CH. X] OF PROTEIDS. 251 



on p. 244 (which are referred to below as the alkali solution 

 and the water solution) and not on the extracts used for 

 non-nitrogenous substances. 



QUALITATIVE EXAMINATION. 



Proteids insoluble in water^ soluble in dilute 

 alkali. 



A portion of the alkali solution is carefully neutralized 

 with dilute acid, a precipitate (soluble in excess of acid) is 

 formed if proteids are present. This precipitate should be 

 filtered, washed, and portions of it tested by the ordinary 

 reactions for proteids (xantho-proteic, Millon's, biuret, etc.) 



Proteids soluble in water. 



Portions of the solution should be tested by boiling 

 after addition of a drop of acetic acid and by the addition 

 of 90 p.c. alcohol — if precipitates are produced they 

 should be filtered off, washed and tested for proteid 

 reactions. 



Whether precipitates are caused or not by the above 

 add to fresh portions of the solution : — 



(1) Potassium ferrocyanide and a drop of acetic acid, 



(2) Aqueous solution of trichloracetic acid. 



Both these reagents give precipitates with proteids 

 and they will frequently cause precipitates when the 

 solution does not change on boiling or on addition of 

 alcohol. 



If proteids are present add cupric acetate (aqueous 

 solution) as long as it causes a precipitate, and filter. 



