78 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Cystine is usually not present in sufficient amount in proteins to cause 

 any considerable error in this determination, but if a keratin be under 

 investigation a correction must be made ; 1 8 per cent, of its nitrogen 

 is evolved when it is boiled with alkali ; the nitrogen figure of cystine 

 is obtained later in the process. 



8. Total Nitrogen of the Hexone Bases and Cystine. The solution re- 

 maining from the arginine determination is transferred to a Kjeldahl 

 flask of 500 c.c. ; 35 c.c. of sulphuric acid are added and 0-25 gram 

 of copper sulphate. The nitrogen is then estimated in the usual way. 



The sum of the No. of c.c. decinormal acid neutralised + the No. of 

 c.c. of decinormal acid previously neutralised in the arginine deter- 

 mination, 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 of Benedict's method. 

 10 c.c. of the solution of hexone bases + cystine from operation 

 (6) are placed in a porcelain dish of 7 to 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 per cent, hydro- 

 chloric acid and diluted to 150 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 milligrams barium sulphate. 



i milligram BaSO 4 corresponds to 0*06 milligram cystine nitrogen 

 in the 10 c.c. analysed, i.e., to O'3 milligram cystine nitrogen in the 

 total solution of bases + cystine. 



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

 used for this estimation (p. 69). The reaction must be allowed to 

 proceed for half an hour at 20 C., or longer at a lower temperature; 

 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. 



