Quinone-imine Dyes 



113 



product as a bacterial stain. It must be repeated that for all 

 staining purposes, except possibly some lines of vital staining, 

 medicinal methylene blue is an eminently satisfactory grade. 

 When ordered for staining purposes it is best specified under the 

 name: methylene blue chloride. 



There is just one salt of methylene blue, other than the chloride, 

 which is recommended by the Commission — namely meihylene blue 

 thiocyanate. This form of the dye, however, is not recommended 

 as a stain but as an oxidation-reduction indicator for use in milk. 

 Thornton and Sandin (1935) have shown that this salt has distinct 

 advantages over the chloride in determining the rate of reduction 

 of milk by its bacterial flora; the thiocyanate contains no water 

 of hydration and hence can be obtained in almost 100% dye 

 strength — a fact which makes for more ready standardization of 

 solutions. Accordingly tablets of methylene blue for use in the 

 reduction test in milk, which are put on the market in America 

 are now wholly the thiocyanate. 



PROCEDURES RECOMMENDED BY THE COMMISSION IN WHICH THIS STAIN IS USED 



Polychrome methylene blue. The oxidation of methylene blue 

 into one or more of the compounds of lower methylation de- 

 scribed on the preceding pages takes place in any methylene bliie 

 solution upon standing, especially rapidly if the solution be al- 

 kaline as in the Loeffler formula. All methylene blue solutions 

 therefore, particularly if they have been standing any length of 

 time, contain small amounts of the lower homologs, primarily 

 azure A and azure B. These lower homologs are not only more 



