ACIDITY 311 



Wine manufacturers have long titrated red wine for total 

 acidity, using the color change from red to green as an end point. 

 The pigment which thus serves as an indicator is in the grape 

 skin. 



Such indicators serve well enough for crude determinations; 

 but in laboratory physiological work, where hydrogen ions play 

 so prominent a role, more precise measurements are necessary. 

 Two synthetic indicators, methyl red and phenolphthalein, had 

 begun to take the place of litmus as acid-alkaline indicators before 

 the now more commonly used indicators were introduced. 

 Methyl red and phenolphthalein are sufficient for most titro- 

 metric (normality) purposes, but their pH range is not great. 

 Through the efforts, first, of Friedenthal and Salm, then of 

 S0rensen in Copenhagen, many dyes that change their color with 

 change in acidity of the solution were carefully studied, and their 

 accuracy as pH indicators determined by comparison with other 

 (electrometric) methods of pH determination. Nine dyes thus 

 selected indicate a pH range from 1.2 to 9.6. These indicators 

 are synthetically produced from coal-tar products. One of them, 

 bromphenol blue, is yellow at a pH of 3.1 or less and blue at a 

 pH of 4.7 or more. If the pH of the solution is above 4.7 or 

 below 3.1 the dye next in the series of color changes must 

 be used. 



Dyes change color with change in acidity, because they disso- 

 ciate into a colored ion, with a noncolored or different-colored ion. 

 They do so as a simple salt (phenolphthalein is a sodium salt) or, 

 less often, as ampholytes, i.e., awp/iofenc substances (see page 483). 

 In both cases, the end result is the same — an ion with a color is 

 liberated on dissociation. The degree of dissociation, and there- 

 fore the concentration of the colored ion, depends on the 

 hydrogen (or hydroxyl) ion concentration. A change in pH 

 consequently means a change in the color of the solution. Thiel 

 has shown that the color changes rest upon more complex reac- 

 tions, involving molecules, simple ions, ions of different valence, 

 "Zwitter" (double) ions, and isomeric changes. 



Colorimetric pH determinations are made in a colorimeter, or 

 comparator, which may be of the LaMotte roulette-wheel type. 

 The color of a solution of unknown pH to which an indicator dye 

 has been added is compared with that of a standard solution of 

 known pH and color. 



