30 



OXIDATION-REDUCTION POTENTIALS 



is represented diagrammatically in fig. 6. By means of the potentiometer (P) the 

 E.M.F, (El) tapped off is varied until the galvanometer (G) shows no deflection. At 

 this point the E.M.F. (EJ must just balance the E.M.F. of the complete cell E^-E.t 



P 



Fig. 6 



POTENTIOMETRIC Ejj MEASUREMENT. TqEORETICAI, CiROUIT 



(the negative sign is included since the two half-cells are in opposition). So that at 

 potentiometric balance 



Ei = Ei, -E,t or Eh = E^ -f E3t 



(The salt bridge [S.B.] ensures electrical contact without introducing errors due to 

 liquid : liquid interface potentials, etc.) 



EXPERIMENTAL DETAILS 



The author's methods (Hewitt, 1930, 1) are described in detail, particularly 

 their application to the study of bacterial cultures. The methods described have 

 been found to function reliably, and their description may assist other workers to 

 establish the best conditions for application to their own problems. In working 

 with cultures of pathogenic bacteria it was necessary to work with sterilised glass 

 apparatus, assembled with aseptic precautions, and to sterilise the apparatus after 

 use. The apparatus described was designed with these special objects in view but its 

 simplicity makes it suitable for other biological investigations. 



The Electrode 



The electrode must be, for all practical purposes, unattackable, and must not 

 participate in the oxidation-reduction system in any way, except as an inert carrier 

 of electrons. Although this is possibly an ideal definition, working approximations 

 to it may be obtained in most cases. Electrodes used by different workers are 

 as follows : — 



(1) Platinum. — Plain bright platinum foil or wire has been used by nearly 

 all workers in this field. The author has found bright platinum electrodes 

 the most reliable of those used, and for most purposes uses a platinum wire 

 spiral (s.w.g.26) sealed into glass tubing. The glass tubing is, of course, filled 

 with mercury, and an amalgamated copper wire is dipped into the mercury 

 to connect to the rest of the apparatus. Care must be taken that there are no 

 cracks or defects in the glass tubing. Irregular results may be obtained with a 



