70 



CHEMICAL STATICS 



Spiro and Pemsel (49) employed ammonium sulphate to precipi- 

 tate protein salts; their procedure was as follows: A given weight 

 of protein was dissolved in a measured quantity of standardized 

 NaOH solution, and the protein-base compound was then precipi- 

 tated by the addition of a suitable excess of a saturated solution 

 of ammonium sulphate. The quantity of alkali bound by the 

 protein and precipitated with it was then determined by titration 

 of the filtrate. The following were their results, employing casein : 



Now I have shown (40) employing the potentiometric method 

 {vide infra), which is a static measurement, not involving any 

 change in the dynamical condition or composition of the body 

 of the solution, that in all of the above solutions investigated by 

 Spiro and Pemsel the amount of alkali which was actually bound 

 by one gram of casein while in solution must have been at 

 least 160 X 10~^ gram equivalents. Hence it is clear that in 

 the act of coagulation by ammonium sulphate these salts of casein 

 are very materially altered in composition. 



The method of coagulation by heat has been comparatively 

 little employed for the investigation of protein compounds with 

 acids. It cannot be employed for the investigation of compounds 

 of proteins with bases because although proteins in alkaline 

 solution are denatured by heat the denatured protein is only 

 flocculated in an acid medium (6). It has been shown by Chick 

 and Martin that in acid media the protein coagulum binds a 

 certain proportion of the acid, but whether or not the protein 

 salt, as it exists in the solution prior to coagulation, is flocculated 

 without alteration, cannot at present be definitely stated. 



At the present stage of our knowledge therefore, the direct 

 method of demonstrating the existence of protein compounds 

 cannot be trusted to yield accurate quantitative data, and re- 

 sults and conclusions based upon this method are to be accepted 

 with the greatest caution. 



