324 Keport of the Department of Chemistry or the 



siurn or ammonium chloride. This solubility is due to an exchange 

 of bases, which, for our purpose, can be represented by the following 

 reversible reaction: 



Ca< + 2 NaCl Z 2 Na caseinate+ CaCla 



caseinate 



(insoluble) (soluble) 



That the reaction is a reversible one is supported by the following 

 experimental evidence: Mono-calcium caseinate was prepared and 

 freed from soluble calcium salts by washing and dialysis. The 

 compound was then dissolved in a 5 per ct. solution of calcium-free 

 sodium chloride. That an interchange of bases had taken place was 

 shown by the fact that when the caseinate brine solution was dialyzed, 

 calcium was found in the solution outside the dialyzing tube. This 

 brine solution of caseinate was then dialyzed until free from cal- 

 cium and was then filtered. A solution of calcium chloride was 

 then added to this dialyzed solution and at once a precipitate of 

 calcium caseinate was produced. That this precipitate is a calcium 

 salt can be shown in two ways: (1) By washing and dialyzing until 

 free from soluble chloride and then igniting. Calcium is found in 

 the ash, (2) By washing and dialyzing until free from soluble 

 calcium, then redissolving in 5 per ct. solution of calcium-free sodium 

 chloride and dialyzing. Calcium is found to dialyze out of this 

 brine solution of caseinate. 



There is another point of interest in connection with this com- 

 pound which we will briefly refer to here but consider in more detail 

 in the report of another investigation. When a small amount of 

 acid is added to milk or is formed in milk by lactic fermentation, 

 a substance separates on warming which is very stringy and which 

 easily dissolves in a 5 per ct. solution of sodium chloride. This 

 substance is probably mono-calcium caseinate. 



Preparation of mono- and di-calcium caseinates. — In order to study 

 the composition and properties of these compounds more fully, 

 preparations of mono- and di-calcium caseinates were made. The 

 following method was employed: In 800 cc. of ^7 Ca(0H)2 there 

 were dissolved 20 grams of base-free casein. To this solution was 

 added 400 cc. of .^ HCl; the solution was then dialyzed to remove 

 most of the resulting calcium chloride. Then ^ HCl was added 

 very cautiously under constant agitation of the mixture until a 

 permanent precipitate began to appear, as shown by centrifuge. 

 The solution was then dialyzed again and then more acid was added 

 until a precipitate once more began to form. Alternate dialysis and 

 addition of acid were continued until no more acid could be added 

 without causing a precipitate. The amount of acid necessary to 

 precipitate all of the casein was next determined in an aliquot por- 

 tion, and one-third of this amount of acid was then added. The 



