CH. IV.] CYSTINE. 83 



(vi.) Transfer the precipitate to a porcelain beaker or dish, and 

 boil with 150 cc. of 20 per cent, hydrochloric acid (by 

 volume). Filter. 



(vii.) Boil the residue with another 100 cc. of the hydrochloric 

 acid, filter, and mix the two filtrates. 



(viii.) Boil these with 5 grams, of good decolourising charcoal 

 (see p. 390), and filter. If the filtrate is not practically 

 colourless, the solution must be boiled with more char- 

 coal until a colourless or light yellow fluid is obtained. 



(ix.) Add a hot, concentrated, filtered solution of sodium acetate 

 until the free hydrochloric acid is removed. 



(x.) Allow to stand till quite cold. 



(xi.) Filter off the cystine, wash with cold water, then with 

 alcohol, and dry in the air. 



Yield : 8 to 10 grams. 



Properties of Cystine. It crystallises in characteristic 

 hexagonal plates, which are only very slightly soluble in 

 cold water (i part in 8840 parts of water) and in alcohol. 

 It dissolves readily in mineral acids, but is insoluble in 

 acetic acid. In normal acid urine it is soluble to the extent 

 of i part in 2000. It is readily soluble in dilute alkalies and 

 in ammonia. 



It is precipitated from its solution in sulphuric acid by 

 mercuric sulphate (see p. 90). On reduction it yields 

 cysteine 



CH 2 .SH. 

 CH.NH 2 



COOH. 



This is readily oxidised to cystine by atmospheric 

 oxygen in ammoniacal solution. 



