72 



CHEMICAL STATICS 



dissociation so that it is stable, and precipitation is complete, 

 only in the presence of an excess of the reagent. 



It is obvious that this method is open to all of the objections 

 attaching to the direct method of precipitation considered above. 

 In our present stage of knowledge we are very much in the dark 

 concerning the actual chemical mechanism of the precipitation 

 of proteins by reagents such as phosphotungstic acid and we are 

 by no means certain, indeed it is highly improbable, that only 

 one compound of a given protein with phosphotungstic acid 

 exists or that its composition is independent of the excess of 

 phosphotungstic acid employed. Hence it is not certain that in 

 the substitution of phosphotungstic acid for hydrochloric acid, 

 under the conditions outlined above, one equivalent of phospho- 

 tungstic acid replaces one or even a constant number of equiva- 

 lents of hydrochloric acid. Results obtained by this method are 

 at present, therefore, only of qualitative value. 



The experiments of Erb (11), who employed this method, 

 must nevertheless be quoted here in order to point out a mis- 

 conception which his interpretation of the results involves. One 

 cubic centimetre of a 5 per cent watery solution of vitellin was 

 mixed with 1, 2, 3, etc., cubic centimetres of normal HCl and the 

 volume of each solution was made up to 10 cc. To these were 

 added a constant excess of calcium phosphotungstate, the pre- 

 cipitate was filtered off and washed, and the filtrate titrated. 

 The following is the way in which Erb expresses his results : 



It will be seen that the quantity of HCl which is bound by 

 one gram of vitellin increases with increasing concentration of 

 HCl in its solution. We shall see that this phenomenon is a 

 general one where proteins are concerned. It appears that the 



