270 ELECTROCHEMISTRY 



cent alcohol is a phenomenon of "electric endosmose" rather than 

 of true electrolytic conduction through the migration of ions. 



7. The Chemical Mechanics of the Coagulation of Proteins 

 by Alcohol. — We have seen that when potassium caseinate is 

 dissolved in varying concentrations of alcohol, the opalescence of 

 the solution undergoes an abrupt increase in the neighborhood of 

 70 per cent, indicating the beginning of coagulation, which, 

 however, does not proceed far enough, even on further addition 

 of alcohol, to lead to the actual separation of flocculi. We have 

 also seen that for solutions containing 60 per cent and less of 



alcohol the law — ^^ = constant holds good and that this 



a:^alc. HjO 



implies that almost the sole effect of the alcohol in these con- 

 centrations is to modify the ionic mobility of the casein ions, 

 leaving the degree of dissociation of the caseinate comparatively 

 unaffected. In solutions containing 75 per cent of alcohol, 

 however, the degree of dissociation of the caseinate is profoundly 



diminished and, consequently, the law — = constant no 



a^alc. H2O 



longer holds good. On the basis of the hypothesis and experi- 

 mental data outlined in preceding chapters this phenomenon 

 can readily be interpreted. We have seen that the protein 

 salts dissociate, not at terminal — NH2 or — COOH groups but 

 at — COH.N— groups within the molecule. If, however, the 

 terminal — NH2 and —COOH groups of the molecule are bound 



together, as they are in anhydrides of the type HN.RCOH.NR.CO, 



! ^1 



then this dissociation can no longer occur. We have already 

 (Chap. VI) seen strong reason for concluding that the coagula- 

 tion of proteins is accomplished by dehydration of the protein; 

 the electrochemical behavior of caseinates dissolved in alcohol- 

 water mixture is clearly in harmony with this view. 



As explained in Chap. VI, section 6, equations (i) to (iv), how- 

 ever, not merely "internal neutralization" of the protein molecule 

 may result from the dehydration of its terminal — NH2 and 

 — COOH groups, but also polymerization. The high electro- 

 chemical equivalent of potassium caseinate, dissolved in 75 per 

 cent alcohol, and the opacity of its solution favor the view that 

 this phenomenon also occurs in these solutions. 



From the effects of alcohol upon the rate of solution of casein 



