38 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



made the cathode of a dense current in an acid solution for an hour was 

 quickly washed with neutral ferrous sulphate, and was measured as soon as 

 possible. In half a minute after immersion it had sunk to 0.822 volt, in four 

 minutes later 0.800, and in an hour to constancy at 0.790. Evidently the 

 hydrogen is only very superficially deposited, and most of it is gone before 

 measurement is possible. Longer exposure as cathode might have more 

 effect. Much more striking results were obtained from electrolytic iron 

 obtained from a neutral solution with the help of a current so dense as to 

 deposit hydrogen with the iron. Such a specimen (sample 48), measured 

 about three seconds after the breaking of the current, gave a potential of 0.93, 

 equal to that observed in the quenched iron. In a minute this had sunk to 

 0.85 volt ; but even after an hour it remained as high as 0.83 volt, not sinking 

 to about the normal value 0.798 for several days. 



Thus it is clear that as far as potential measurements can show, the 

 hydrogen driven into iron with the help of electrical energy is essentially 

 similar to that absorbed in the act of quenching, although the latter operation 

 seems to be especially favorable to the occlusion. Probably the action of 

 the red-hot core upon the steam produced by the exterior of the porous metal 

 supplies the nascent hydrogen to the iron at a temperature especially suitable 

 for occlusion. 



It is interesting to note that even without outside electrical assistance 

 hydrogen in this active form may be taken up by iron from acid solutions. 

 Upon immersing the metal in an acid solution of ferrous sulphate, hydrogen 

 is of course evolved, and the potential of the iron while still in the acid is 

 lowered as much as two decivolts evidently by the coating of gas, for 

 hydrogen gas has a potential much lower than iron. But besides this super- 

 ficial effect, a more deep-seated one is occurring, as is easily shown. 



Upon removing this iron quickly to a neutral solution of ferrous sulphate 

 and measuring at once, its potential is found to be above the normal value, 

 the usual cell often reading as high as 0.83 instead of 0.78 or 0.79. In a 

 few hours it settles down as usual to its normal level, evolving in the process 

 bubbles of hydrogen gas. The iron alone could not of course raise the active 

 hydrogen which it absorbs to a concentration above that capable of giving 

 an electromotive force equal to its own (0.79 with the decinormal electrode), 

 but with the help of the osmotic pressure of the ionized hydrogen in the 

 acid more hydrogen is driven in. This excess manifests its potential and 

 changes to hydrogen gas when the surrounding acid is removed. As a 

 change of only tenfold in the concentration of the ionized hydrogen would 

 be expected to produce a change of potential of nearly 0.06 volt, the observed 

 effect is by no means excessively large. Stated in another way, it may be 



