CHAPTER XII 



OXIDATION-REDUCTION POTENTIALS OF DYE SYSTEMS 

 AND THEIR SIGNIFICANCE IN BACTERIOLOGY 



W. MANSFIELD CLARK' 

 Hygienic Laboratory, Washington, D.C. 



INTRODUCTORY 



Various attempts have been made to trace the history of our knowledge of the 

 decoloration of dyes by bacteria. But let it be remembered that the preparation of 

 several of the older dyes involved fermentative reduction and that the origins of these 

 preparative processes are lost in antiquity. The Chinese, importing synthetic indigo 

 from America, now avoid the necessity of the fermentative splitting of the glucoside 

 of the indigo plant, but still employ the fermentative vat process they imported from 

 India in ancient days. So it was that the decoloration of indigo and also the decolora- 

 tion of other dyes, during the events known as "fermentations," were common knowl- 

 edge during the period when the theory of fermentation took shape. 



Had a century or two in either direction separated the rise of bacteriology from 

 the rise of modern chemistry, certain important aspects of fermentative dye de- 

 coloration might not have been burdened with a nomenclature which somewhat ob- 

 scures their significance. However, the technical and physiological importance of 

 oxygenation had already established a dominating point of view at the time the 

 theory of the vat process took form. It was the period in which great classes of chemi- 

 cal transformations were formulated in terms of the gain or loss of oxygen. As carbon 

 dioxide is a "higher" oxide of carbon than carbon monoxide, the process 



2CO-fO.-^2CO. 



was naturally called "oxidation," while the reverse process resulting in the reduction 

 of degree of oxidation was called "reduction." So when the cloth came from the indigo 

 vat and was blued obviously by atmospheric oxygen, an oxidation process was said 

 to have occurred, and by the same token the decoloration was regarded as a reduction. 



The fact that organic chemistry represents the molecule of indigo as differing 

 from that of indigo white by the loss of two hydrogen atoms does not perturb the 

 formalist. Nothing is simpler than to assume the hydrogen to be removed by com- 

 bining with oxygen to form water. But when oxidation of indigo white is accom- 

 plished by halogens, ferric salts, and other "oxidizing" reagents the postulated chem- 

 istry of the reaction is very complex. 



It is not the purpose of this chapter to review either the chemistry of dyes, the 

 theory of their oxidative-reductive transformations, or the details of the uses to which 

 these processes are put in bacteriology. There will be described briefly a picture which 

 is useful. 



' Now at the School of Medicine, Johns Hopkins University, Baltimore, Md. 



i88 



