VISCOSITY 



321 



increasing concentration. For the majority of crystalloids the 

 value of A is not greatly in excess of unity, while for sodium 

 caseinate, as we see, it is of the order of 10^^. This fact alone 

 would lead us to suspect that the mechanism which produces 

 the viscosity of these solutions is different in nature from that 

 which produces the viscosity of solutions of crystalloids. Sackur 

 has endeavored to ascertain which constituent of the solutions of 

 sodium caseinate plays the greater part in determining their 

 viscosity. He arrived at a conclusion the correctness of which 

 more recent investigations have fully established, by a process of 

 reasoning, however, which recent investigations have shown to 

 be in some respects fallacious. He argued that the viscosity of 

 these solutions might be attributable, primarily, to undissociated 

 sodium caseinate, or the product of its hydrolytic dissociation,* 

 i.e., free casein, or to caseinate ions. Hydrolytic dissociation 

 would, he believed, be diminished by the addition of alkali and 

 increased by the addition of acid. In the former case, according 

 to his view, the number of caseinate ions should be increased, in 

 the latter decreased. He found that the addition of alkali in- 

 creased and the addition of acid diminished the viscosity of a neutral 

 (to phenolphthalein) solution of sodium caseinate; the following 

 are among his results: 



hence, he argued, the viscosity of these solutions is primarily 

 attributable to caseinate ions. 



We have seen, however (Chaps. IV and VIII), that the caseinates 

 do not undergo hydrolytic dissociation in solution. In fact the 

 initial solution to which Sackur added alkali in order to suppress 

 hj^drolytic dissociation was neutral to phenolphthalein and there- 

 fore could not have contained more than 10~^ N NaOH derived 

 from hydrolytic dissociation of the sodium caseinate. We have 

 also seen (Chap. IX) that on adding alkali to a solution of a casein- 



* Since free casein is insoluble this possibility may be dismissed. 



