Electrodialysis as a Means of Characterizing Ampholytes

In the course of our studies of water soluble vitamins and bioses we have had frequent occasion during the past 5 years to enquire whether the physiological activity of an extract of a foodstuff was due to a single substance or a plurality of substances. Information was also often needed as to whether the substances involved were colloidal or crystalloidal, and if the latter, whether acidic, basic, amphoteric or non-electrolytic. This paper describes a method which often suffices to secure the answer to these questions by a single simple experiment, thus laying a good foundation for any attempt at isolation. The method also indicates the approximate isoelectric point of the substance in case it proves to be an ampholyte, a fact which is often of great service in suggesting the proper choice of precipitants. Kerr has cited an instance in which the method was of use, namely in detecting β bios. The method consists of electro-dialysing the solution in question in a multiple compartment cell. A convenient form of such a cell is illustrated in Fig. 1. After electrolysis the contents of the compartments are separately removed, their H' ion concentrations are determined and the solutions are assayed for the constituents of interest by chemical means if available, or by physiological test such as an animal feeding experiment. If the substance sought is an ampholyte it may be concluded that the pH of that portion of the solution which contains the substance in maximum concentration approximates the isoelectric point of the substance. For at any pH greater or less than that of the isoelectric point, the substance will be ionized more strongly as a base than as an acid or vice versa and will therefore migrate toward the region at which ionization as acid and as base are equal, namely the region of its isoelectric point.