The bond connecting one of the sulfonic groups with an amino 

 group attached to a different benzene ring is assumed to exist in 

 order to account for the fact that altho only two of the sulfonic 

 groups are neutralized Avith sodium, the compound acts as tho it 

 has no free acid. In other words, it is a case of intramolecular salt 

 formation. Now when the tri-sodium salt is formed, this bond is 

 broken down, whereupon the quinoid ring disappears and the fol- 

 lowing compound is produced: 



SOrXa 



H.X 



XH. 



XaS03 \_/ 



This compound, it will be seen, is a carbinol in structure, and as it 

 lacks the quinoid ring it is colorless; but it is very readily converted 

 into the di-sodium salt by the addition of acid, whereupon the color 

 again appears. This property makes acid fuchsin of use as an 

 indicator. The decolorized solution of acid fuchsin neutralized 

 with sodium hydrate is called the Andrade indicator. It is used 

 quite extensively in bacteriological work, because of the striking 

 reaction when its color is restored by acid-forming bacteria. As an 

 indicator to show hydrogen-ion concentration at all accurately, 

 however, it is found to have much less value than the phthalein 

 and sulphonphthalein dyes (see pp 83 to 86.) 



Acid fuchsin is a widely used plasma stain, which has also been 

 recommended for a number of special uses. Among the best known 

 are: the Van Gieson connective tissue stain, in which it is used with 

 picric acid after haematoxylin to differentiate smooth muscle from 

 connective tissue; the Ehrlich-Biondi stain, in which with methyl 

 green and orange G it is employed in histology and for staining 

 blood smears; and the Ehrlich tri-acid stain for blood, which is a 

 "neutral" combination with orange G and methyl green. In plant 

 histology it is used to stain the cortex, pith and cellulose walls; 

 while the Pianese stain (with malachite green and martins yellow), 

 originally applied to cancer tissue, is now used by plant pathologists 

 in studying infected vascular plants. It is used with methyl green, 

 bv Altmann, Benslev and Cowdrv as a stain for mitochondria. 

 To the pathologist it is quite valuable as a constituent (with anilin 

 blue and orange G) of the Mallory connective tissue stain. 



HOFFMAN VIOLET C. I. NO. 679. 



Synonyms : Dahlia. Iodine violet. Red violet. J^iolet R, RR, or J^RN. 



Hoffman violet is derived from fuchsin bv the introduction of 

 methyl or ethyl groups (generally the latter) into the amino groups. 

 Thus the tri-ethvl rosanilin has the formula: 



65 



