H. ECKWEILER, H. M, NOYES, AND K. G. FALK 



299 



The values for the isoelectric points of some of the substances are 

 given in Table II. The significance of these points will be discussed 

 in detail later in this paper. The values calculated by means of the 

 expression 



i 



I = 



kw 



in which ka and h represent the ionization constants of the ampho- 

 lyte substance acting as an acid and as a base, and kw the ionization 

 constant of water (very nearly 10~^^ at the temperatures used) are 

 given and also the values determined experimentally by means of the 

 potentiometer (values from Table I), and by the indicator methods. 



TABLE II. 

 Isoelectric Points of Some Amino-Acids and Peptides. 



Glycine 



Alanine 



Leucine 



Glycyl-glycine . 

 Alanyl-glycine . 

 Alanyl-alanine . 



ka* 



1.8X10-10 

 1.9X10-10 

 1.8X10-10 

 1.8X10-8 

 1.8X10-8 



kh* 



2.7X10-12 

 5.1X10-12 

 2.5X10-12 

 2.0X10-11 

 2.0X10-11 



Isoelectric points. 



Calcu- 

 lated. 



PH 



6.1 

 6.2 

 6.1 



5.5 

 5.5 



Poten- 

 tiometer. 



pH 

 5.4 



5.3 

 6.1 

 5.6 

 5.2 

 5.6 



Indicator. 



pU 



6.0 



6.15 



7.0 



5.5 



5.15 



6.15 



* Winkelblech, K., Z. physik. Chem., 1901, xxxvi, 546. Euler, H., Z. physiol. 

 Chem., 1907, li, 219. 



Before discussing these results, a number of points connected with 

 the experimental measurements will be taken up briefly; first for the 

 results given in Table I, then for those in Table II. 



Recently it was stated^ that "There is hardly any use attempting 

 the measurement of unbuffered solutions, if indeed there would be 

 any significance to the measurement were it accurate." This state- 

 ment apparently necessitates further explanation, possibly basing the 

 definition of the term buffer in some way upon the slope of the titra- 

 tion curve. Otherwise, solutions of hydrochloric acid, etc., would be 



' Clark,^ p. 184. C/. also a similar statement on p. 34. 



