rENERAL CHEMICAL CHARACTERS OF PROTEINS 47 



tant, the filtrate after precipitation of the protein should contain 

 calcium chloride, excess of calcium phosphotungstate and hydrochloric 

 acid. As hydrochloric acid is much stronger than phosphotungstic 

 the calcium salt of the latter is decomposed, and the filtrate contains 

 as a matter of fact chiefly calcium chloride and phosphotungstic acid. 

 The latter acid cannot be accurately titrated with the use of rosolic 

 acid, which was employed by Cohnheim and Krieger as an indicator. 

 Certain of the conclusions drawn by these observers, therefore, as to 

 the dissociation of the hydrochlorides of proteins are erroneous. 



Another source of error has also been discovered by von Rhorer, 

 viz., that the bulky precipitates can adsorb certain quantities of acid 

 from solution. If a large excess of acid be present part of this 

 excess will be carried down ; it can, however, be recovered by suffici- 

 ently washing the precipitate. The statement of Cohnheim and 

 Krieger that the amount of acid entering into combination with 

 the protein depends on the amount originally present in solution is 

 erroneous ; the larger the quantity in solution the larger the quantity 

 carried down by the precipitate ; by thoroughly washing the latter, 

 and estimating the acid in the washings, the error due to adsorption 

 can be readily eliminated. 



Von Rohrer found that the precipitation method yields reliable 

 results if calcium picrate or potassium mercuric iodide be employed 

 as precipitants (provided, of course, that the precipitates be thoroughly 

 washed). By means of the latter reagent, he estimated the equiva- 

 lent combining weight of crystallised egg-albumin as 981, whereas 

 Sjoqvist estimated it for dialysed egg-white at 800 (see p. 41). 



Owing to its simplicity, the method seems worthy of further 

 application. 



The Salt Formation of Individual Proteins. 



The salt formation of some individual proteins has been investi- 

 gated in detail. Many of the general principles mentioned above 

 are well illustrated by these investigations. The following will receive 

 some more detailed consideration : 



(a) The salt formation of edestin, according to the investigations 

 of Osborne. 



(&) The salt formation of caseinogen, according to the investiga- 

 tions of Lacqueur and Sackur. 



(c) The salt formation of serum-globulin, according to the investi- 

 gations of Hardy. 



The Salt Formation of Edestin (Osborne). 



Owing to the ease with which edestin is obtained in quantity, and 

 to the fact that it can be recrystallised and obtained in an apparently 

 homogeneous state, it forms a very suitable substance for investigation. 



Osborne noticed that all the crystalline edestin preparations ob- 

 tained by the deposition of the protein from warm salt solutions are 

 capable of neutralising small quantities of potassium hydroxide 

 solution when phenol phthalein is used as an indicator ; some even 

 are slightly acid to litmus. Out of twenty preparations examined, 



ftr 

 eight required 0-2 to 0-5 c.c. - - potassium hydroxide to produce a 



