6 THEORY OF COLLOIDAL BEHAVIOR 



protein in 100 c.c. solution capable of combining with an acid or 

 alkali, is also a definite function of the hydrogen ion concentration. 



2. THE ISOELECTRIC POINT OF PROTEINS 



The conception of the "isoelectric point" of proteins was 

 introduced before its chemical meaning was recognized and it 

 attracted attention because it was connected with the precipita- 

 tion of colloids, a phenomenon on which the interest of a number 

 of investigators had been focussed. The conception of the 

 isoelectric point of proteins, which is due to W. B. Hardy, 1 

 must be considered as the starting point for the physical chem- 

 istry of proteins. This author found in 1899 that white of egg 

 diluted with eight or nine times its volume of distilled water, 

 filtered, and boiled when put into an electrical field migrated 

 in an opposite direction according to whether the reaction of the 

 fluid was acid or alkaline. When the fluid had an alkaline 

 reaction, the particles moved in an electrical field from the 

 cathode to the anode; when the fluid was acid, the direction of the 

 motion of the particles was the reverse, namely, from the anode 

 to the cathode; when the fluid was neutral the movement of the 

 particles under the influence of a current was so slight that it was 

 difficult to detect. 



"I have shown that the heat-modified proteid is remarkable in that its 

 direction of movement [in an electric field] is determined by the reaction 

 acid or alkaline, of the fluid in which it is suspended. An immeasurably 

 minute amount of free alkali causes the proteid particles to move against 

 the stream while in presence of an equally minute amount of free acid 

 the particles move with the stream. In the one case therefore the 

 particles are electro-negative, in the other they are electro-positive. 

 Since one can take a hydrosol in which the particles are electro-negative 

 and, by the addition of free acid, decrease their negativity, and ulti- 

 mately make them electro-positive it is clear that there exists some 

 point at which the particles and the fluid in which they are immersed 

 are isoelectric. 



"The isoelectric point is found to be one of great importance. As it 

 is neared the stability of the hydrosol diminishes until, at the isoelectric 

 point, it vanishes, and coagulation or precipitation occurs, the one or the 

 other according to whether the concentration of the proteid is high or 



1 HARDY, W. B., Proc. Roy. Soc., vol. 66, p. 110, 1900. 



