430 hostetter: electrolytic reduction of iron 



oxide and persulfuric acid. The amount of peroxide formed at 

 the cathode by the action of nascent hydrogen on oxygen diffusing 

 from the anode depends partly on the cathode material.^ Thus 

 Pd, under given conditions, yields most hydrogen peroxide, Pt, 

 and Hg come next, while metals, which do not readily absorb 

 hydrogen, as Au, Ag, Zn, Cu and Pb yield but very little. With 

 oxidizable anodes, such as Mg, Zn, etc., hydrogen is the sole 

 product of the anode.* In the presence of ferrous sulfate, however, 

 the amount of hydrogen peroxide formed in the cathode chamber 

 must necessarily be low. With the reducing action of the current 

 any peroxide would oxidize the ferrous sulfate to ferric and this, 

 in turn, would be reduced by the current. This was tested out 

 experimentally with 1 mg. of iron present and found to be true. 

 Therefore, the time of electrolysis after complete reduction can 

 produce no excess consumption of permanganate due to peroxide 

 formation. 



Considering the above facts, it was decided to use a gold dish 

 of 800 cc. capacity as cathode (effective surface, 200 sq. cm.). 

 Inasmuch as soluble anodes yield no oxidizing materials in the 

 solution, Mg and amalgamated Zn anodes were first employed. 

 By this means, the reduction is made very rapid and the use of 

 a porous cell around the anode is dispensed with. Since, however, 

 it is difficult to obtain these metals free from impurity, such as 

 iron — thus introducing a correction — the use of soluble anodes 

 was abandoned and a Pt anode with porous cell substituted. 



The use of a porous cell as an anode chamber introduces two 

 sources of error: (1) The absorption of iron salt by the cell itself; 

 (2) diffusion of iron into the anode chamber where it is immedi- 

 ately oxidized. The first is probably negligible since the cell 

 "seasons" rapidly. The second is made neghgible in the follow- 

 ing manner. W^hen the reduction is complete in the cathode 

 chamber — as shown by test for ferric iron with thiocyanate — the 

 current is stopped, the contents of the anode chamber poured 

 into the cathode chamber and, after filling the anode chamber 

 with fresh acid, the current is allowed to run some 5 to 10 minutes 



' M. Traube, Ber. 15: 2434. 1882. 



' Cf. Elsasser, Ber., 9: 1818. 1876; 11: 587. 1878. 



