312 



PROTOPLASM 



The first pH color indicators were plant products, and now 

 plants again make their contribution to the list of indicators. 

 Pratt and Swartout investigated pigments of fruits and vege- 

 tables and found that apricots, carrots, peaches, pears, persim- 

 mons, and tomatoes failed to yield pigments with indicator 

 characteristics, while red apples, a variety of berries, black 

 cherries, grapes of all colors from red to black, certain plums, 

 pomegranates, and prickly pear (cactus) fruit proved to contain 

 pigments of indicator value. The pH range of several of these 

 pigments is given in the following table : 



Concentration vs. Activity. — Acidity in terms of hydrogen 

 ions, or pH, is usually defined and expressed as a concentration. 

 When S0rensen recommended the symbol pH+, which has since 

 become pH, he thought that he was measuring pH = —log CH+, 

 i.e., the negative logarithm of the hydrogen-ion concentration. 

 That concentration is a factor is evident from the fact that with 

 increase or decrease in concentration of an acid or an alkali, a 

 different pH value results; but while it is still convenient to 

 express pH as a quantity factor, what we actually measure (by 

 color or electrometrically) is activity; therefore, instead of pH 

 being an expression of concentration, it is an expression of 



activity (aH+), and pH = log ^fT+- (The true situation is a 



little different from that as just stated; we measure neither 

 concentration nor actual activity but activity with standards 

 V)ased on concentration, owing to a theory that appears invalid 

 today but was acceptable when first expressed by S0rensen on 

 the basis of the then existing ionization theory of Arrhenius.) 



