354 PIGMENTS 



Preparation. 



To prepare a solution of phycoerythrin the red sea-weed, 

 Ceramium rubrum, which is one of the best to use, is washed 

 in ordinary water to free it from sea salts and adhering sand. 

 It is then soaked in distilled water ; in two days most of the 

 pigment will have diffused out. The solution is filtered through 

 glass wool, and a few drops of eucalyptus oil or carbon bi- 

 sulphide are added as an antiseptic, for putrefaction soon 

 sets in. 



It is a matter of great difficulty to obtain a pure sample of 

 phycoerythrin, for, in an aqueous solution, it passes over into 

 an irreversible gel,* even when kept at 0° C. This, of course, 

 renders ordinary filtration extraordinary slow, and thus in- 

 creases the difficulty of purification. 



The solid phycoerythrin may be prepared from the aqueous 

 solution by concentrating it under reduced pressure at a 

 temperature not higher than 38° C. ; any precipitate which 

 comes down during this process must be filtered off. Methyl- 

 ated spirit is then added to the concentrated solution until 

 the fluorescence disappears. The precipitated phycoerythrin 

 is allowed to settle and the more or less clear supernatant 

 fluid is filtered off, again treated with alcohoh and filtered. 

 The operation is repeated until the red colour has entirely 

 disappeared from the solution. The precipitates are washed 

 by decantation with 70 per cent alcohol ; the pigment, in a 

 pasty mass, is placed in a clock glass and dried in a vacuum. 



Reactions. 



1. Phycoerythrin is precipitated from its solution by 

 alcohol, by small quantities of mercuric chloride, and by 

 saturation with ammonium sulphate or magnesium sulphate. 



2. When dilute acids are added gradually, the fluorescence 

 first disappears, leaving a somewhat opalescent solution of 

 a lilac-pink tint. After the lapse of two days a pink precipitate 

 comes down. 



3. Ammonium hydrate in small quantities removes the 

 fluorescence ; in excess, a yellowish-brown coloration results. 



* See Section VIII. on the Colloidal State. 



