THE PROTEIN SUBSTANCES. 73 



The reactions which have just been explained are the most 

 important and characteristic of all which have been suggested 

 for the recognition and identification of the proteins. Numer- 

 ous other reactions are known, however, which are easily 

 observed. Several of these are color tests, depending on the 

 formation and combination of furfuraldehyde, but they need 

 not be described as in principle they do not differ much from 

 the Molisch test. 



QUANTITATIVE DETERMINATION OF PROTEINS. 



The above tests serve for the recognition of proteins but not 

 for their determination, and for the latter purpose it may be 

 said further that no one method is perfectly suited to all cases. 

 Many of the simpler protein bodies are determined by com- 

 plete coagulation, followed by weighing of the precipitate 

 formed. This involves several operations, all of which must 

 be very carefully performed. For example, a pure native 

 albumin in solution may be coagulated by adding a few drops 

 of acetic acid and boiling thoroughly. The coagulum is col- 

 lected on a weighed paper filter or in a Gooch funnel, thor- 

 oughly washed, dried and weighed. Instead of drying and 

 weighing the precipitate it may be decomposed according to 

 the Kjeldahl process, in which the nitrogen is converted into 

 ammonia by digestion with sulphuric acid. The ammonia 

 is easily separated and measured. The nitrogen is 14/17 of 

 it. By multiplying the nitrogen found by the factor 6.25 we 

 obtain the original protein content on the assumption that 

 these bodies contain 16 per cent of nitrogen. This method is 

 now commonly followed in the determination of crude protein 

 for many technical and scientific purposes. But in many cases 

 an error is naturally introduced because of the uncertainty of 

 the factor; 6.25 is the mean value for the native proteins and 

 the closely related bodies. 



