14 W. H. E M I G 



Acid dyes, Colour Index Numbers. 



151. Orange II. 



252. Crocein Scarlet MOO. 



280. Biebrich Scarlet. 



286. Crocein Scarlet 7B. 



692. Acid Fuchsine. 



707. Aniline Blue. 



Many of the acid dyes are sodium salts of a sulfonic acid. The 

 principal salt-forming radical is the hydroxyl group. The radical 

 - S0 3 Na, which is not considered an auxochrome, renders the dye 

 more soluble in water and more acidic. If possible, 0.01 per cent 

 sulfuric acid (1 ml. of 1 per cent) should be added to each 100 ml. 

 of the dye solution. A few dyes are precipitated by sulfuric acid, 

 whereupon either acetic acid, formic acid, sodium sulfate, or potas- 

 sium dichromate becomes the chemical assistant. With many of 

 these dyes the amount of ionic dissociation of the molecule in water 

 is so slight that the stain is relatively inactive. The intensity of 

 the stain is increased by the presence of an acid, decreased with an 

 alkali. 



The routine procedure with an unfamiliar acid dye requires 

 three tests. The first object taken from the stain at the end of 15 

 minutes is dehydrated in alcohol and cleared in xylene. An exami- 

 nation of the finished product shows whether or not a 15-minute 

 interval is sufficient. If the color is too light, the second section 

 is tested at the end of an hour. The third section, in turn, remains 

 in the stain 12 to 24 hours. The activity of acid dyes does not 

 extend beyond 24 hours. Naturally, if the first test at the end of 

 15 minutes gives too intense a color a shorter interval is indicated. 



Acid dyes are especially suited for proteins, either in plant or 

 animal tissues. Within plants the proteins are more abundant in 

 parenchyma, cambium, phloem, and immature spores. It is impos- 

 sible to give a definite time limit for any particular dye because the 

 time differs both with the kind of tissue and with the fixative. 

 Orange II is sometimes too weak to color mature parenchyma in 24 

 hours, yet the meristem of root tips fixed in sublimate-dichromate- 

 acetic is deeply stained within a few seconds. On animal tissues, 

 Orange II requires an average of three to five minutes although 

 sections may be found that are not overstained in 24 hours. 



