ACIDITY 309 



replaceable hydrogen atoms) . A pronounced change in normality 

 (titratable acidity) of the buffer mixture represents a very slight 

 change in pH. 



A similar state of affairs exists when an alkali is added to an 

 acetic acid and sodium acetate buffer. The alkali, e.g., sodium 

 hydroxide, will dissociate into Na+ and 0H~ ions. Water is 

 only slightly dissociated; few H+ and OH" ions can, therefore, 

 exist together in an aqueous solution. The 0H~ ions from the 

 sodium hydroxide combine with the H+ ions to form water. 

 This will rob the acetic acid equilibrium CH3COOH ;:^ CH3COO- 

 + H+ of some of its H+ ions and leave unbalanced CH3COO- 

 ions, which are then taken care of by the free Na~ ions from the 

 sodium hydroxide to form more sodium acetate. The acetic acid 

 dissociation constant K must be maintained; consequently, 

 more of the acetic acid will dissociate. K is now constant again, 

 with a slight decrease in total concentration of acetic acid, which 

 does not appreciably affect the pH value but which titration will 

 show to have taken place. 



It is thus evident that considerable quantities of acids or alka- 

 lies may be added to solutions containing buffer salts without 

 materially changing their pH values. 



METHODS OF MEASURING pH 



Color Indicators. — The chemist has long used litmus as a 

 color indicator of the acid or alkaline state of a solution. Litmus 

 when acid is red; when alkaline, blue. At best, it indicates 

 simply whether the solution is neutral, slightly acid or alkaline, or 

 strongly acid or alkaline. Litmus is of plant origin, coming 

 from Roccella tinctoria and other lichens. Cochineal, from the 

 insect of that name, is an indicator of animal origin which has 

 also been used for acidity determinations. 



An interesting example of the indication of pH by color is 

 that of flowers the color of which is due to acidity values of the 

 particular pigments that they contain. The change in the color 

 of flowers as they open and wither, and probably of leaves in 

 autumn, is due to change in pH. The pigments may be extracted 

 and used as indicators. Anthocyanin (the soluble pigment of 

 red and blue flowers) from the tulip has a pH range of 5.3 to 9.2. 

 One of the earliest references to natural pigments as acid-alkaline 

 indicators is that of Robert Boyle, who, in 1664, called attention 



