CHAPTER III 

 THE QUANTITATIVE ESTIMATION OF THE PROTEINS 



1. The General Principles Underlying the Quantitative 

 Estimation of Proteins. — The extraordinary bulk of the protein 

 molecule in comparison with the molecules of the simpler sub- 

 stances which were the first to claim investigation by analytical 

 chemists, places unusual difficulties in the way of the estimation 

 of proteins by the traditional methods of analytical chemistry. 

 Although the proteins act as acids and bases, yet their amphoteric 

 character seriously interferes with direct titration by acidimetric 

 methods, for both the acidic and basic functions of the proteins 

 are weakened and modified by their simultaneous presence in the 

 same molecule, and it is but rarely, in cases such as those afforded 

 by casein and the various members of the protamin group, that 

 the predominance of the one function is sufficiently great to 

 permit of its utilization in the determination of the protein by 

 direct titration (1) (18) (32). Even in these cases we are addi- 

 tionally hampered by the great magnitude of the combining- 

 weight of the protein, leading to a proportionate enhancement 

 of the normal margin of acidimetric inexactitude, and also by the 

 fact that many of the indicators which are customarily employed 

 in alkalimetry or acidimetry form compounds with proteins, either 

 ceasing to function as indicators, or displaying novel or abnormal 

 color-changes or even directly entering into and disturbing the 

 very equilibrium, between protein on the one hand and an acid 

 or base on the other hand, which we desire to measure. 



Since the nitrogen-content of the proteins is high and nitrogen 

 is one of the elements which we are able to determine with the 

 greatest precision, nearly all the efforts of analytical chemists 

 in this field have until recently been aimed at reducing the esti- 

 mation of protein to an estimation of nitrogen. The problem 

 thus resolved itself into one of separating the protein in question 

 from other proteins and from nitrogenous contaminations. In 

 cases in which this preliminary isolation is not difficult of attain- 



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