Xanthene Dyes 199 



The dyes of this group have no value in practical dyeing, al- 

 though they are all weakly acid dyes. Similarly they are of little 

 use as biological stains, except occasionally in vital staining. Their 

 real value is as acid-base indicators. Even when they are em- 

 ployed as vital stains, they serve primarily to study the reaction of 

 tissues or body fluids. They are also valuable indicators in 

 micro-injection of cells. 



Hydrogen-ion indicators. The sulphonphthaleins, together 

 with phenolphthalein, are among the most useful indicators known 

 to the chemist. Quite a long series of them has been prepared, 

 which in general show their deepest color in alkaline solutions and 

 turn yellow on the addition of acid. Some of them, such as 

 thymol-sulfonphthalein (thymol blue), show two colors besides 

 yellow, one in strong acid solutions and the other in strong basic 

 solutions, while in solutions near the neutral point they are yellow. 

 That these color changes are due to alterations in the structure of 

 the molecule, such as the disappearance and reappearance of the 

 quinoid ring, is generally assumed; but in the case, of these com- 

 pounds the relation of structure to color is complicated and has 

 not yet been worked out to general satisfaction. 



The colorimetric measurement of H-ion concentration depends 

 upon these color changes. The color change of an indicator takes 

 place within a short range on each side of that point in the H-ion 

 scale at which the dye is 50% dissociated, i.e., occurs half in the 

 form of the undissociated dye and half in the form of free ions. 

 This point on the pH-scale (at which an indicator is 50% dis- 

 sociated) is important, and corresponds to the invert logarithm of 

 its dissociation constant. The dissociation constant of an in- 

 dicator, in other words, is a quantity whose invert logarithm is the 

 pH-value of the point at which the indicator is half dissociated. 

 The approximate invert logarithm of the apparent dissociation 

 constant is denoted by chemists by the symbol pK; the value of 

 this quantity is given in the case of each of the indicators listed 

 below. 



For a short distance on each side of the dissociation constant, 

 every shade of the indicator corresponds to a definite pH-value; 

 this zone is known as the sensitive range of the indicator. Roughly 

 speaking, the sensitive range of any indicator extends for about 

 0.8 pH to each side of its pK- value. Throughout its sensitive 

 range, an indicator can be used to determine the H-ion concentra- 

 tion of a solution by comparing its shade with that produced in 

 standards of known reaction. The various indicators of this group 

 differ in their strengths as acids (i. e. the extent to which they dis- 

 sociate) ; and the greater their strength the lower the pH-value of 

 the point at which they are 50% dissociated. It is plain, therefore 

 that different indicators may be selected according to their dis- 

 sociation constants to cover successive portions of the H-ion scale. 



