H. ECKWEILER, H. M. NOYES, AND K, G. FALK 307 



Because of insufficient data, the application of these relations is 

 possible only in isolated cases. For simple substances, including 

 amino-acids, etc., the isoelectric point as defined is probably identical 

 with the hydrogen ion concentration of the pure substance dissolved 

 in water. For more complex substances such as proteins, with a num- 

 ber of different acid- and base-combining groups, there will probably 

 ordinarily be an overlapping of actions. The isoelectric point will 

 then be the hydrogen ion concentration which involves a minimum 

 combination with added acid or alkali, where the protein exists most 

 nearly uncombined. The isoelectric point of a substance obviously 

 shows the relative strengths of the substance acting as an acid and 

 as a base. 



Equation (1) requires that the isoelectric point of an ampholyte 

 does not change with change in concentration. ^^ Some results with 

 glycine and asparagine show definite if small changes in the hydrogen 

 ion concentrations of solutions of these substances on dilution,^* 

 The use of equation (2) may help to explain these variations. 



The agreement between the isoelectric points calculated by means 

 of equation (1) and those found experimentally is surprisingly close in 

 many cases. For substances such as glycine, etc., where the values 

 of the acid and basic dissociation constants are not far removed from 

 each other and the isoelectric points in the neighborhood of the 

 hydrogen ion concentration of the solvent, this is not unexpected. 

 For a substance like aspartic acid, for which ka^^ = 1.5 X 10~^ and 

 kb^^ = 1.2 X 10 "^2, the calculated isoelectric point according to equa- 

 tion (1) is very nearly (H+) = 10~^N. The value found by the indi- 

 cator method was (H+) = lO'^-^N. This can only mean that there 

 is some sort of compensation with terms (b) and (c) of equation (2) 

 resulting in the calculated values differing to only minor extents 

 from those given by equation (1). It must be remembered, however, 

 that this compensation is not a necessary conclusion in every case as 

 far as known at present, but that differences may be shown by the 

 two equations. 



24 Cf. Tizard, H. T., /. Chem. Soc, 1910, xcvii, 2490. 

 2^ Quoted by Clark,^ p. 30, from results of S. P. L. Sorensen. 

 ^^ Winkelblech, K., Z. physik. Chem., 1901, xxxvi, 546. Lunden, H., Z. physik. 

 Chem., 1906. liv. 532; /. Biol. Chem., 1908. iv, 287. 



