2io Cellulose 



groups are chiefly two, viz. aldehydic and acid ; and hence the 

 employment of sulphurous acid and bisulphites on the one 

 hand, and alkalis on the other, for the purposes as indicated in 

 the table. 



The application of these principles is well illustrated by 

 taking any of the processes in series of variations. Thus the 

 sulphite processes : 



(a) Sulphurous acid alone is capable of resolving wood into 

 cellulose (insoluble) and non-cellulose (hydrolysed), and soluble 

 derivatives. The acid, however, owing to its feeble hydrolysing 

 power, requires to be used in 7 p.ct. solution (SO 2 ) prepared 

 under pressure ; and, again, to prevent reverse action, the limit 

 of temperature employed is 105. (See 'The Pictet-Brelaz 

 Process of Preparing Wood Cellulose,' Cross and Bevan. 

 Spon : London, 1889.) 



The hydrolysing action of the SO 2 .Aq, ordinarily very 

 feeble, is perhaps also more powerful in relation to aldehydic 

 condensations. 



(b) The bisulphites. The addition of the base lowers the 

 hydrolysing action of the acid ; a higher temperature (150-160) 

 is therefore required. The base, however, serves to saturate 

 acid groups, and the process is further aided by sulphonation 

 in the CH=CH groups ; the presence of the excess of bi- 

 sulphite prevents reversal by condensation of aldehydic groups. 



(c) Neutral sodium sulphite. In this case the hydrolysing 

 action is still feebler, and a higher temperature is required 

 (160-180). 



In presence of the lignocellulose complex, undergoing 

 decomposition, the sulphite is dissociated, the base going to 

 acid groups, and the acid sulphite residue to aldehydic groups. 



In all the above processes, moreover, the resolution is aided 

 by deoxidation of the lignocellulose constituents, a certain 

 proportion of sulphate being formed. 



