PECTINS 199 



5 c.c. of 10 per cent caustic soda are added ; after gently 

 agitating the flask, the cork is inserted and the mixture is 

 left for five minutes ; now add 2-5 c.c. of dilute sulphuric acid, 

 gently agitate once more, and distil over 17 c.c. of liquid ; 

 this is once more distilled, about 11 c.c. being collected. The 

 resulting hquid is tested for methyl alcohol * 



5. Pectin solutions are precipitated by copper sulphate 

 or lead nitrate but not by barium chloride, ferric chloride, 

 or lead acetate. Also they are precipitated by baryta or 

 lime water or basic lead acetate. 



Alkalis readily attack pectins with liberation of methyl 

 alcohol and formation of the alkali metal salt of pectic acid ; 

 neutral calcium and barium salts do not precipitate unless 

 alkali is present. 



6. Aqueous solutions of pectins are precipitated by 

 alcohol, but the precipitate can be redissolved in water. 



7. Pectins are insoluble in ammoniacal solution of copper 

 hydroxide. 



Microchemical Reactions. 



The fact that these pectic substances are akin to cellulose, 

 and occur in conjunction with it, renders its identification 

 by microchemical means somewhat difficult. Mangin f gives 

 the following methods : — 



1. Methylene blue, Bismarck brown, and fuchsin stain 

 pectic substances, lignified and suberized walls, but not pure 

 cellulose. If sections thus stained are treated with alcohol, 

 glycerine, or dilute acids, the lignified or suberized walls 

 retain their coloration, whilst the pectic substances are de- 

 colorized with rapidity. 



2. Crocein and nigrosin stain lignified and suberized walls, 

 but do not stain pectic compounds. 



3. Crocein, naphthol black, and orseille red stain pure 

 cellulose, but do not stain pectic substances ; similarly, pectic 

 compounds are unstained by congo-red and azo-blue, whilst 

 cellulose and callose are. 



* Particulars of this test will be found in Sucharipa : " Die Pektin- 

 stoffe," Braunschweig, 1925. 



t Mangin : " Compt. rend.," 1889, 109, 579 ; 1890, no, 295, 644. 



