40 Biological Stains 



of the American Pharmaceutical Association. In this table there 

 are listed the solubilities of 50 samples representing 25 com- 

 mercial certified stains. They are of interest In showing approxi- 

 mately how much deviation to expect between commercial samples 

 of the same dye and about how nearly the solubilities of such 

 samples approach those of the recrystallized dyes listed in Table 6. 

 Frequently the data In Table 7 may prove of more assistance In 

 determining how much dye to employ in making a saturated solu- 

 tion than those given In Table 6. 



It would, of course, be still more useful if the solubilities in 

 water and in alcohol of each batch could be printed on the label. 

 Unfortunately, however, no practical method of furnishing this 

 information to the manufacturers and of publishing it on the labels 

 has yet been devised. 



INFLUENCE OF IMPURITIES ON INTENSITY OF STAINING 



Impurities present in any dye sample or in the solvent not only 

 Influence the solubility of the dye but may have a great effect on 

 the intensity of staining. An impurity present may alter the 

 H-ion concentration of the staining fluid; while acid dyes stain 

 better In more acid solutions, and basic dyes in more alkaline solu- 

 tions. If, moreover, the impurity is a mineral salt or the salt of an 

 organic acid, it may have an effect on intensity of staining even 

 if the reaction of the solution is not altered. Theories to explain 

 these facts are discussed in Chapter III. 



The practical lesson from this is that poor staining results with 

 any dye sample are as often to be explained by the presence (or 

 absence) of impurities In the dye, or in the water used as a solvent, 

 as by some irregularity in the dye itself. In many instances the 

 impurities normally present In some dye may be necessary to 

 assure its proper behavior as a stain; and too great effort in the 

 way of purification may be detrimental. This seems to have been 

 unquestionably the case in respect to rose bengal. It has been 

 found (Conn and Holmes, 1928) that a sample of the latter dye, 

 too highly purified for good results if dissolved in distilled water, 

 may stain satisfactorily in tap water containing a certain amount 

 of calcium. It may well happen that the user of some dye which 

 gives too weak staining can Increase its Intensity of action by add- 

 ing minute amounts of some mineral salt to the staining solution 

 or by altering the H-ion concentration. In trying this experiment 

 great caution must be employed, for large effects are sometimes 

 produced by minute changes, and it is easy to bring about too 

 weak staining on the one hand, or overstaining on the other if too 

 much of any salt, acid, or base be added. 



