CHAPTER VI. 



PRECIPITATION OF VEGETABLE PROTEINS. 

 A. Precipitation by Neutral Salts. 



MANY of the seed proteins, like those of animal origin, are precipitated 

 by adding neutral salts to their solutions up to sufficient concentration. 

 A few of them are precipitated by saturating their solutions with sod- 

 ium chloride or with magnesium sulphate. All of them are precipitated 

 by saturating their aqueous solutions with ammonium sulphate or with 

 sodium sulphate at 33. Fractional precipitation with ammonium 

 sulphate is discussed 'in detail in Chapter VIII. 



B. Precipitation by Dilution or by Dialysis. 



Precipitation by dilution or by dialysis is frequently employed in 

 separating many of the seed proteins. From what has been said be- 

 fore it is evident that the degree of precipitation obtained by this means 

 depends largely on the reaction of the solution. Most of the precipi- 

 tates are protein salts and usually separate only from solutions which 

 contain a small quantity of acid. It is for this reason that many diluted 

 solutions of proteins yield a precipitate only after carbonic acid has 

 been passed through them, for the small proportion of hydrogen ions 

 thus liberated in the solution is sufficient to lead to the formation of 

 their insoluble salts. That such salts are formed by the action of car- 

 bonic acid has been shown by Osborne (3 1 3) by the following experi- 

 ment with neutral edestin, which is more soluble in a very dilute saline 

 solution than are its salts. Such a solution when diluted with water 

 until it shows a slight turbidity and then saturated with carbonic acid 

 yields a crystalline precipitate of edestin chloride. This precipitate 

 when washed free from chlorine requires a considerable quantity of 

 decinormal potassium hydrate solution to make it neutral to phenol- 

 phthalein. The solution in which the neutralisation is effected contains 

 over 90 per cent, of the added potassium in the form of chloride, which 

 shows that the precipitate produced by carbonic acid is edestin chloride, 

 formed in consequence of the slightly acid reaction imparted to the 

 solution by the carbonic acid. Neutral solutions of legumin and many 



35 3* 



