HEAVY METALS 141 



10-^ equivalents of base. This corresponds, therefore, with the 

 direct interaction of one molecule of silver nitrate with one molecule 

 of the caseinate, according to the equation: 



H 



I 

 HaN.R.CONa.N.R.COOH + AgNOg 

 I 



OH H 



I 

 = HzN.R.COAg.N.R.COOH + NaNOj. 

 I 

 OH 



This compound is sparingly soluble in water (0.47 per cent) 

 and its solution requires the addition of a small amount of NaOH 

 before it becomes neutral to phenolphthalein.* If to this solu- 

 tion, neutralized to phenolphthalein, more AgNOs be added, a 

 fresh precipitate is obtained containing a higher percentage of 

 silver than the first, and the additional silver content is exactly- 

 equivalent to the quantity of alkali which was added to the first 

 to render it neutral to phenolphthalein. The active mass of 

 caseinate in this reaction is directly proportional to the number of 

 — COH.N— groups which have been opened up by combination 

 with a base, i.e., which are electrolytically dissociated. Conversely, 

 undissociated —COH.N— groups cannot react with neutral silver 

 nitrate. 



The compounds which proteins form with zinc sulphate have 

 been studied by Lippich (72) who considers that the reaction 

 between zinc sulphate and protein results in an equilibrium which 

 is determined by a variety of factors amongst which he enumerates 

 ionization, hydrolytic dissociation, complex salt formation, etc. 

 He finds that serum proteins combine with zinc in alkaline solu- 

 tion to form insoluble salts which dissolve in excess {\ saturated) 

 of ammonium sulphate. This soluble salt he believes to be of the 

 type ZnS04-protein, the insoluble salt formed at a higher degree 

 of hydration (absence of a dehydrating agent such as ammonium 

 sulphate) being of the type Zn-protein. 



The compounds which copper salts form with amino-acids, 



* Since AgOH is a very weak base, a caseinate of silver corresponding to the 

 caseinate of a strong base wiiich is neutral to phenolphthalein, would neces- 

 sarily be acid to phenolphthalein. 



