200 



Biological Stains 



Fortunately nearly all of them have different dissociation con- 

 stants; so that by employing a series of them one may determine 

 the H-ion concentration of solutions of any reaction ordinarily 

 encountered. 



There are two general types of indicators, those which show 

 only one color in the useful range, and those which change from one 

 color to another. The phthaleins, such as phenolphthalein, change 

 from colorless substances in their acid ranges to colored compounds 

 in their alkaline ranges. The sulfonphthaleins, on the other 

 hand, change from yellow to some deeper color. With the first 



TABLE 3 



COLOR CHANGES OF THE INDICATORS OF CLARK AND LUBS AND OF COHEN 



*In 95% ethyl alcohol. 



f Approximate invert logarithm of apparent dissociation constant. 



type of indicator the H-ion concentration may be determined by 

 the alteration in intensity of color, with the second type by altera- 

 tion in hue. The two-color indicators, such as the sulfonphthaleins, 

 are more satisfactory because one can measure changes in hue 

 more accurately than changes in color intensity, especially when 

 one must depend on rather crude methods of colorimetry as is 

 usually the case when indicators are employed. 



The sulfonphthalein indicators are especially valuable as indi- 

 cators for still other reasons. Unlike the azo compounds, such as 

 methyl red, they are very stable chemically, while they are less 



