30 



Scientific Proceedings (125) 



from which they are derived. The increase in acidity may be 

 titrated against a standard alkali solution and serve as a measure 

 of the amino acids and ammonia present in the sample. 



Sorensen (1907, 1908) pointed out that the reaction of amino 

 acids with formaldehyde is a reversible one. A considerable ex- 

 cess of formaldehyde is required to effect complete conversion 

 of the amino acids into their methylene derivatives, and, since 

 water is a product of the reaction, the presence of too much 

 water serves to throw the reaction back in the opposite direction. 

 Sorensen found it necessary to add as much as 10 c.c. of formalin 

 (40 per cent, formaldehyde) to 20 c.c. of amino acid solution. 



The amino acids are ampholytes. If a pure monoearboxylic 

 amino acid is dissolved in distilled water the solution will be 

 found to have a hydrogen ion concentration at or near the isoelec- 

 tric point of the amino acid, a fact also noted by Eckweiler, 

 Noyes and Falk 5 (1921). If it is titrated with alkali and the 

 titration curve plotted with amounts of alkali added as abscissae 

 and hydrogen ion exponents as ordinates the curve will be seen 

 to drop almost vertically toward the alkaline side as the first 

 drops of alkali are added. This represents a portion of the 

 "isoelectric zone" (Michaelis 6 , 1914, p. 40). As more alkali is 

 added the curve assumes a more nearly horizontal position and 

 if the curve is continued it again assumes a nearly vertical posi- 

 tion as the point of complete neutralization of the amino acid is 

 approached (at about P H 11.3 in the case of glycine). This lat- 

 ter vertical portion of the curve may be called the "zone of neu- 

 tralization. " When formalin is added the amino acid solution 

 becomes more acid not because the carboxyl groups are increased 

 but because the amino groups are destroyed. The titratable 

 acidity of the acid methylene derivative is the same as that of the 

 amino acid but the point of neutralization of the methylene de- 

 rivative lies at a higher hydrogen ion concentration (at about P H 

 8.0 in the case of methylenglycine) than does that of the amino 

 acid. For the formol titration of an amino acid the solution 

 should be reduced to a hydrogen ion concentration within its 

 isoelectric zone and after the addition of formalin the titration 



5 Eckweiler, H., Noves, II. M., and Falk, K. O., J. Gen. Physiol, 1921; 

 iii, 291. 



6 Michaelis, L., Die Wasserstoffionc-nkonzentration, Berlin, 1914. 



