"ISOELECTRIC" CONDITION 209 



a compound such that less than 1 per cent of the acid remains 

 uncombined (as estimated by gas-chain measurements). One 

 gram of casein combines with 90 X 10~^ equivalents of KOH to 

 form a compound such that less than | per cent of the KOH 

 remains uncombined (59). If, now, these salts were appreciably- 

 subject to hydrolytic dissociation, or even if they yielded CI' and 

 H+ ions respectively, then on mixing two volumes of a solution 

 of the ovomucoid salt with one volume of a solution of the casein 

 salt (each of the same percentage concentration) the K+ pro- 

 vided by the caseinate would be exactly neutralized by the CI' 

 provided by the ovomucoid salt and it might be anticipated 

 that free uncombined casein would be precipitated. Nothing 

 of the sort occurs, however. If to 25 cc. of a 2 per cent solution 

 of the casein salt are added 50 cc. of a 2 per cent solution of the 

 ovomucoid salt the mixture is no more opalescent than its con- 

 stituent parts and the conductivity of the mixture is the sum of the 

 separate conductivities of the two protein salts* If the mixture 

 be allowed to stand in the presence of toluol for a considerable 

 period at 36 degrees, however, after 24 to 45 hours a marked 

 increase in its opalescence is observed; after two or three days 

 traces of casein begin to be deposited, and after three to four 

 days all of the casein is found to have been precipitated. The 

 precipitation of the casein is accompanied by a marked increase 

 in the conductivity of the mixture, attributable to the setting 

 free of KCl, It is therefore evident that at the beginning the 

 mixture must contain only minute traces of K+ and CI' ions and 

 that the protein salts only yield up these ions with extreme 

 slowness. 



4. The "Isoelectric" Condition of Proteins at Certain H+ and 

 OH' Concentrations. — If we assume that the proteins dissociate 

 H+ and OH' ions according to the formula: 



NH3OH /NH3OH 



and 



R^ ^Il( +H+ 



^COOH ^COO' 



.NHsOH /NH3+ 



R^ t=^R( +0H' 



^COOH ^COOH 



* Conductivity of a 0.5 per cent solution of potassium caseinate containing 

 90 X 10"^ equivalents of KOH per gram at 30 degrees equals 38.0 X 10"' 

 reciprocal ohms. Conductivity of a 1 per cent solution of ovomucoid chloride 



