2o8 THE CARBOHYDRATES 



condition of chemically pure cellulose. For this purpose * 

 the cotton wool has to be extracted successively in a Soxhlet 

 for six hours with 96 per cent alcohol and with ether to remove 

 fats and waxes ; it is then boiled for several hours with i per 

 cent caustic soda under specified conditions involving the 

 rigorous exclusion of air in order to avoid oxidation. After 

 washing with water and acetic acid, and finally with water, 

 it is dried. The resulting product, known as standard cellu- 

 lose, should have the following composition :— 



a-Cellulose ..... 99-8 



Ash ....... 0-03-0-06 



The term a-cellulose is applied to cellulose which is in- 

 soluble in 17-5 per cent caustic soda. In other plant material 

 such a-cellulose is accompanied by varying proportions of 

 two other modifications known respectively as j8- and y- 

 cellulose ; these forms are less resistant to chemicals than the 

 a form, but may only differ from it in the degree of polymeri- 

 zation, of dehydration or even of dispersion. f The relative 

 proportions in which these forms occur in a given sample of 

 cellulose may be determined by extraction with 17-5 per cent 

 caustic soda whereby a-cellulose remains undissolved while 

 the jS and y modifications go into solution ; on acidifying the 

 filtrate with acetic acid, the jS-cellulose is precipitated while 

 the y- remains dissolved. The evaluation of a cellulose for its 

 suitability for technical purposes is largely dependent upon 

 the results of such an analysis. 



PROPERTIES OF CELLULOSE. 



Pure cellulose is a white hygroscopic substance, which 

 absorbs about 6-12 per cent of water, which it loses again when 

 heated to 100° ; it is insoluble in water at ordinary pressure, 

 but when heated with water in sealed vessels at 500° F. it 

 is dissolved completely with decomposition. 



SOLUBILITY OF CELLULOSE. 

 Cellulose is insoluble in all ordinary solvents, but when 

 treated with zinc chloride in the presence of water it is con- 



* Corey and Gray : "J. Ind. Eng. Chem.," 1924, 16, 853, 1130. 

 t See Schwalbe and Becker ; " J. prakt. Chem.," 1919, lOO, 19. 



