Phenyl Methane Dyes 163 



fuchsin which stains the mitochondria; (see Lee, 1937, p. 166). It 

 is an ingredient of the Ehrlich triacid mixture (with orange G and 

 acid fuchsin) for staining blood smears. Botanists find it a 

 valuable stain, combined with acid fuchsin, for lignified xylem. 

 One of its most valuable uses today is in the Pappenheim (1899) 

 stain, in which it is combined with pyronin and used for staining 

 the gonococcus and mast cells as well as by Unna in studying 

 chromolysis. It is also a useful chromatin stain for protozoa, and 

 is employed in weak acetic acid solution for staining fresh ma- 

 terial beneath the cover glass. 



When the foreign supply of dyes was first shut off, this stain 

 proved one of the most difficult to obtain in satisfactory quality, 

 largely due to the looseness with which the seventh methyl group 

 is attached and the resulting instability of the compound. At first 

 certain green dyes of an entirely different nature were furnished, 

 but as soon as an investigation of the dye was begun manufac- 

 turers proved perfectly able to produce methyl green; the difficulty 

 came in obtaining the right degree of purity. Samples were finally 

 furnished so pure that they lacked completely the necessary meta- 

 chromatic staining quality; and it proved necessary to add a certain 

 small percentage of the violet dye to obtain the proper results. 

 This problem seems to have been solved at present and satisfactory 

 methyl green is available. The chief problem now is to standardize 

 it. With other stains this can ordinarily be done on the batch 

 basis, approving some batch large enough to meet the demand for 

 a period of years. With methyl green this cannot safely be done, 

 on account of its instability. Hence large batches are impractical; 

 and the stain ought to be sold with the caution that the dye does 

 not keep indefinitely without change. 



This instability complicates standardization by spectro tests. 

 Fig. 19, p. 145, shows a spectrophotometric curve of this dye in 

 comparison with those of light green SFY and fast green FCF. 

 The three are much alike with two maxima at about 430 and 630. 

 All the green dyes show disagreement between dye content deter- 

 minations by means of TiCls titration and comparisons of the 

 density at the peak of the chief maximum. Methyl green has an 

 additional source of inconsistency in its assay: the ratio between 

 two points of the curve equal distances each side of the maximum 

 fluctuates, due apparently to the fact that solutions change on 

 standing. 



There seems, since 1930, to have been a change in the nature 

 of the methyl green samples supplied as biological stains. One of 

 the companies admits that its methyl green is not C. I. No. 684, 

 but the following: 



