CORRECTNESS OF THE CHEMICAL VIEWPOINT 55 



about 3.0 or a trifle below. When too much acid is added, i.e., 

 when the pH is 1.6 or possibly a little above, casein precipitates 

 out again from a 1 per cent solution. 



Figure 9 gives the titration curves for HC1 and H 3 PO 4 , drawn 

 out within those limits of pH within which the casein salts are 

 soluble in a 1 per cent solution. The curves show that- about 

 three times as many cubic centimeters of 0.1 N H 3 PO 4 as of 

 0.1 N HC1 are required to bring 1 gm. of originally isoelectric 

 casein in a 1 per cent solution to the same pH; or in other words, 

 H 3 PO 4 combines with casein in molecular proportions, as should 

 be expected if casein phosphate is a true chemical compound. 



It was not possible to plot the corresponding curves for casein 

 sulphate and casein oxalate since these salts are too sparingly 

 soluble. This is true also for casein salts with other acids, e.g., 

 trichloracetic acid. 



From all these experiments we draw the conclusion that acids 

 combine with crystalline egg albumin, gelatin, and casein (and 

 probably proteins in general) by the same forces of- primary 

 valency by which the same acids combine also with crystalloidal 

 substances, e.g., NH 3 or NaOH. 



3. In the preceding experiments we started with isoelectric 

 protein and determined the number of cubic centimeters of 

 0.1 N acid required to bring the protein solution to a definite 

 pH. It seemed of interest to confirm our results by the reverse 

 titration; namely, by starting with a protein-acid salt of a 

 definite pH and determining how many cubic centimeters of 

 0.1 N NaOH are required to bring a solution of a protein-acid salt 

 to a definite pH, e.g., 7.0. This method requires, however, cer- 

 tain corrections which will become clear from the following con- 

 siderations. The experiments were made with gelatin solutions 

 containing about 0.8 gm. of originally isoelectric gelatin in 100 c.c. 

 solution. When we add different quantities of 0.1 N acid, e.g., 

 HBr, to 0.8 gm. of isoelectric gelatin, melt, and make a 0.8 per 

 cent solution by adding enough water to bring the volume to 

 100 c.c., there is in solution a mixture of two substances, namely, 

 free hydrobromic acid and gelatin bromide. The total amount 

 of Br contained in 10 c.c. solution can be determined by titrating 

 for Br; part of this Br is in combination with protein and part is 

 in combination in the free HBr, The latter part can be 



