ELECTROLYSIS 183 



of base until the proportion of base to casein in the film in im- 

 mediate contact with the anode falls to that which obtains at 

 "saturation" of the base with casein. Any additional casein 

 thus migrating into the film in contact with the anode must be 

 precipitated as uncombined casein. The cations, containing the 

 potassium, migrate to the cathode and there react with water, 

 liberating KOH, casein and hydrogen, the casein reacting with 

 the excess of KOH to again form potassium caseinate and to 

 again participate in carrying the current in each direction. 



Hence the electrochemical equivalent which is actually meas- 

 ured in solutions of all reactions is that of casein at "saturation" 

 of the base with the protein. 



Rejecting the data obtained in the solutions alkaline to phenol- 

 phthalein, on account of the possible error arising from hydro- 

 lytic decomposition of the protein due to the excess of alkali, 

 the average of all the determinations yields the value 0.0242 

 0.0019 for the electrochemical equivalent of casein. 



Multiplying this by the Faraday constant, 96,530, we obtain 

 the weight of casein in grams which transports one atomic charge. 

 This is 2336 =t 183. 



Now at "saturation" of a base by casein, the proportion of 

 base to casein is 11.4 X 10~ 5 equivalents per gram,* correspond- 

 ing, if at this reaction casein combines with only one molecule 

 of base, with the molecular weight of 8772. If we assume that 

 at "saturation" of the base with casein two, three, or four, etc., 

 molecules of base are bound up in one molecule of caseinate the 

 molecular weight of the casein would be two, three, or four, etc., 

 times 8772. Either of two assumptions may now be made: 



(i) The potassium caseinate dissociates into potassium and 

 casein ions. If this be the case then the weight of the casein 

 anion must be that of the molecule of casein, i.e., a multiple of 

 8772, and the valency of the casein ions must be a multiple of 



8772 

 2336 183 



i.e., of 3.8 dz 0.3, or, in round numbers, 4. 



(ii) The potassium caseinate dissociates into two protein ions 

 of approximately equal weight. If this be the case then the 

 weight of the casein anion must be half that of the molecule of 



* Cf. Chap. V. Also T. Brailsford Robertson (24) (27). 



