CLARK: REDUCTION POTENTIALS 255 



PHYSICAL CHEMISTRY. — Reduction potentials of mixtures 

 of indigo and indigo white, and of mixtures of methylene blue 

 and methylene white. '^ W. Mansfield Clark. Dairy 

 Division, Bureau of Animal Industry, United States Depart- 

 ment of Agriculture. 



Oxidation-reduction indicators have been used to some extent 

 in volumetric analysis, but it is in various biochemical studies 

 that they find their widest and most interesting applications. The 

 reduction of methylene blue by bacteria in milk is regarded as 

 one of the first observable evidences of bacterial action. Not all 

 bacteria reduce this dye, and this differential property is there- 

 fore used in the identification and biochemical classification of 

 species. An important class of bacteria seem to require the 

 complete absence of oxygen before they will grow, and since 

 reduced methlyene blue and reduced indigo can be used to detect 

 minute traces of oxygen these indicators have played a most 

 interesting part in the development of conceptions regarding 

 the so-called anaerobic state. Anaerobiosis is a large subject 

 whose various phases have not yet been satisfactorily resolved. 

 There is abundant evidence, however, that one or another phase 

 of it is intimately associated with the reducing tendency of 

 cellular activity in general. The recognition of this tendency has 

 come about in large measure through the use of reduction indi- 

 cators such as methylene blue. It has even been claimed that 

 reduction of this dye by a tissue indicates the presence of life and 

 the failure of reduction indicates the death of the cells. Yet there 

 seem to exist different powers of reduction in various tissues, as 

 was shown in Ehrlich's classic work and confirmed by some of 

 the more recent staining investigations of the histologists. 



So far as the writer is aware such indicators have not been 

 regarded in their possible relation to oxidation reduction poten- 

 tials in a manner analogous to the now well-systematized relation 

 of hydrogen-ion indicators to hydrogen-electrode potentials. 

 That such a relationship if established will aid in the interpreta- 



^ Published by permission of the Secretary of Agriculture. Received February 

 12, 1920. 



