248 J. M. Lewis.- 



solution is a matter of great importance since serious errors 

 would be introduced if reaction occurred between it and that 

 in the electrodes. Some error is inevitable where solutions 

 which are not identical are in contact, and in work which neces- 

 sitates extreme accuracy, due allowance is made for this, but 

 for routine biochemical estimations " contact potential " may- 

 be ignored. This has been made possible by the use of a satur- 

 ated solution of KG as a connecting fluid. In such a solution 

 the speed of the dissociated ions is equal or nearly so; further- 

 more, where the saturated calomel electrode is employed, the 

 solution is identical with that of the electrode, in which case 

 error can occur only at the point where the fluid in the hydrogen 

 electrode comes into contact with the connecting solution. For 

 more detailed information the student is referred to the article 

 by Michaelis. (7) 



The Hydrogen Electrode. 



The hydrogen electrode consists of a small element of pure 

 platinum, or sometimes gold, which has had a layer of " plati- 

 num black " deposited upon it, and great care must be taken 

 that the electrode is at all times completely platinised, and is 

 not contaminated. The form given to the electrode varies ac- 

 cording to the choice of the worker; thus it may be a small 

 rectangular piece of foil, as used by Hildebrand (10) and 

 Clark (16), or again it may be simply a fine platinum wire, the 

 form preferred by Walpole (17) and Barendrecht (13). Un- 

 der any circumstances, the electrode, when saturated with pure 

 hydrogen, is placed in the solution in such a way that while part 

 of it is immersed, a considerable portion is still exposed to the 

 gas. Willsmore carried out his measurements with a rectangu- 

 lar oiece of foil, which dipped for a considerable distance into 

 the solution (19). It has been shown, however, that a steady 

 e.m.f. is obtained more quickly when there is a minimum con- 

 tact between the electrode and the solution. The reason for 

 this is not at once apparent, but it is supposed to depend on 

 certain peculiar physical conditions at the surface of the liquid, 

 whereby a rapid diffusion from the surrounding solution to the 

 point of contact is prevented, the local conditions maintaining 

 an equilibrium, which is not quickly upset (16). As minimum 

 contact is easily obtained with the electrode devised by Wal- 

 pole, this electrode, modified by Barendrecht, has been most 



