376 PROTOPLASM 



terms of acidity. The pH value at which latex (rubber) particles 

 do not migrate is, as determined by Moyer, about 4.1 for certain 

 species of plants. The isoelectric point is, therefore, expressed 

 as pH 4.1. 



Other properties of colloidal suspensions often change at the 

 isoelectric point. Stability is one of these. At the isoelectric 

 point, most colloidal suspensions settle out. The point of 

 minimum or maximum turbidity of a colloidal dispersion (mini- 

 mum where the suspension was originally coarse and has cleared 

 by setthng or rising of the particles; maximum, where the suspen- 

 sion was originally fine and has become cloudy because of 

 aggregation of the particles) may indicate the point of zero 

 potential and therefore the isoelectric point. Only in so far 

 as the minimum or maximum value of a selected property of a 

 solution indicates the point of zero potential, and therefore the 

 point of no migration in an electric field, is it the isoelectric point. 



The isoelectric point is rarely a point but a zone; that is to 

 say, if the ''point" at which particles will not migrate in an 

 electric field is determined and expressed in pH, it will be found 

 to be a range, say between pH 4.4 and 4.6, in which no migration 

 occurs. There is the further fact, discovered by Powis, that 

 coagulation does not take place only at zero potential but may 

 occur at a higher value. The potential at the surface of colloidal 

 particles is usually, in a stable solution, about 40 to 50 mv. 

 Precipitation and allied phenomena may occur at any potential 

 below 30 mv. At this value, an oil emulsion, which had an 

 original potential of 46 mv. at the surface of its oil globules, 

 may separate. Rather than refer to so high a potential value 

 as the isoelectric point, especially as the "point" is a long range, 

 it is instead termed the critical potential. Northrop found that 

 certain bacteria agglutinate at 11 mv.; this is their critical 

 potential. Usually, the potential at which a system becomes 

 unstable, i.e., the critical potential, is of more significance than 

 the isoelectric point. It better characterizes the system. 



While isoelectric points and reversibility are possessed by all 

 types of colloidal systems, they are primarily characteristic of 

 amphoteric substances such as protein and lecithin, which can 

 be reversed with the hydrogen ion. It was in connection with 

 the cataphoretic behavior of albumin that the term isoelectric 

 point was coined. The property is not so typical of colloidal 



