

 132 COLLOID CHEMISTRY OF THE PROTEINS 



is insoluble, and is precipitated as such. The alkali caseinate 

 described above, as it is capable of more accurate definition, is 

 particularly suitable for investigations of this kind, as in solution 

 the concentration of free hydroxyl ions and of alkali ions is so 

 small as not to come into consideration during electrolysis. 



If a second electrolytic cell in which the products can be 

 easily estimated, e.g., a silver voltameter, is included in the 

 main circuit with the solution under investigation, the ratio of 

 the weight of silver deposited to the quantity of protein precipi- 

 tated can be determined. The equivalent of the latter can then 

 be deduced from Faraday's law. Thus in the case of the two 

 caseinates 



I. K 3 . (Caseinate) '" and 

 II. K 3 . ([Caseinate] '" . Casein) 



when introduced into such a circuit in separate cells, the same 

 current will precipitate at the anode twice as much casein in 

 solution II. as in solution I. 



As it happens, T. B. Robertson,* inter alia, has made a 

 number of measurements of the electro-chemical equivalents in 

 such mixtures of casein and alkali. Solutions containing 

 50 X io- 5 gm. equivalents (0-5 millimol) and 100 X io~ 5 gm. 

 equivalents (i millimol) of potassium hydroxide per gram of 

 casein respectively correspond accurately to the caseinates 

 mentioned above. 



Robertson's measurements of the electro-chemical equivalent 

 of a protein are, so far, the only ones of their kind, and it is to be 

 regretted that he is so much under the influence of his theory of 

 the formation of protein ions that he has failed to make the 

 deductions which would be warranted by the importance and 

 accuracy of his work. 



His apparatus comprised a U-tube with platinum wires 

 leading into it. The two limbs of the U-tube could be connected 

 by a three-way stop-cock, or could be separately emptied. The 

 anode was a spiral of platinum wire of such a length as to 

 permit of a suitable current density, and at the same time retain 



* " Physical Chemistry of the Proteins," p. 176, where the literature 

 is quoted. 



