CHAPTER V 

 SALTS OF ALBUMIN AND ACIDS 



THE reaction of albumin with acid, in a i per cent, solution 

 of the former, undergoes a change when the acid concentration 

 exceeds io~ 3 N. Below that concentration, the evidence for 

 combination is complex, but with greater quantities of acid the 

 process becomes more simple, and reveals itself as the mere 

 saturation of the basic valencies of the protein. 



The plain fact that proteins combine both with acids and 

 bases has been known for a long time. On the other hand, 

 quantitative measurements of the extent of combination 

 awaited the clear conception of the terms acid and base, which 

 only physical chemistry could furnish. The results of the 

 application of titrimetric methods to the determination of the 

 capacity of proteins for combination with acids and bases 

 (Spiro and Pemsel) were only made clear by the progressive 

 understanding of the function of indicators (W. Ostwald, 

 H. Friedenthal, E. Salm and others). Certain proteins, such 

 as casein, have been shown to form approximately neutral salts 

 when titrated with strong alkalis, using phenol phthalein as 

 indicator, the point of colour change in this instance being near 

 the true point of neutrality in the physico-chemical sense 

 (Laqueur and Sackur). Many other indicators, such as phloro- 

 glucin vanillin, give an idea of the maximum combination 

 of acids with albumin, which, although only rough, is sufficient 

 for many purposes (J. Christiansen). The work of S. P. L. 

 Sorensen,* in particular, has shown that, in order to arrive at 

 useful conclusions as to hydrion concentrations by the use of 

 indicators, special precautions must be taken when proteins 



* Compt. rend. lab. Carlsberg, 1907, 7, i ; 1909, 8, i ; Biochem. 

 Zeitsch., 1907, 7, 45 ; 1909, 21, 131 ; 1909, 22, 352. For summary and 

 literature, see Ergebn. d. Physiol., 1912, 12, 303 ; and L. Michaelis, " Die 

 Wasserstoffionenkonzentration." 1914. Berlin. J. Springer. 



