1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 359 



stances, such as peptone, beef extract, blood, etc., act in a similar 

 manner. A good account of buffer action appears in Clark's work 

 (II, 15, p. 116). 



COMPARISON BETWEEN FULLER'S SCALE AND HYDROGEN-ION 



CONCENTRATION 



Having briefly and perhaps inadequately described what is 

 meant by hydrogen-ion concentration and how to measure it, the 

 next question worthy of attention is, how does the older titration 

 method, such as Fuller's (II, '95), compare with the determina- 

 tion of hydrogen-ion concentration. There are two main reasons 

 why the two methods yield different results ; first, the difference in 

 degree of dissociation of different electrolytes; and, second, the 

 buffer action of various substances. As previously described, 

 when equal amounts of normal solutions of hydrochloric and 

 acetic acids are titrated the same amount of alkali is utilized, 

 although the two acids have entirely different P H values, since 

 one dissociates very strongly and the other but w r eakly; that 

 is, titrating with sodium hydroxide, in the case of weak acids, 

 gives no indication as to the actual acidity or the hydrogen-ion 

 concentration. When a nutrient medium is neutralized with 

 strong alkali the figure obtained gives no indication of the actual 



to uiivun ^^ ^b ulv vutuxnw & 



acidity present in the medium but gives an expression of the 

 total acidity, including the "active" acidity and the "reserve" 

 acidity, and this "reserve" acidity remains an unknown quantity 

 made up of undissociated acid molecules as well as the acid held 

 in union by the buffers present in all ordinary nutrient media. 

 Titrating with sodium hydroxide then gives a measure of the total 

 acidity, while the hydrogen-ion concentration measures the true 



or "active" acidity. 



Why is it insufficient to titrate for total acidity? The following 

 illustration taken from Sorensen (II, '09 ) answers this question. 

 He and other investigators found that certain enzymes, such as 

 invertase, catalase, pepsin, and others, show optimum activity in 

 the presence of a certain amount of acid, but the quantity of acid 

 necessary for this activity could not be definitely ascertained by 

 the ordinary titration method. The reason for this is clear. A 

 solution of an enzyme, such as invertase, contains substances 

 which are capable of combining with acids, so that the optimum 

 acidity depends, for one thing, on the substances, buffers, going 

 with the enzyme. However, without measuring the hydrogen- 

 ion concentration it is not possible to get a measure of the exact 



