NEUTRALITY OF THE TISSUES AND TISSUE-FLUIDS 273 



acid, using methyl orange as an indicator, by the time the red color 

 appears these compounds have been successively decomposed, and, in 

 fact, some proportion of the acid employed in the titration has actually 

 entered into combination with the proteins which are now acting as 

 bases instead of acting, as they do in normal blood, as weak acids. 

 The Titratable Alkalinity of the blood, therefore, bears no relationship 

 to its actual alkalinity or Hydroxyl ion Concentration. It does, how- 

 ever, bear some relationship, as we shall see to the power of the blood 

 to maintain its neutrality, in other words to the "Alkali-reserve" of 

 blood. 



FIG. 11. Modified Cottrell hydrogen electrode. (After Schmidt.) 



In order to ascertain the actual reaction of a complex mixture of 

 weak and strong acids and bases such as the blood or other tissue- 

 fluids, therefore, a method of measurement must be employed which is 

 static and not dynamic, i.e., which leaves the state of the* blood unal- 

 tered in respect to the balance of acids and bases which it contains. 

 For this purpose no method is better adapted for obtaining accurate 

 results than the electrometric or Potentiometric Method. The principle 

 of this method has already been explained upon page 154. For the 

 degree of accuracy usually required in biochemical or physiological 

 researches the apparatus employed by Hoagland (1) and illustrated on 

 page 272 (Fig. 10) is undoubtedly the simplest and most convenient. 

 For solutions not containing volatile acids, the Cottrell form of elec- 

 trode as modified by C. L. A. Schmidt is the best (Fig. 11), but when 

 the fluid to be investigated contains carbon dioxide which would be 

 18 



