36 OXIDATION-REDUCTION POTENTIALS 



finally to earth. With this simple arrangement the potentials of eight cultm-es may 

 be read in four minutes. With rapidly growing cultures during the logarithmic 

 phase of proliferation, half-hourly readings of the potential should be taken, but in 

 cultures which are continued for several days readings every few hours may be 

 sufl&cient. 



It is frequently desirable to obtain a record of the changes in electrode potential 

 throughout the life history of a culture. Unfortunately the active life of a culture 

 is rarely limited to the length of a normal working day so that an automatic recording 

 apparatus would be of considerable advantage to the investigator. Bacterial cultures 

 are, however, very susceptible to polarisation effects, so that such an automatic 

 apparatus requires careful design. Automatic recording was described by Cannan, 

 Cohen and Clark (1926), but might be improved by the use of modern apparatus. 



Summary of Chapter II 



The general principles and methods of application of dyes to the study of 

 electrode potentials are described together with the relevant data of a number of 

 dyes suitable for use as oxidation-reduction indicators. The serious difficulties 

 attending the use of dyes are mentioned, and the methods and apparatus for the 

 direct potentiometric determination of electrode potentials are given with full 

 experimental details. 



