6 BULLETIN" 158, U. S. DEPARTMENT OF AGRICULTURE. 



It should also -be stated that although the results from the Van 

 Slyke analysis are expressed in the usual way, arginine X, histidine 

 N, etc., that it is not intended to convey the impression that these 

 fractions contain pure arginine, histidine, etc., since as will be shown 

 later, other compounds are included under these analytical terms. 

 However, the nitrogen so expressed is that which is contained in 

 compounds which give the various reactions upon which the Van 

 Slyke method depends. 



Two 20-gram samples of base goods were extracted for analysis. 

 The first sample was extracted with boiling water until the extract 

 ceased to give an acid reaction. The second sample was boiled for 

 24 hours with hydrochloric acid, sp. gr. 1.115, the resulting solution 

 was filtered by suction and the insoluble residue washed with hot 

 water until the washings ran free from chlorides. The two extracts 

 were then concentrated to the consistency of a sirup in vacuo to expel 

 the free volatile acid, and each was finally made up to a volume of 

 250 c. c. 



Total nitrogen. — Total nitrogen in solution was determined by sub- 

 jecting 50 c. c. of the solution to Kjeldahl analysis. The water ex- 

 tract contained 1.372 per cent and the hydrochloric-acid extract 1.435 

 per cent of the base goods. 



Amide nitrogen. — Amide nitrogen was determined by distilling in 

 vacuo the remaining 200 c. c. of solution, to which were added 100 c. c. 

 of 95 per cent alcohol and 20 c. c. of a 10 per cent suspension of cal- 

 cium hydroxide, as described under the determination of ammonia. 

 The water extract contained 0.374 per cent and the hydrochloric acid 

 extract 0.882 per cent. 



Humin nitrogen. — The residue from the amide nitrogen determina- 

 tion was used for the determination of humin nitrogen. The precipi- 

 tate, formed by the addition of calcium hydroxide, was filtered off and 

 washed with distilled water in the same manner in which Van Slyke 

 directs that the phosphotungstic acid precipitate be washed. The 

 washing was continued until no reaction for chlorides or alkalinity 

 was obtained. The nitrogen remaining in the precipitate and in the 

 filter paper was then determined by Kjeldahl analysis. The humin 

 nitrogen was 0.031 per cent for the water extract and 0.074 per cent 

 for the hydrochloric acid extract. 



Diamino acid nitrogen. — The combined filtrate and washings from 

 the humin precipitate were neutralized with hydrochloric acid, con- 

 centrated in vacuo to a volume of about 100 c. c. and then transferred 

 to a 300 c. c. Erlenmeyer flask. To this solution were added 18 c. c. of 

 concentrated hydrochloric acid together with 15 grams of purified 

 phosphotungstic acid * and the whole diluted with water to a volume 

 of 200 c. c. The flask was placed on a steam bath and heated until 



1 Winterstein, Zeit. physiol. Chem., 34, 153 (1901). 



