METHOD OF CATALYSIS 79 



Schmidt (45) who, with its aid, has been able to sharply dis- 

 tinguish between the precipitation of a protein through the 

 formation of a true, insoluble compound, and flocculation of the 

 protein through alteration of its physical condition or degree 

 of hydration; the so-called " salting-out" effect. 



The details of recent investigations by the potentiometric 

 method and their interpretation will be found in the chapters 

 dealing with the electrochemistry of the proteins. 



8. The Method of Catalysis. In this method, as in the 

 above, direct measurements of the number of hydrogen or 

 hydroxyl ions bound by the protein are secured. The measure- 

 ment is not, however, altogether a static one, since a foreign 

 substance is added to the solution of the protein, and the rate 

 of change of this substance is the quantity actually measured. 

 When the change in this substance leads to the setting free of 

 an acid (as, for example, in the catalysis of methyl acetate) the 

 method is open to serious objection. 



As is well known, many reactions are accelerated by hydroxyl 

 and hydrogen ions, and the velocity-constants of these reactions 

 (such as the inversion of sugar by acids, the saponification of 

 methyl acetate by alkalies, etc.) are directly proportional to the 

 concentration of the hydroxyl or hydrogen ions in the reacting 

 system. If, therefore, we act upon such a mixture with a given 

 concentration of an acid, or of a base, as the case may be, and 

 observe the velocity of the transformation, and, if we then act 

 upon a similar mixture with the same quantity of acid, to 

 which, however, a weighed amount of protein has been added, 

 and again determine the velocity of the transformation, the 

 difference between the two velocities affords a measure of the 

 numbers of H ions (or of OH ions, as the case may be) which 

 have been bound through the introduction of the protein. This 

 method was employed by Cohnheim (8) who states that it was 

 first used for this purpose by Hoffman, at the suggestion of 

 Wilh. Ostwald. 



As a reaction which is accelerated by H + ions Cohnheun em- 

 ployed the inversion of cane sugar. This reaction, in the presence 

 of free acid, obeys the monomolecular formula 



