DISTRIBUTION OF NITROGEN 103 



prefers to estimate the total nitrogen in another portion, 5 c.c., of 

 the solution. 



The sum of the number of c.c. of decinormal acid neutralised + 

 the number of c.c. of decinormal acid previously neutralised in the 

 arginine determination, multiplied by 0-0028, gives the total nitrogen 

 content. 



9. Estimation of Cystine. Cystine is estimated by determining 

 the total sulphur content by Denis' modification [1910, i] of Benedict's 

 method [1909-10]. 10 c.c. of the solution of hexone bases + cystine 

 from operation (6) are placed in a porcelain dish of 7-10 cm. diameter 

 with 5 c.c. of a solution containing 25 grams copper nitrate, 25 grams 

 sodium chloride, and 10 grams ammonium nitrate per 100 c.c. The 

 mixture is evaporated to dryness on the water-bath and then gradually 

 heated to redness, at which temperature it is maintained for ten 

 minutes. The residue is dissolved in 10 c.c. of 10 percent, hydro- 

 chloric acid and diluted to 1 50 c.c. 10 c.c. of a 5 per cent, solution 

 of barium chloride are slowly added to the boiling solution and the 

 barium sulphate is filtered off and washed in the usual way. A 

 correction must be made for the amount of sulphate in the reagents 

 which should not exceed I -5 mgm. barium sulphate. 



I mgm. BaSO 4 corresponds to 0*06 mgm. cystine nitrogen in the 

 10 c.c. analysed, i.e., to 0*3 mgm. cystine nitrogen in the total solution 

 of bases + cystine. 



10. Estimation of Amino Nitrogen. 10 c.c. of the solution are 

 usually used for this estimation (p. 89) ; I c.c. or 2 c.c. if the micro 

 apparatus be used. The reaction may be allowed to proceed for half 

 an hour at 20 or longer at a lower temperature without shaking, 

 or for five minutes with vigorous shaking ; similarly for the blank 

 determination for the reagents. 



A correction must be made for cystine which gives 107 per cent, 

 gas ; as before it applies only to those proteins keratins which con- 

 tain a large amount of cystine. 



Calculation of Histidine Value. 



Total nitrogen of bases minus total amino nitrogen gives the total 

 non-amino nitrogen, D. This is contained in the arginine and histi- 

 dine. 



Since three-fourths of the arginine nitrogen and two-thirds of the 

 histidine nitrogen are in this form, the total non-amino nitrogen minus 



