Nitro and Azo Dyes 81 



c60 METHYL ORANGE C. I. NO. I4i 



Synonyms: Orange III. Helianthin. Gold orange MP. 



Tropaeolin D. 



N = N 



/\/ 

 NaSOa 



Ci4Hi4N303SNa; Mol. Wt. 327.333 



{A weakly acid dye; absorption maximum 506-507 in 



acid solution) 



Solubility at 26°C: in water 0.52%; in alcohol 0.08% 



This dye has httle use as a stain, but is widely employed as an 

 indicator, as it is red in acid, and orange in alkaline solutions. Its 

 chief value as an indicator is that it is sensitive to mineral acids 

 without being affected by carbonates or most organic acids. It has 

 been used as an indicator for cell sap by Pfeffer (see Krause, p. 

 2305), and occasionally as a counterstain in histology. Under 

 the name of gold orange, it has been employed by Newcomer 

 (1938) as a counterstain to crystal violet in staining pollen tubes. 



A spectral curve of this dye, compared with orange II and 

 orange G, is given in Fig. 7, p. 74. 



c64 PRONTOSIL 



Synonym : Prontosil red. 



NH2SO2 

 C12H14N5O2SCI; Mol. Wt. 327.789 



This is a red substance employed principally as a therapeutic 

 agent. It is essentially a basic dye in its chemical structure, al- 

 though not used as a dye. It is referred to here only because it has 

 been called for by Carter (1930) as a vital stain for insects and 

 plants. 



