THE STRUCTURAL FEATURES OF CELLULOSE 73 



reasoning, that the chains will differ, perhaps widely, in length. Where- 

 as, therefore, in simple inorganic molecules we are accustomed to 

 think in terms of a molecular weight which can be determined with a 

 considerable degree of accuracy, and which can be considered to 

 represent the weight, relative to hydrogen, of each and every molecule, 

 in cellulose we must be prepared to think in terms of an average mole- 

 cular weight. This carries with it two implications. If the molecular 

 chains of cellulose do vary considerably in length (as in fact they do) 

 then firstly, different methods of determining molecular weights will 

 give different results and, secondly, the physical behaviour of two 

 celluloses with the same "molecular weight" may be different if the 

 range in molecular weights is different. Before discussing these points 

 it will be as well briefly to look into the methods used for molecular 

 weight determination in cellulose. 



The determination of molecular weights 



It should be noted at the outset that molecular weight determinations 

 can be made only in solution, and in substances like cellulose this 

 imposes strict limits on the accuracy with which the molecular weight of 

 untreated cellulose can be estimated. This is true since there is no 

 solvent known for cellulose which does not cause some degenerative 

 breakdown (i.e. decrease in chain length) during solution. Cellulose is 

 dissolved usually in cuprammonium; or it may be nitrated and the 

 nitrocellulose dissolved in acetone, etc., and clearly the resulting chain 

 length will depend on the treatment and the care with which the various 

 operations involved are carried out. 



The actual methods used can be classed as physical and chemical (or 

 analytical) and these will be discussed in turn. Some results of the 

 various methods are collected in Table III, where the striking variations 

 in the estimated value of the molecular weight bears testimony to the 

 difficulties involved. The results in Table III are given in terms of the 

 "degree of polymerization" i.e. the number of glucose residues united 

 in one chain, since this gives a clearer mental picture of the condition 

 arising in cellulose than would a statement of the molecular weight 

 itself. The actual molecular weight can be derived by multiplying the 

 figure given here by 162. 



(a) Osmotic pressure determinations. — It should be clear that since 

 osmotic pressure depends only on the number of particles of a solute 

 in a solution and not on their size, then measurement of the osmotic 

 pressure of a solution of known concentration (gm. per litre) will enable 

 the molecular weight to be determined. The method is of general 



