was measured by the well- 

 known compensation method of Poggendorff. 10 



A d'Arsonval galvanometer served as zero instrument, being sensitive to 

 o.ooi volt with the resistance used. A Helmholtz cell was used as a standard 

 of comparison. Its exact value was 0.993 international volt, as determined 

 by a comparison with carefully prepared cadmium normal elements. These 

 agreed accurately among themselves, and were assumed to possess at 

 21.50 C. the value 1.0185 volts. They agreed also with the standards of 

 the Jefferson Physical Laboratory. 



As a standard electrode the mercury-calomel-potassium chloride electrode 

 first suggested by Ostwald 11 was considered the best for the reasons he so 

 convincingly puts forward in his argument with Wilsmore. 12 In the present 

 research tenth normal instead of normal potassium chloride was usually 

 used as the electrolyte. All values of electromotive force are given for the 

 whole cell, Fe, FeS0 4 , KC1, HgCl, Hg, a definite measurable quantity, 

 instead of for the single potential difference Fe, FeS0 4 , because of the uncer- 

 tainty as to the absolute value of the decinormal electrode. If 0.56 volt is 

 supposed to be the value of the normal calomel electrode, the value 0.612 

 volt for the tenth-normal electrode should be used, as recently determined 

 by Sauer 18 and verified by us. Subtracting this from the values given, a 

 value comparable to the commonly accepted values of single potential differ- 

 ences will be obtained. 



A special arrangement of the decinormal electrode was adopted, in order 

 to prevent its contamination with iron. To the connecting-tube, used for 

 immersion in the ferrous sulphate cell, was attached above a siphon-inlet 

 from a large bottle of decinormal potassium chloride on the shelf above. 

 The fact that it was possible to wash out the tube of the decinormal elec- 

 trode by means of this fresh solution removed the possibility of variation 

 through the backward diffusion of normal ferrous sulphate or other electro- 



8 Sackur, Z. f. Electrochem., 10, 843 (1904). 

 10 Ostwald-Luther, Physico-Chemische Messungen, p. 367. 

 "Lehrbuch, 2, 945 (1893). 



12 Z. f. Physik., ch. 35, 333 (1900) ; ibid., 30, 91 (1901). 



18 Z. f. Physik., ch. 47, 184 (1904). That is, N/10 electrode = 0.612 volt if the N/l 

 electrode is called 0.560 volt. 



